Invasive Plants

A non-native species is one which has been brought by humans to an area it did not grow naturally. The term tends to be used for species which have been moved long distances – often between continents – in relatively modern times. Some of these species may become “invasive” – spreading rapidly and displacing native species and damaging ecosystem function.

The term “weed” refers to a plant which is growing where humans don’t want it to. It may be native, non-native or invasive. It could be a plant which competes with our food crops, or something which grows in a crack in a sidewalk. In other words, it is a context-dependent term.

Non-native plants can be invasive because of their particular life history – they can be fast growing, or may produce a lot of seeds, or may spread quickly via their roots. They are often species which have evolved to take advantage of disturbance from floods, ice storms, or a tree falling over. In their new location, they lack diseases, predators/grazers, and so can spread rapidly, outcompeting native plants.

There are many non-native plants which are common in gardens which do not have these growth characteristics, and are unlikely to ever become particularly invasive. However, there are others which might be somewhere in between, and can suddenly become invasive after a particular pattern of weather. For example, in the north, a series of shorter winters with less snow cover can allow some non-native plants to expand rapidly and gain a root-hold into new areas. The same can be true elsewhere with changes in patterns of droughts, rain, heat and cold. So we never like to “trust” a non-native plant to continue to behave forever!

Furthermore, non-native plants simply don’t support our native ecosystems to the same extent that native plants do, so we see them as a wasted opportunity to grow lovely native plants which will benefit more insects and more birds.

Some native plants can behave like an invasive species; in native plant parlance we tend to use the word “aggressive” when referring to a native plant, rather than “invasive”. In some contexts, this can be excellent – for example, if you have removed a non-native invasive species, then selecting some aggressive native species can be a great way to kick-start the restoration process.

In a garden context, it can be a little challenging, but sometimes, if you are able to hang on for a few years, some aggressive native species will sort of calm down a little, and you can be ready to slot in some additional native species to keep the diversity high. If not, simply cutting and digging to give other species some space is a good option too. It can be difficult to predict what native species may be aggressive, as it can be quite site and region specific.

It helps to see the word as a combination between native + cultivar. A nativar is a native plant that has been specifically bred and cultivated, usually for a characteristic desirable for humans. Characteristics can be size of the plant, growth habit, shape or color of the flower, leaves or fruit, etc. It is essentially a native plant that has been modified or encouraged by humans. Sometimes, some of these artificially selected characteristics prevent the plant from supporting wildlife.

Every invasive plant individual you remove from your space provides an opportunity for a native plant to become established instead, supporting more wildlife. It also reduces the number of invasive seeds produced and sent off into the environment, protecting your surrounding area.

Whenever you travel, including locally between natural areas, take the time to wash you footwear, and remove any plant fragments from the rest of your clothing, bags, walking poles, and pet fur. Choose native plants for your garden, and report non-native invasive plants via iNaturalist.

Nativars are generally better to plant than non-native plants, but its really best to get the “real thing” – the pure native variety from a reputable source. Although some nativars provide similar benefits as pure native plants, some do not. Some have obviously different flower structures which may exclude pollinators, different flower colour which may not attract some pollinators, while even subtle differences in chemical composition of leaves and stems can render plants unpallatable or lacking in nutrition.

Planting a variety of native plants suitable for your space means you will support a small ecosystem, supporting both prey and predatory insects. Although you will see insects causing damage to plants, this is something to be celebrated – you are supporting animals! Planting good diversity is also a good way to minimize the impacts of plant diseases.

We recommend using either a field guide, iNaturalist or SEEK, to identify plants.

Sometimes local county cooperative extension offices will offer identification guides to invasive plants commonly found in your region.

Unfortunately, there are many invasive plants still commonly for sale in the US and Canada. In some places, some regulated invasive plants must have a warning label. We recommend taking the time to read the scientific name on the label, and Googling it to see what you can learn about the plant before making a purchase. You may want to make your plant purchases at a specialized native plant store instead.

