How to maintain a garden free of invasive species
It’s long been known that certain imported garden plants can be a problem for our native species. But why are certain species particularly problematic, while others are not? And which species should you avoid in your garden? You’ll find the answers here.
Translation by Julian Lee
Go directly to: Siberian squill | Moneywort | Snow-in-summer | Myrtle | Garden lupin | Large yellow loosestrife | Goat’s beard | Himalayan balsam | Beach rose | Red elderberry | Common broom | Hollyberry cotoneaster
Here you can download the entire campaign. Note: campaign materials are in Norwegian
What makes a plant a threat?
At garden centres, plant nurseries, and flower shops we find many wonderful species to take home to decorate our houses and gardens with. Many of these plants are native to other countries, and the majority of ornamental plants available for purchase are cultivars. Cultivars are plants that have been artificially bred to showcase specific desired qualities and appearances. Often they have also been bred to be resistant to diseases and pests, and sometimes also to be easy to grow and to require little maintenance. Cultivars that are sold in Norway frequently have good tolerance to our cold and wet climate in order to make them great garden plants. It is often these ones which can survive and begin to spread in our surroundings, if they can spread through hardy seeds, tubers, or division.
As a result, one needs to pay special attention to a plant’s hardiness, competitiveness, and ability to spread itself when one is evaluating which plants to have in the garden. Some non-native plants are so invasive and hardy that they can pose a threat for the natural diversity by spreading from the garden and into nature, and out-competing native species. These are the plants that we most should try to avoid in our gardens.
Luckily, someone has done the work for us
Wondering whether a plan could pose a threat to our local natural habitat and native species? An expert panel in Norway’s Species Database (Artsdatabanken) conduct scientific evaluations of which species pose a threat to Norwegian nature. The information and risk evaluations of the non-native species are published in the Non-Native Species List of 2018 (Fremmedartslisten 2018) on artsdatabanken.no and can be used by anyone. The next Non-Native Species List will be published in 2023.
How does the Non-Native Species List work?
Experts conduct thorough objective and quantitative risk evaluations of non-native species based on documented data and surveys. There are nine criteria which are considered in the evaluations (grouped into the two categories of “Ecological Effect” and “Invasion Potential,” see the Explanation of Terms section of this article), and the risk evaluations are conducted on species which established themselves in Norway after 1800 or those which likely will establish themselves here within the next 50 years.
The “Invasion Potential” and “Ecological Effect” are the most important parameters which determine whether a non-native species poses a threat for our local biodiversity or not. Additionally, a species must be able self-sustain a population for more than 10 years before it is considered to be of risk. It is how high a species scores in these categories which determines how high of a risk the plant has for damaging Norwegian biodiversity.
When the species are categorised, they are given a risk rating of SE (extremely high risk), HI (high risk), PO (potential risk), LO (low risk), or NK (no known risk). Species marked NR in Norway’s Species Database have not been evaluated for risk based on the criteria of the Non-Native Species List. The species categorised as SE (extremely high risk) and HI (high risk) are those that we first and foremost want to prevent from spreading further in nature. If you have such species in your gardens and backyards, we urge you to remove the plant and discard of roots and plant parts in your general waste collection points. Then we can avoid further spread of these species into our surrounding nature.
It’s also worth noting that species marked PO (potential risk) have the potential to pose a threat in the future. Some of these species have a high spreading capability and high ecological effect if given the opportunity to spread themselves, but have not been planted in significant amounts to date, and as such have not spread much yet. Here, it can be smart to use the precautionary principle and err on the side of caution. If possible, try to avoid species that have the potential to be added to the Non-Native Species List in the future.
Do note that not all non-native species are damaging to our environment. But there are also species which can be damaging even if they have not had their risk rated yet.
What can you as a conscious garden owner do?
Even though the Non-Native Species List is a great resource, we know that it can sometimes be cumbersome to use when you are at your local garden centre buying plants, have received some new plants from a fellow gardener, or simply wish to remove non-native and damaging plants from your garden. As such, we’ve curated a selection below of common garden plans from the Non-Native Species List that are rated as SE (extremely high risk) or HI (high risk). These are the worst of the worst, so if you find them in your garden, we recommend you to remove them. Consider replacing them with local species and species which support your local insects, bees, and other pollinators.
In the case that you do not want to remove the plants completely, it is extremely important that you prevent them from spreading by carefully controlling them when they prepare to produce seeds.
It’s also very important that you discard roots and plant pieces from these species in your general waste collection points, and not in your compost or yard waste. Sadly, this is one of the most common ways that we unintentionally spread non-native species.
Demonstrating to garden centres that you wish to avoid non-native species is also important, as the power of consumers is greater than you might think. Request clear and good labelling of non-native species where you shop, and ask for safe and nature-friendly plants.
Four rules of thumb
- Don’t purchase plants with a risk rating of SE (extremely high risk), HI (high risk), or PO (potential risk) for your garden. Instead, look for more local-friendly plants at your nearest plant nursery or garden centre.
- If you do have a non-native and invasive plant in your garden, remove all parts of the plant and discard them in the general waste collection bins, or deliver them to your nearest waste disposal services. Do ensure that you alert them that these are invasive species that you are dropping off, and that they must not be composted.
- Don’t throw plant cuttings over the fence or outside of your garden, whether or not they come from invasive or native species.
- Remove flower stalks before they go to seed and maintain your plants well to restrict their growth, both inside and outside of your garden.
Are you wondering what plants you have in your garden? You can use the app Artsorakalet from Norway’s Species Database (Artsdatabanken), or the PlantSnap or PlantNet. You can also ask others on Norwegian Facebook groups such as “What is this?” (“Hva er dette?”) and others.
Tuber division: A tuber is a swollen part of a stem or root, which has the ability to produce another plant. If parts of the tuber are detached or broken off, a new plant can grow from these alone.
Clonal growth: Plants can spread and create new plants without the use of seeds, and plants produced in this way are genetically identical to the original plant. Examples of this include growth from root sprouts, tubers, or other pieces of plants which have the capability to grow new roots and plant parts.
