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.
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
Moneywort, or creeping jenny (Lysimachia nummularia L.) (Norwegian: Krypfredløs)
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.
Artikkelen ble sist oppdatert: 02.05.2022