Non-native and invasive plants will be used by our native wildlife to some extent. However, they do not always provide the nutrition that our animals need – this is the case in terms of pollen, nectar, and leaves, so insects and birds may eat various parts but do not benefit as much as they do from native plants. Also, some invasive plants may support a lot of pollinators for a brief period while they are flowering, but provide nothing for the rest of the year – whereas a native plant is more likely to support a whole ecosystem in various ways throughout the year.

Birds will nest in invasive plants, but careful scientific research has shown that in some species, those nests fail more often than nests in native plants due to subtle differences in plant structure which makes it easier for predators to climb to nests.

Some invasive plants are indeed edible. We don’t necessarily have that expertise here, so you may want to search for reputable sources of information about edible plants in the wild. By definition, many invasive plants may grow too vigorously to be adequately controlled by harvest and consumption, and in fact some plants grow even more vigorously when they are cut and not further treated.

Be cautious about harvesting from the wild, as you may need land owner permission to do so. In the case of invasive plants, be careful not to interfere with any control which might be occurring. Or, consume chemicals that could be have used in management that may be residually left on the plants themselves.

There are many lovely native plants which have evolved to do well on shady forest floors. Good options are very regional and site specific. Get to know what is native in your area by using a field guide or plant ID app, and find a native plant nursery which sells the species you like.  Local cooperative extension offices may have guides or educators who can help, too.

One tool we use frequently is the Lady Bird Johnson Wildflower Center. In an advanced search you can specify your region and enter the conditions of the growing site and it will provide you with a list of natives that would be good choices.

Invasive species can be traceable along major global trade routes, although as the world becomes more and more connected, understanding these routes becomes more difficult. For example, there are invasive species from North America and Asia in Europe; North America has invasive species from Europe, Asia, and South America; New Zealand has invasive species from all over the world but is also the source of some. What allows a plant to become invasive in a new region has more to do with its reproductive characteristics than its country or ecosystem of origin.

Canadian prairies and grasslands are composed of many beautiful native plants, many of which can be grown in a garden setting. Visit a local natural area and get to know what grows there using a field guide or app, and then find a reputable native plant vendor to purchase those species.

For any region, search for “Invasive Plant Council” or similar to look for resources close to you.

Spreading the word is a great thing to do! Lead by example – take good care of your garden, show that it is tended, and pro-actively manage plants which may encroach on sidewalks or into your neighbors’. Get to know what your neighbor likes – maybe they love a certain color, or are interested in butterflies. Maybe they have kids who are excited about plants and animals. Its good to find some common ground, and build slowly from there. Share your knowledge as you learn, and spend lots of time listening as well.

Always obey local laws about pesticide use, and follow the label very carefully. Some labels will include information on timing to most effectively kill the plant, but this may not always coincide with the best time to avoid non-target impacts.

Depending on your latitude, habitat type, region, and plants to be controlled, very late summer through fall and winter are generally the safest times to avoid nesting birds, and can also be a very effective time to kill invasive plants. Check in with your local breeding bird atlas to get a good understanding of bird nesting seasons in your area, and think about the species likely to be using the habitat in question. You can also spend some time simply watching the patch to get a sense of what is happening.

Do consider other wildlife as well – amphibians may also be on the move terrestrially, and there may be local species of concern/endangered species guidelines or legislation to pay attention to.

As with many plants which have vigorous root systems, manual control methods are only somewhat successful. Repeated cutting and digging over many years may eventually be effective. Targeted herbicide application directly to the plant is likely to be the most effective and cause the least disturbance. Always obey local laws about pesticide use, and follow the label very carefully. Search for “[species name] management” to find resources online.

Goutweed is one of several persistent non-native, invasive plants commonly found in gardens. As with many invasives, regular, dedicated effort across several years is likely to be required. Manual digging of the entire plant and root system can work, especially for relatively small areas. Careful, targeted application of a systemic herbicide such as glyphosate is very effective and avoids disturbing the soil.