Invasion potential: This is defined as the extent to which a species can spread itself and survive in nature, or become naturalised to a new area. The invasion potential is calculated from a plant’s viability in Norway (whether it has the ability to survive and spread year-round), growth speed (how fast a species can spread, in meters), and colonisation ability within the ecosystem (how much of an ecosystem a species can establish itself within).
Note: In the campaign, we have referred to “invasion potential” (invasjonspotensiale) as “spreading risk” (spredningsfare) instead.
Ecological effect: This quantifies how much a species disrupts the ecological balance in the new location where it tries to establish itself. The ecological effect describes the negative impact that the invasive species has on endangered or key species and on other various native species. It also describes the impact on threatened or rare types of ecosystems, as well as the impact from transfer of genetic material or of parasites and pathogens from the invasive species to endangered or threatened native species.
Note: In the campaign, we have referred to ecological effect (økologisk effect) as “natural disturbance” (forstyrrer naturen).
The responsibility of plant nurseries and garden centres
A sincere “thank you” should be given to all garden centres which have removed a wide range of non-native invasive species from their inventories. The investigations which we have done do, however, reveal that there still do exist some foreign invasive species for sale.
Our challenge to all those who sell and distribute plants for outside gardening and landscaping purposes:
- Operate your business in such a way that it is not detrimental to the local ecosystems and biodiversity
- Remove all non-native invasive species from your inventory
- Instead select plants to sell that do not have risk rating of SE (extremely high risk), HI (high risk), or PO (potential risk)
- Purchase your plants and products from local gardeners and nurseries which breed garden plants of varieties that occur naturally in Norway
- Refrain from selling harmful pesticides and insecticides
- Reduce the sale of soil with a high peat content and increase the sale of peat-free soil
- Promote the sale of plants and seed mixes which are beneficial to the insects and wildlife of your local region
Ban on 28 invasive non-native plant species
The Norwegian Environmental Agency (Miljødirektoratet) has instituted a ban against 28 dangerous vascular plants. This means that it is not legal to sell, clone, or spread these plants. If you have any of these plant species in your garden, we recommend that you remove them and discard all plant parts into the general waste collection, or deliver them for destruction at your local waste disposal services.
The complete list over banned plants:
- Alaskan dogwood or red osier dogwood (Swida sericea) (Norwegian: Alaskakornell)
- Alpine laburnum (Laburnum alpinum) (Norwegian: Alpegullregn)
- Balsam poplar (Populus balsamifera) (Norwegian: Balsampoppel)
- Cotoneaster (Cotoneaster monopyrenus) (Norwegian: Blomstermispel)
- Diel’s cotoneaster (Cotoneaster dielsianus) (Norwegian: Dielsmispel)
- Snow-in-summer (Cerastium tomentosum) (Norwegian: Filtarve)
- Two-row stonecrop (Phedimus spurius) (Norwegian: Gravbergknapp)
- Crack willow (Salix x fragilis) (Norwegian: Grønnpil)
- Common laburnum (Laburnum anagyroides) (Norwegian: Gullregn)
- Large-leaved lupine (Lupinus polyphyllus) (Norwegian: Hagelupin)
- Bohemian knotweed (Reynoutria x bohemica) (Norwegian: Hybridslirekne)
- Japanese barberry (Berberis thunbergii) (Norwegian: Høstberberis)
- Wild lupine (Lupinus perennis) (Norwegian: Jærlupin)
- Canada goldenrod (Solidago canadensis) (Norwegian: Kanadagullris)
- Giant hogweed (Heracleum mantegazzium) (Norwegian: Kjempebjørnekjeks)
- Giant goldenrod (Solidago gigantea) (Norwegian: Kjempegullris)
- Giant knotweed (Reynoutria sachalinensis) (Norwegian: Kjempeslirekne)
- Himalayan balsam (Impatiens glandulifera) (Norwegian: Kjempespringfrø)
- Japanese knotweed (Reynoutria japonica) (Norwegian: Parkslirekne)
- Wide-leaved bellflower (Campanula latifolia macrantha) (Norwegian: Prydstorklokke)
- Beach rose (Rosa rugosa) (Norwegian: Rynkerose)
- Nootka lupine (Lupinus nootkatensis) (Norwegian: Sandlupin)
- Phedimus hybridus (Phedimus hybridus) (Norwegian: Sibirbergknapp)
- Spreading cotoneaster (Cotoneaster divaricatus) (Norwegian: Sprikemispel)
- Boreal chickweed (Cerastium biebersteinii) (Norwegian: Sølvarve)
- Persian hogweed or Golpar (Heracleum persicum) (Norwegian: Tromsøpalme)
- Common waterweed or Canadian pondweed (Elodea canadensis) (Norwegian: Vasspest)
- Western waterweed or Nutall’s waterweed (Elodea nuttallii) (Norwegian: Smal vasspest)
Read more about how you can remove invasive non-native vascular plants.
Invasive species that are rated SE (extremely high risk) or HI (high risk)
Siberian squill, or wood squill (Othocallis siberica) (Norwegian: Russesblåstjerne)
Risk rating: HI (high risk), high invasion potential, and low ecological effect.
The Siberian squill is a beautiful but hardy bulb-plant in the asparagus family which grows naturally in southwestern Russia, the Caucuses, Turkey, Syria, and Iran.
It has hanging dark blue bell-shaped flowers that are roughly 2,5 cm in size. The petals have a dark centreline and a white tip, and the anthers are grey-blue. The plant itself has two to four small and narrow shiny leaves at its base.
The Siberian squill is often used as an early flowering spring plant because it begins to bloom as early as March and April. It can form huge flower carpets and is therefore often used as a ground covering in planters under trees and rhododendrons.
It spreads itself mainly via garden waste and self-spreading from its seeds and bulbs. Ants can also contribute to its spread when they carry its seeds. If Siberian squill spreads itself in the wild, it can quickly form large flowering swaths in nutrient-rich grasslands and open forest as it has an extremely high invasion potential. It can cover large areas of forest floor, and by doing so, can have suffocating effect and displace other native plants.
- The garden owner is responsible for ensuring that the plant does not spread outside the garden. The best option is to remove the plant and replace it with a local insect-friendly plant.
- Because it is a bulb plant, it is very important to remove all bulbs from the soil, as well as any small roots. It can be a good idea to do this before the plant begins to flower, so as to hinder further seed creation and spread. In many cases, this process needs to be done more than once, as there can be other plant parts left behind in the ground.
- It is smart to remove the flowers before they begin to seed. Discard the entire flower, and any seeds, in the general waste.
- This plant should not be placed in the compost, or in other places where it can spread into the wild
- As a good alternative, you can instead plant snake’s head fritillary (Fritillaria meleagris; in Norwegian: Rutelilje). It is an old and well-known garden plant which can occasionally spread, but cannot form populations which can survive over ten years (in the Norwegian climates). It is also not as capable of spreading as the Siberian squill. As a result of these characteristics, the snake’s head fritillary is not considered to be a plant which poses significant risk. This flower is especially beneficial to bumblebees, and is a beautiful addition to any garden.
- If you are looking for a spring-blooming flower with the same colour as the Siberian squill, the Siberian bugloss / great forget-me-not (Brunnera macrophylla; in Norwegian: Forglemmegeisøster) can be a good alternative
Risk rating: SE (extremely high risk), and medium ecological effect
Moneywort is a plant from continental Europe and the Caucuses which spreads quickly in the wild and displaces other species. This can lead to the native species being out-competed when moneywort establishes itself in an ecosystem. Moneywort is a ground-covering plant which grows early in the spring. For a more nature-friendly garden, it is smart to remove this plant and instead plant some local insect-friendly plants.
The moneywort plant can be recognised by its 10 to 60 cm long creeping and rooting growths which blanket the ground in a carpet of foliage. The leaves have a characteristic lime-green colour, and are rounded and grow symmetrically from the stalk. That is to say that the leaves grow directly opposite each other on each side of the stalk. From the corners of the leaf stems grow two bright-yellow cup-shaped flowers which bloom between June and September.
- The garden owner is responsible for ensuring that the plant does not spread outside the garden. The best option is to remove the plant and replace it with local insect-friendly plant.
- Moneywort forms creeping shoots which crawl across the ground and can take root, and as such, the plant can be difficult to get rid of. You should remove all plant parts and roots, and continue to monitor the area over the coming years, especially if the plant was rather established at the time you began to remove it.
- If the garden waste contains moneywort plants, or parts of plants, never discard it outside of the garden or in the compost. All plant parts should be discarded in the general waste collection or delivered for destruction at your local waste disposal services.
- Broad-leaved thyme / lemon thyme (Thymus pulegioides L.; in Norwegian: Bakketimian) or Breckland thyme / wild thyme (Thymus serphyllum; in Norwegian: Kryptimian) are good alternatives to moneywort. In addition to smelling good, these plants are edible herbs which can be used for seasoning.
- Lady’s mantle (Alchemilla vulgaris) is also a good choice, as it thrives in the same growth conditions and can be found naturally all over Norway. It is a beautiful plant whose leaves and flowers can be added to summer salads (except for the alpine lady’s mantle – Alchemilla alpina; in Norwegian: Fjellmarikåpe). Lady’s mantle does well in both shaded and sunny areas. Try to avoid planting garden lady’s mantle (Alchemilla mullis; in Norwegian: Praktmarikåpe) as it has an extremely high spreading risk and a negative ecological effect.
Snow-in-summer / boreal chickweed (Cerastium tomentosum L. / Cerastium biebersteinii) (Norwegian: Filtarve / sølvarve)
Risk rating: SE (extremely high risk), with high invasion potential and medium ecological effect. Forbidden to spread, sell, or distribute further.
Snow-in-summer and boreal chickweed belong to the carnation family. They originally come from Italy and Crimea, and have been grown since the 1800s as a decorative plant. These two plants are so similar that they are treated as one in the Non-Native Species List, and they have presumably also formed hybrids.
Snow-in-summer and boreal chickweed grow in 10-40 cm high thick carpets, and are often used as an edge growth or groundcover plant. They can be distinguished by the silver-grey-green hairy leaves and white flowers, which have five petals which split just before the edge of the petal. The plants form small bouquets containing between three and 15 flowers, which bloom between May and June. Snow-in-summer and boreal chickweed spread quickly by clonal growth and via seeds. They have creeping side shoots which form new roots.
These plants are evaluated to be extremely high ecological risk due to their high invasion potential in areas with rocky and shallow soil, particularly in calciferous soils in the boreal zone, where it can displace vulnerable or endangered species. Snow-in-summer and boreal chickweed have several negative ecological effects related to displacement, changing of local ecological conditions, and introgressive hybridisation (resulting in several different genes that are very different from the parent genes). These plants are now found over all of Norway, with the exception of Trøndelag and north of Alta.
It is forbidden to sell, spread, give away, or discard waste from snow-in-summer and boreal chickweed outside of your garden. If you have these plants in your garden, we recommend that you remove them and discard all parts in the general waste collection, or deliver them for destruction at your local waste disposal services
How to remove:
- All plant parts must be removed. That is to say, remove all leaves, flowers, stems, and roots from the garden. Be careful not to drop any plant pieces on the ground, or you risk that it will spread to other areas. This process may need to be repeated if there are plant parts or seeds left in the soil. Check again in the following years to ensure that nothing has sprouted up again.