Always obey local laws about pesticide use, and follow the label very carefully. Mowing or cutting in spring followed by tarping can be somewhat effective. Once you have achieved control, research what native plants might do well in that spot and replant – but stay vigilant for goutweed reappearing. A physical barrier like a big chunk of wood or metal partially buried might go some way to stopping local, vegetative spread.

As with many plants which have vigorous root systems, manual control methods are only somewhat successful. Repeated cutting and digging over many years may eventually be effective. Targeted herbicide application directly to the plant is likely to be the most effective and cause the least disturbance.

Always obey local laws about pesticide use, and follow the label very carefully. Search for “creeping bellflower management” to find resources online e.g. https://dnr.wisconsin.gov/topic/Invasives/fact/CreepingBellflower

Depending on the species, and the area, you can simply dig out each non-native individual as you detect it. The sooner you are able to do it, the sooner your native plants can benefit from the extra space and reduced competition. You can also search online for “best management practices” for each non-native species.

Vinca and English ivy can be very persistent. In a garden context, you can simply keep digging them out and gradually enlarge the invasive-free patch. You will need to keep a close eye on the cleared area and expect to remove individuals for several years before it becomes truly clear. You can also search online for “best management practices” for each species.

As with many plants which have vigorous root systems, manual control methods are only somewhat successful. Repeated cutting and digging over many years may eventually be effective. Spending a few years cutting might reduce a larger patch to a more manageable level so you can then dig out the remaining roots. However, targeted herbicide application directly to the plant is likely to be the most effective and cause the least disturbance, and allow you to add native plants soonest.

Always obey local laws about pesticide use, and follow the label very carefully. Search for “crown vetch management” to find resources online e.g. https://dnr.wisconsin.gov/topic/Invasives/fact/CrownVetch

Invasive bush honeysuckles have shallow roots compared with other similarly-sized deciduous shrubs and can be remarkably easily uprooted, either by hand or via the use of a tool like a “honeysuckle popper”. This does cause soil disturbance so as with any invasive species management, keep a close eye on the site and be prepared to treat other invasive plants which may then appear. When done properly, chemical control can be much more targeted and cause less disturbance, as long as it is done in very late summer and through the winter to avoid disturbing nesting birds.

Always obey local laws about pesticide use, and follow the label very carefully. Look online for “bush honeysuckle best management practices”.

You must -by law- follow the label of any pesticide carefully, especially regarding personal protective equipment, concentration and species controlled. There are several herbicide options for cut stump control of deciduous invasive trees and shrubs. The technique is best applied in very late summer through fall when the plant is drawing its resources back underground.

Purple loosestrife (Lythrum salicaria) is non-native and invasive in the US and Canada. It is an example of fairly successful biocontrol (by an introduced leaf beetle), though it can still be quite prevalent in some years. In a garden context, it is definitely worth removing so you don’t contribute to its spread. Cut off the purple loosestrife flowerhead before it sets seed, and dig the plant and its roots out. Depending on your area, there are some lovely native loosestrifes e.g. fringed loosestrife (Lysimachia ciliata) which is yellow, or pink alternatives such as dense blazing star (Liatris spicata).

This species of non-native holly is evergreen, and spreads both by seed and via roots. It has the potential to significantly alter forest structure and composition because it is so different from the native species in the natural plant community. As such, it is important to manage it and avoid contributing to the invasion. There are many beautiful native plants that would be better alternatives to plant in your garden.

As with many invasive plants, Japanese knotweed requires several years of sustained effort to eradicate. A combination of mechanical and chemical techniques, administered at the correct time of year, are necessary.

Always obey local laws about pesticide use, and follow the label very carefully. You can read more about control methods here: https://extension.psu.edu/japanese-knotweed

As with many plants which have vigorous root systems, manual control methods are only somewhat successful. Repeated cutting and digging over many years may eventually be effective. Targeted herbicide application directly to the plant is likely to be the most effective and cause the least disturbance.