- If you deliver the plant parts to a waste disposal service, it is important to specify that they be destroyed or disposed of in the general waste collection, as foreign invasive species should not be composted
A beautiful alternative to these plants is woodland sage, also known as steppe sage (Salvia nemorosa; in Norwegian: Steppesalvie). Woodland sage thrives in dry and sunny locations, but can also do well in partial shade. It works well both as a decorative edge flower or as a main flower in your garden. When it flowers, woodland sage grows faster and will not spread as fast, but in return you will create an environment which welcomes many friendly local pollen-spreading insects. There are pink, purple, and white variants of the plant. Yarrow (Achillea millefolium; in Norwegian: Ryllik) is another good and eco-friendly plant, but avoid noble yarrow (Achillea nobilis; in Norwegian: Engryllik) as it is rated high risk on the Non-Native Species List.
Lesser periwinkle, common periwinkle, or myrtle (Vinca minor) (Norwegian: Gravmyrt)
Risk rating: SE (extremely high risk).
This plant species Vinca minor is known by many names, including the lesser periwinkle, dwarf periwinkle, common periwinkle, myrtle, or creeping myrtle – for simplicity, it will be referred to by myrtle throughout this article. Myrtle is a perennial ground-covering plant in the dogbane family which is native to continental Europe. It forms tight mats and spreads via creeping stems which grow roots. Myrtle is wintergreen-coloured and has shiny elliptical leaves. The plant releases a milky-white sap when stems or leaves are broken. This invasive garden plant produces purple-blue funnel-shaped flowers between May and June; these flowers have five petals surrounding the stigma.
Myrtle spreads easily via garden clippings and by the creeping root stalks. When it establishes itself somewhere, it crowds out other low-growing plants. It occurs in nature in areas which have many threatened species. For example, it is a direct threat to the endangered rock cinquefoil plant – Drymocallis rupestris (Norwegian: Kvitmure). The risk ratings in the Norway’s Species Database (Artsdatabanken) include other feral species of myrtle, including the greater periwinkle – Vinca major (Norwegian: Storgravmyrt).
- As the Norwegian Environmental Agency (Miljødirektoratet) names this plant on their forbidden list, it is against the law to sell, distribute, or spread the plant. We recommend removing the entire plant, along with all plant parts and roots.
How to remove:
- The plant spreads via rooting growths and not via seeds (in Norway). When removing, loosen the surrounding soil first, so that the entire plant and its roots can be removed and discarded in the general waste collection. It must not be tossed outside of the garden, or placed in the compost, as it will then take root and begin to grow in those locations instead.
- Remove any new growths a few times in the season, and monitor the area the following year as well. If there is a significant amount of plant waste to be discarded, you should contact your local waste disposal services to ensure that it will be discarded as general waste, or destroyed. Make sure that when transporting it there to not lose any plant parts along the road.
When you have removed myrtle from your garden, you can plant an eco-friendly groundcover instead. If you would like a plant with blue flowers, you could plant Siberian bugloss / great forget-me-nots (Brunnera macrophylla; in Norwegian: Forglemmegeisøster) instead, or you could just choose a completely different type of plant which attracts beautiful butterflies and friendly bees.
Source: Norwegian Species Database.
Garden lupin, or large-leaved lupine (Lupinus polyphyllus) (Norwegian: Hagelupin)
Risk rating: SE (extremely high risk); high invasion potential and high ecological effect.
Garden lupins are 50 to 150 cm tall perennial plants in the pea family. Lupins (or lupines) have tall stalks which are full of monosymmetric flowers which are arranged in rows in long clusters. The leaves are pointy and branch out, with long hairs on the underside. Lupins can be found in blue, pink, and white variants, and they bloom between May and June. The garden lupin originated in North America, but is now naturalised in large parts of Europe after it was imported in 1826.
A lupin plant can produce several hundred seeds; these seeds can remain viable for as long as 50 years. It can therefore be problematic to stir up or mix the soil in areas where lupins have already gone to seed. Luckily, the seeds don’t spread very far away from the plant by themselves, but they can be transported accidentally when soil is being transported and by machinery. Because the seeds can remain viable for up to 50 years, old meadows of lupins can easily spring up again if the conditions become right and the seeds suddenly germinate. New plants can also sprout from the short rhizomes of the plant. Torn pieces of roots and rhizomes can contribute to spreading lupins beyond the field and garden, and they are now found in large areas across all of Norway.
Garden lupins are well suited for improving the soil, because the nitrogen-fixing tubers on the roots, in addition to dead plant material, contribute to enriching soil and increasing its nutrient contents. As a result, in Norway it is often used along roads, land patches, and along train tracks as a sort of green fertiliser and as a decorative plant in gardens. Because the nitrogen-fixing tubers of lupins improve and change the soil quality, this can lead to other species being able to establish themselves in areas where they would not have been able to grow under normal conditions. For example, this occurs along riverbanks where it can outcompete near-endangered species such as German / false tamarisk (Myricaria germanica; in Norwegian: Klåved) by transforming the structure of the areas where it grows, changing the sediment composition, and altering the species’ nutrient supply.
- Luckily, it is now forbidden to sell, plant, distribute, or spread garden lupins. As a responsible garden owner, you can remove the flower stalks before they begin to go to seed, and in general by containing growth and spread of the plant within your garden. Also ensure that you handle the clippings carefully, and do not discard them in the compost or outside of your garden.
- However, we still encourage you to remove the plant, along with all plant pieces, roots, clippings, and seeds. It is very important that you do this carefully, so that no plant pieces are spread by movement of the plant clippings and waste when you do this. Deliver the garden waste to a known local waste disposal services for destruction. Be sure to label the waste as containing lupins so that it is understood that you are dropping off non-native invasive species, and not regular garden waste.
- If there are large areas with garden lupins on your property, it can be necessary to cut down or remove them twice a year for up to three to five years. It is important to cut down the lupins before they begin to bloom, so as to prevent the development of seeds, and then to repeat it again after two months for the best effect. Alternatively, if done only once a year, it should be done before flowers appear. All plant waste must be delivered to a trusted waste disposal services for destruction.
- If you have had garden lupins in your garden earlier, you must assume that the seeds and rhizomes which remain in the ground can sprout up again. Pay careful attention to your garden and weed it thoroughly multiple times in the season, and over the course of several years. If anyone offers you lupins, we would advise you not to accept them.
Did you know that the Nootka lupin / sand lupin (Lupinus nootktensis; in Norwegian: Sandlupin) and wild lupin (Lupinus perennis; in Norwegian: Jærlupin) are also problematic species in Norway? These lupin variants are registered as extremely high risk (SE) and high risk (HI), respectively, on the Non-Native Species List.
If you are very attached to lupins, you can instead consider annual and heat-loving lupin species which would not be able to establish themselves in Norway in the next 50 years, given that the climate conditions continue as they have. You can find more information on various species at the Norwegian Species Database.
Preferably though, you could instead (and with higher ecological safety and peace of mind) plant catmint, also known as Faassen’s catnip (Nepeta x fassenii; in Norwegian, Prydkattemynte). Catmint is evaluated as being unable to form long-lasting growths over several years (in Norway) and as a result does not pose a threat to the local biodiversity. That said, do be mindful of where you discard your garden waste when the plant becomes too large. Catmint should also not be discarded outside of your garden. With catmint, you have a friendly plant with beautiful violet flowers, also in clusters, which attract bees and other beneficial visitors the entire summer. Trimming it down right after the first bloom will also help ensure a second bloom toward the end of the summer.
Risk rating: SE (extremely high risk); high invasion potential and medium ecological effect
The large yellow loosestrife is native to south-eastern Europe and thrives in damp and wet forests. It grows to between 30 and 100 cm tall. The flowers are 10 to 16 mm wide, with five petals, and they grow in pairs from the corners of the leaves, in a ring around the stalk. The petals have deep lobes and are yellowish-orange in colour; the plant flowers between July and September. The leaves are lanceolate-shaped.
The large yellow loosestrife is an old garden perennial which was first observed having escaped into the wild in Norway in 1865, but it quickly spread through the end of the 1950s when it became a very popular garden plant in Hordaland, Rogaland, and Oslo / Akershus. Since then, it has also spread to other parts of Norway as it began to be more widely planted in other counties; it now has permanent populations growing in some areas.
This plant poses a threat for the local biodiversity because it outcompetes local species and displaces other species. Examples of displacement caused by the large yellow loosestrife have been seen in boggy forests and in semi-natural meadows, but it can also have a negative effect in drier and denser forests.
The large yellow loosestrife can easily establish itself in areas outside of gardens, such as slopes, ditches besides roads, scrub areas, and the edges of forests. It has previously spread itself via garden waste and movement of soil to gardens and for road construction.
- The garden owner is responsible for ensuring that the plant does not spread outside of the garden. It is best to remove the plant and replace it with an insect-friendly plant instead.
- The large yellow loosestrife spreads over short distances by clonal growth via rhizomes, as well as through seeds. As such, you must remove all plant parts and roots. It is likely that you may need to repeat the process over the coming years. This plant spreads easily if waste containing plant clippings and pieces of it are not handled properly; if you toss it somewhere, it will grow there. It must be dumped in the general waste collection bins for incineration, or delivered to a local waste disposal services for destruction. It can be smart to check if it has established itself in areas around your garden. If it has, remove it here as well, or register it in the Norwegian database used to track observations and spottings of various species (www.artsobservasjoner.no).
As an alternative, you can plant the orange coneflower / perennial coneflower (Rudbeckia fulgida; in Norwegian: Praktsolhatt); this plant poses no known risk in Norway. The flowers are a bit different as they are daisy-shaped, but they will look wonderful in your garden and are loved by bumblebees. The orange coneflower grows to between 50 and 100 cm tall, and blooms between July and August.
Source and link to the Norwegian Species Database’s Non-Native Species List 2018.
The large yellow loosestrife has not been documented in western Norway / Vestlandet. Do you have this plant in your garden, or have you seen it in the wild? Register the plant here. (LINK: https://www.artsobservasjoner.no/)
Fun fact: Did you know that the large yellow loosestrife is one of Norway’s oldest garden plants? Source.
Risk rating: SE (extremely high risk); high invasion potential and medium ecological effect
The goat’s beard plant is a one to two meter high plant with 20 – 50 cm long shoots of small cream-white flowers that bloom between June and July. It is also called buck’s beard or bride’s feathers, and its Norwegian name of skogskjegg directly translates to “forest beard.” It grows large and thick with dense foliage, and is often used in gardens between flowers, or as a sort of garden hedge during the summer months. The stalks are straight and do not have branches; there are two to three offshoots with leaves on each stalk (compound pinnate leaves). These leaves are egg-shaped and pointy at the end, with serrated edges down the side. After the plant flowers, the flowering stalks turn brown.
The goat’s beard plant belongs to the rose family (Rosacea) and has been a popular perennial tall-growth garden plant since the 1700s. It is unclear where the plant truly originates from, but it is found naturalised in the wild in southeast Asia, central Europe, and western parts of North America. Goat’s beard has spread quickly in Norway ever since it was first discovered having escaped into nature here in 1923. The reason that it has only recently escaped into the wild in Norway, despite having been present in gardens for many years earlier, is the introduction of female plants – which led to an explosive growth in seed production. The goat’s beard plant, as the scientific name suggests, is dioecious, meaning that there are separate male and female plants – and both must be in proximity for seeds to grow. The male plants can be recognised by their white flowers, whereas the female plants have more off-white or yellowish-white flowers.
In Norway, goat’s beard can now be found in forests, at the forest edge, in scrublands, by ravines, and in open meadows across nearly the entire country. Because of how tall and dense they grow, the plants can make it difficult to pass through, and can outcompete native species. The goat’s beard plant can pose a threat for vulnerable and endangered species in areas where they both grow.
- If you have goat’s beard in your garden, the plant can contribute to undesired spread of the species – even if you only have a male plant – as there can be female plants nearby. If a new goat’s beard plant appears in places other than where you had planted it originally, you can be sure that there are others of both genders nearby (the seeds can be spread by the wind and by animals over moderate distances).