Always obey local laws about pesticide use, and follow the label very carefully. Search for “creeping bellflower management” to find resources online e.g. https://dnr.wisconsin.gov/topic/Invasives/fact/CreepingBellflower

English ivy can be very persistent. In a garden context, you can simply keep digging it out and gradually enlarge the invasive-free patch. You will need to keep a close eye on the cleared area and expect to remove individuals for several years before it becomes truly clear. You can also search online for “best management practices” for the species.

Garlic Mustard seeds can survive for several years in the soil. They are also easily transported by animals, shoes and tires. In a garden context, simply being persistent in digging out each individual before it sets seed is likely the best approach. Although digging (you would just need a hand trowel) will cause soil disturbance which will stimulate any other Garlic Mustard seeds to germinate, in some ways this is the best way to get closer to actually eradicating it from a patch.

Just make sure you keep checking back on the area each spring and take out new individuals as soon as you see them. You can also do this work throughout the summer and fall if the seedlings are visible, but the critical point in the year is early spring to stop those individuals from flowering and seeding that year. Early spring can be an easier time to dig as the soil is perhaps more likely to be moist and soft. If you can’t get to digging each one out in the spring, try to focus on completely clearing a patch, and then switch to cutting off the flower heads of the remaining individuals. You can work on those patches next year.

As with many plants which have vigorous root systems, manual control methods are only somewhat successful. Repeated cutting and digging over many years may eventually be effective, but the spreading rhizome root system may take several years to truly remove. The hardest work is likely to be in the beginning, and, as long as you spot them quickly, re-sprouting parts can be more easily dug out. Targeted herbicide application directly to the plant is likely to be the most effective and cause the least disturbance. Always obey local laws about pesticide use, and follow the label very carefully.

Poison hemlock (Conium maculatum) is native to Europe and North Africa, but non-native and invasive in many parts of North America. It is biennial, meaning an individual typically lives for two years, flowering in its second year. It is best to prevent it from producing seed, although repeated cutting will be required as they will keep trying to flower even after cutting. Seeds can survive for 2 – 6 years in the soil, so as with many invasive species management projects, you will need to sustain management annually for several years until the seed bank is depleted. Unfortunately, seeds may well keep being brought in by animals, or wind and water, so it is worth always being vigilant. Do take care to protect yourself from this highly toxic plant.

As with many invasive plant management projects, repeated management over several years is likely to be required. It is good to start at the “invasion front”, so in your case, where it is starting to reach your ferns. You can dig them out with a hand trowel, and keep pushing that invasion front back and out of your yard. You may want to add more native plants as you remove it, to help keep it out in the future. Digging can be easier after rain when the soil is moist and soft. A key goal is to prevent it from setting seed, so digging around the time it is flowering may be prudent. Here is some information from more of an agricultural perspective:  https://cals.cornell.edu/weed-science/weed-profiles/deadnettles  

You might able to lay some sort of a physical barrier along your property line, such as logs, plants or even sheet metal, half buried into the soil, to try to keep it out in future.

Burning non-native plant material can certainly be an option, as long as you follow any local guidelines on timing and precautions. However, for many species it may not be necessary. For woody invasive plants like honeysuckles, you can stack them so that the roots dry out in the air. This creates a brush pile which may be used by wildlife – there are several species of butterfly which overwinter as adults in brush piles for example. Keep an eye out for any bits which re-sprout, or seedlings which establish underneath and be prepared to take action. If you don’t have the space to do this, you can burn it or send it off to landfill.

Depending on the species, you can put the cut material into a black container or heavy duty bag and leave it in the sun for several weeks. This “solarizes” it and and kills the live parts.

Alternatively, you could submerge them in water for a similar period of time – this can be effective to kill bulbs. After a few weeks, you can send the material to landfill. If you compost at home, you can add the solarized or drowned material to it, but this needs to be done with extreme care in case some fragments do survive.