- In such a case, we would advise you to remove the entire plant, along with the roots, before seed production can begin. Ensure that you do this before the plant begins to flower, and at latest, before seed development. If the goat’s beard plant has already begun to bloom, it is smart to remove the blooming parts and discard them in the general waste collection.
- You can prevent spread of the plant in and outside of your garden by not discarding flowering parts of the plant or roots outside of your garden or in the compost; if you do not do this, you can inadvertently help propagate new plants. Other parts of the plant, however, can be composted safely. Do not discard plant material outside of your garden, though.
When you have removed the goat’s beard plants from your garden, there will be lots of space for something new. You could, for example, plant fern species which are already native to Norway – or how about a beautiful and vibrant hydrangea or berry bush?
Risk rating: SE (extremely high risk); high invasion potential and medium ecological effect
The Himalayan balsam, as the name suggests, is native to the Himalayas and can grow up to 2,5 meters tall. Its Norwegian name of kjempespringfrø translates to “large spring-seed” and may be a reference to its ability to spread it seeds outwards via an explosive seed pod. They are annual plants with thick stalks and sturdy branches. The flowers have five pink petals which fade into lighter tones, and three sepals which resemble petals; they bloom between July and September. The flowers are monosymmetric and the four lowest petals are fused together at the tip.
After flowering, a five-chambered fruit with many seeds is formed. This can be flung up to seven meters away from the plant with just the slightest touch. The seeds can enter streams and rivers and colonise large portions of the riverbank.
Ants enjoy the seeds of the Himalayan balsam, and flying pollinating insects are attracted to the sugary nectar. Because of this, the presence of this plant can attract insects away from native species and lead to their slow displacement. In addition, the Himalayan balsam spreads quickly and creates a lot of shade, which can lead it to outcompete other species in places where it establishes itself. It can also spread via garden trimmings and movement of soil masses.
The Himalayan balsam thrives in humid, wet, and fertile soil – for example, by riverbanks, ditches, and in valleys. It is not very drought tolerant. The small and short tubers it produces bind only the top part of the soil, and change the topsoil so that it erodes easier.
- It is forbidden to spread this plant outside your garden, or to sell or distribute it further. We recommend that you remove it, as it will guaranteed spread itself otherwise.
- To remove the plant, you must cut it down before it begins to flower; if you do not do this, you risk spreading the seeds while you are working at removing it. You should try hard to not move around soil masses more than necessary, as these may contain seeds. The plant clippings are safe to compost as long as you ensure that they have been removed before the plant has begun to flower, and thus do not contain seeds.
- Roots can be discarded in the general waste collection, but it is important that they do not come into contact with soil. For the plant clippings themselves, avoid placing them where they can come into contact with water.
- This process may need to be repeated several times per season, as it is likely that there could be seeds in the soil which may sprout when the conditions are right.
- You can also supplement these activities by thermal treatment; simply pour boiling water over the areas where the plant previously grew. We would either way recommend cutting down the plant first.
- After cutting down the plant more than once, it can be wise to place cardboard or a ground covering over the area to prevent growth until the next year. After one more year, it is safe to plant something else in that location.
How about planting Chinese peony (Paeonia lactiflora; in Norwegian: Silkepeoner), garden tree-mallow (Malva thuringiaca; in Norwegian: Poppelrose), or Asian bleeding-heart (Lamprocapnos spectabilis; in Norwegian: Løytnantshjerte) instead? If you would like a plant which grows a bit taller, you can plant larkspur instead (note: Candle larkspur – Delphinium elatum, or Ridderspore in Norwegian – has been observed having escaped into the wild in Norway, and has been categorised as Low Risk on the Non-Native Species List). Always ensure that you do not discard garden plants outside of your garden, because they can potentially spread unintentionally if you do. Ensure that the soil is completely free of Himalayan balsam seeds before you plant something new in that location.
Read more about the Himlayan balsam on the Non-Native Species List.
Risk rating: SE (extremely high risk)
The beach rose is a rose bush which is originally from East Asia. It grows extremely quickly, and can be identified by its shiny creased leaves, and the numerous small and sharp thorns which are tightly clustered together all over its branches. The dark pink or white individual rose flowers bloom between June and September.
You can also recognise the beach rose from its large and round rose hips / buds which look almost like small tomatoes, in comparison to the narrow and small rose hips on wild roses.
Beach rose plants are often used in road medians or alongside roads, but it is also used as a decorative plant in gardens. If you find an example which has escaped into the wild, you should register your find at the Norwegian Species Database site (artsdatabanken.no).
The beach rose is salt-tolerant, and therefore poses a threat to nature as the rose hips / buds can float down rivers and out into the ocean, after which they can establish populations of beach roses on sandbars and along the beach. Here, they transform their surroundings and displace native species. The seeds can also be spread by birds, which consume the rose hips. The beach rose can also pose a larger threat to birdlife as a whole; when it establishes itself somewhere it can change the food and conditions that local birds depend on, in the process significantly disturbing local bird populations.
- The beach rose is one of the 28 forbidden vascular plant species in Norway. If you have this plant in your garden, you have the responsibility to ensure that it does not spread itself via root fragments and seeds. It is also forbidden to sell or distribute the plant.
- To prevent the beach rose from spreading, one must restrict the growth of new branches, and prevent the rose hips / buds from leaving the garden to other areas via rivers and drainage systems. You should therefore cut off flowers before they form new rose hips / buds, or remove all the buds and discard them in the general waste. Alternatively, you can make rose hip jam from them and discard the boiled seeds afterwards. Either way, removing all of the flowers or rose hips can be a laborious process which must be repeated year after year.
- If you would like to simply cut down the plant, you must do this at least four times a season; otherwise, new shoots will just propagate upwards form the roots, and you will end up with an even larger problem.
- We recommend removing the entire plant, including its roots. All plant parts should be burned or delivered to local waste collection services. Make sure you remove all rose hips, roots, and plant material from the ground, so as to prevent the growth of new shoots. You will likely need to check your garden for a few seasons afterwards to ensure that no new shoots have come up.
As a substitute for beach rose, there are many beautiful rosebushes to choose from at your local garden centre. However, you should avoid the cinnamon rose (Rosa majalis; in Norwegian: Jomfrurose), the redleaf rose (Rosa glauca; in Norwegian: Doggrose), and the Kamchatka rose (Rosa kamchatica; in Norwegian: Kamtjatkarose). We encourage you to choose a cultivar of Norwegian wild roses instead. The apple rose (Rosa villosa; in Norwegian: Hurdalsrose) is another pretty rose you can choose as well.
Risk rating: SE (extremely high risk); high invasion potential and medium ecological effect
Red elderberry belongs to the moschatel family – Adoxaceae – which is native to southern and central Europe, as well as Siberia. Red elderberry is a large bushy tree which can grow from one to four meters tall. It produces five to six centimetre-wide bunches of small light-yellow flowers between April and June. The flowers are radially-symmetric and have five stamens (holding the pollen); after flowering, juicy red berries with three to five seeds are produced. The berries are mildly poisonous, but are still spread over large distances by birds. The leaves are long are irregularly serrated.
The red elderberry was introduced to Norway as a decorative bush in the 1700s, but it became naturalised later in the 1800s. It is one of the few non-native bushes which thrives in both coastal and inland habitats. Red elderberry can establish itself in shallow soil, or deeper nitrogen-rich soil – in forests, in areas which have been cleared of trees, around farms, along roads, and abandoned plots. It has been registered in all counties in Norway up to Nord-Trøndelag and up along the entire coast. Red elderberry can grow in areas with full sun, partial sun, or full shade.
In forests, red elderberry can create a shrub layer which would not have been found in that habitat otherwise. This can interfere with the growth and rejuvenation cycles in the forest, as well as impeding the growth and propagation abilities of native species with fruit or berries, such as rowan.
- It is forbidden to spread red elderberry outside of your garden, give it away, or sell it. We recommend removing the entire plant, including the roots, before it begins to bloom. You will want to avoid spreading fruit with seeds in the process. Drop off the plant at your local waste disposal services. Remember to be on the lookout for new sprouts later in the year, and in the coming years, and to remove them as well.
Also be cautious of European elder, sometimes called black elder or simply elder (Sambucus nigra). This plant has not been sufficiently studied, and according to the 2018 Non-Native Species List, it would possibly have been placed in the high risk category if it were also spreading quickly in the wild. There is still some uncertainty around its ecological effect as we do not quite know the effect that this plant has on native species with regards to the consumption of its berries by birds and the spread of seeds as a result.
More about red elderberry.
More about European elder.
Note: European elder can occasionally be found for sale at some garden centres; this does not necessarily mean that it is safe to plant in your garden.
Guelder rose (Viburnum opulus; in Norwegian: Korsved) is a good alternative to red elderberry if you are looking for a plant with similar growth patterns and size; it is also found naturally in Norway. Alternatively, you can consider the growing conditions in your garden where you removed the red elderberry plant from, and select another native or friendly species which prefers the same conditions.
Risk rating: SE (extremely high risk); high invasion potential and high ecological effect
The common broom is a thornless bush with yellow flowers which grows between half a meter and two meters tall, and spreads quite easily. It is easy to recognize by its green furrowed branches and silky-smooth bright-yellow flowers. The radially-symmetric flowers with an upper-lip and under-lip grow individually from the junctions between the leaves and the stalks. The leaves themselves grow individually or in groups of three. After flowering is complete, the stigma rolls inwards. The seed pods curve inwards as well after they burst open.
The issue with the common broom plant can be a bit confusing. The first documented observation of the plan in Norway was in 1875, and it was initially assumed that it was native to Denmark and Norway. The Non-Native Species List, however, clarifies that as practically all known populations of the common broom in nearly all locations where it is found in Norway are imported or from plants which have escaped into the wild, the common broom is thus considered a non-native species. In Norway, the plant has escaped into the wild in large portions of southern Norway and Nordmøre. The common broom is also problematic and invasive in Denmark and other parts of Europe.
The common broom spreads itself via birds, the wind, and movement of soil masses – and it has a very high invasion potential. In addition, it has a large ecological effect when it displaces native species. The common broom can also alter the area it establishes itself in to the degree that it forms a new shrub layer in areas where there were not shrub layers before – for example, in coastal moorlands, sand dunes, and semi-natural meadows. The plant fixes nitrogen and leads to an increased supply of nutrients. This can make areas which were previously suitable only for native plants that could thrive in low-nutrient habitats now viable for other types of plants with higher nutrient requirements. These plants will then displace the original ones in a competition for sunlight and space.
- If you have common broom in your garden, we recommend removing the entire plant, along with the roots, and either discarding it in the general waste collection or delivering it to your local waste disposal services. It is important to never throw away parts of this plant into the garden waste collection, as it can form new roots and the plant can then spread further.
- The common broom is unfortunately still for sale, and we hope that will change soon. If you find this plant in your local garden centre, it is important to let them know that it is an invasive non-native species. Instead, ask for plants that are either native or at least non-invasive.
We recommend planting mock-orange (Philadelphus; in Norwegian: Skjærsmin) instead. Virginal mock-orange (Philadelphus x virginalis; in Norwegian: Jomfruskjærsmin) will not be able to escape into the wild under Norwegian climate conditions and is safe to have in your garden.
Risk rating: SE (extremely high risk); high invasion potential and high ecological effect
The hollyberry cotoneaster is one of many cotoneaster species which is rated as extremely high risk in the Non-Native Species List. It is a shrub in the rose family with a long lifespan that can grow up to five meters tall, and it looks more like a tree once it grows past a certain age. Hollyberry cotoneaster can be recognised by its leaves which appear “dented” or “embossed” due to the veins in the leaves. It has relatively large leaves, roughly five centimetres long, and branches which are rather smooth and featureless. The leaves range from red-brown to light green in the spring, and become darker green as summer approaches.
The flowers of the hollyberry cotoneaster have small white or light-red petals in small clusters. These turn into plump and round red berries from August onwards; they resemble rowan berries but are a little bit larger and deeper red in colour.
Hollyberry cotoneaster is a tetraploid, which means that it can spread asexually via seeds. It is therefore not dependent on other individuals of the same species nearby. This increases the ease at which it can spread itself past other species which require sexual reproduction to spread. Birds spread its seeds as they eat large quantities of the berries that the hollyberry cotoneaster produces. Because the plant has such bright red berries, it can attract birds away from other fruit-bearing native species (such as rowan), thus limiting their ability to spread seeds.
The hollyberry cotoneaster is a hardy plant with a frost tolerance down to -21 degrees Celsius, so it thrives quite well in the Norwegian climates. Populations of the plant are registered across all of southern Norway, and the species is particularly widespread along the coast. It is found in sunlit areas, such as in scrublands, mountains, and along the edges of forests. It can form a thick scrub layer in forests which would otherwise not have such a scrub layer, in the process making the forest impassable. As well, it also reduces the amount of sunlight available to native species in those areas. Hollyberry cotoneaster is native to the Sichuan province in China, and has previously been a popular garden bush. However, it is now forbidden to sell, grow, spread, or distribute, because it has quickly established itself throughout Norway and its spread has gotten out of control.
In addition, hollyberry cotoneaster is a host and spreader of fireblight, a plant disease caused by the North American bacteria Erwinia amylovora. This contagious disease can cause huge economic impacts in the fruit industry, as the bacteria first and foremost attacks plants in the Maloideae sub-family in the rose family. This means that, among others, apple, pear, aronia, rowan, and quince plants are susceptible to fireblight.
- It is forbidden to sell, spread, and discard hollyberry cotoneaster outside of your garden. Monitor for symptoms of fireblight if you have this plant in your garden or locally. The bacteria which cause fireblight result in infected trees withering. This can be recognised by that the end of the branch and downwards toward the trunk turn brown / black, shoots and flowers quickly wither, and young shoots bend. In addition, in some cases slimy drops containing large amounts of the bacteria will form on the bark. It is very important to monitor if specimens of the hollyberry cotoneaster or other trees in the rose family are infected with fireblight. If one discovers or has the suspicion that there is a case of fireblight, the land owner or land user is obliged to contact the Norwegian Food Safety Authority (Mattilsynet) to report this.
- Fireblight spreads via air, water, insects, and movement of plant materials. As such, it is not advised to move around clippings or waste from hollyberry cotoneaster. They can be collected and left to compost / decompose on the site. If there are cases of fireblight, any nearby colonies of honeybees should be moved to prevent them aiding in accidental spread of the disease.
How to remove:
- It is most effective to remove hollyberry cotoneaster by uprooting the entire plant, assuming this is possible – this is easiest for smaller and younger plants. If the plant is simply cut down, it will form new shoots from the roots; these must be continually cut down. The seeds are also viable for germination for roughly five years. New plants can easily be pulled out of the ground, and this should be done on a regular basis.
- In the event of fireblight, the plant should be shredded / woodchipped and composted on site. The bacteria will eventually die, and then not pose any more threat. Be careful to not spread any berries when you clean up; it can be smart to work with the plant before the berries are formed in the fall. Wash all tools to prevent spreading the bacteria further via them.
Alternatives to hollyberry cotoneaster:
- Rowan (Sorbus aucuparia; in Norwegian: Rogn) is a great alternative to hollyberry cotoneaster. Rowan is native to Norway; it is not a threat to local biodiversity, and it also provides sustenance to pollinating insects in the summer and food to birds throughout the fall. In addition, you will get beautiful leaves, pretty flowers in the summer, and spectacular fall colours in your garden. Did you know that the young shoots of rowan taste of marzipan?
- Even though rowan is a great alternative, it’s important to be aware that if you have confirmed a case of fireblight on your property (or suspect one), rowan can also be susceptible to the disease. In such cases, it’s better to plant trees from outside of the rose family. Here, goat willow (Salix caprea; in Norwegian: Selje) can be a good alternative. It is native to Norway, and is one of the key sources of sustenance for wild bumblebees in the spring, when nothing else is available for them.
More about hollyberry cotoneaster and fireblight:
Facts about fireblight – Norwegian Food Safety Authority (Mattilsynet)
Fact sheet on non-native species – UIB
Alert us if you suspect a case of fireblight – Norwegian Food Safety Authority (Mattilsynet)
Fact sheet on hollyberry cotoneaster – Norwegian Species Database (Artsdatabanken)
Fact sheet on fireblight – Norwegian Species Database (Artsdatabanken)
Other cotoneaster species to avoid:
- Diel’s cotoneaster (Cotoneaster dielsianus; in Norwegian: Dielsmispel) (SE) (Forbidden)
- Spreading cotoneaster (Cotoneaster divaricatus; in Norwegian: Sprikemispel) (SE) (Forbidden)
- Rockspray cotoneaster (Cotoneaster horizontalis; in Norwegian: Krypmispel (SE)
- Shiny cotoneaster (Cotoneaster lucidus; in Norwegian: Blankmispel) (SE)
- Showy cotoneaster (Cotoneaster multiflorus; in Norwegian: Blomstermispel) (HI) (Forbidden)
Hollyberry cotoneaster, spreading cotoneaster, and rockspray cotoneaster are all susceptible to the bacteria that causes fireblight.
Some cotoneaster species are native to Norway:
- Did you know that there are two native cotoneaster species in Norway? They are common cotoneaster (Cotoneaster integerrimus; in Norwegian: Dvergmispel) and dark cotoneaster (Cotoneaster laxiflorus; in Norwegian: Svartmispel). The latter is a vulnerable species in Norway.
Sources: artsdatabanken.no, Bo Mossberg og Lennart Stenberg (2014) Gyldendals Store Flora.
Supporters: Vestland County Municipality, the Norwegian Environmental Agency (Miljødirektoratet), Bergen Municipality.