5 invasive plant species and what to do with them

garlic-mustard-plant Photo by Mr. Meijer/Shutterstock

Invasive plants are a problem, but sometimes it’s hard to tell what they are and what to do about them. Any why are they bad, anyway? Plants that are considered invasive aren’t native to an ecosystem, so, for a number of reasons, spread rapidly and choke out native plants. This can create a monoculture, an ecosystem where one type of organism dominates, reducing biodiversity and sending ripples of disruption throughout the natural area. Not all non-native plants are invasive, but the ones that are can cause serious problems for an ecosystem. Here are some of the common ones and what you can do to stop them.   

Japanese knotweed

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Native to eastern Asia, Japanese knotweed has been in North America for more than a century, but that still doesn’t make it a native plant. Used to prevent erosion, it has characteristic large green leaves, a stem that resembles bamboo and an extensive root system that can spread up to 10 metres and break through concrete. Because it spreads quickly, it can negatively affect wildlife habitat as it form dense patches of undergrowth that block up to 90 per cent of sunlight.

Japanese knotweed is tough to deal with — not only is its persistent root system hard to eradicate, but its fruit has “wings,” which help it spread seeds. Chunks of root can also be carried on water and start new populations when they land. The best thing to do is not to plant it in the first place — it’s actually illegal to purposely grow it anywhere, and you’re also not allowed to buy, sell, propagate or trade it. And don’t be fooled. It’s also known as Mexican bamboo, fleeceflower, Japanese polygonum or Huzhang.

Once Japanese knotweed is established, you may only be able to manage it, rather than eradicate it altogether. Make sure it doesn’t spread by mowing or digging young plants. Tarping — laying a dark-coloured tarp over the area for up to three growing season — can help block sunlight and “cook” the roots. Make sure you keep cuttings away from water sources. To help stop the spread of the plant, make sure you clean off mud, seeds and plant parts from clothing, pets, vehicles, lawn equipment and tools. Make sure you don’t compost any plant waste, it should be placed in thick black plastic bags and left in direct sunlight for a week, then either burn the plant pieces or send them to a landfill.

Invasive phragmites

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Phragmites australis (European common reed) has been called “Canada’s worst invasive plant” by Agriculture and Agri-food Canada. The fluffy-topped reed can be found in ditches, wetlands and beaches, and chokes out native plants both by spreading aggressively and by releasing a toxin from its roots that kills other plants. It has root systems that can extend up to 30 metres, sending up shoots every 30 centimetres, making it almost impossible to eradicate. Phragmites are particularly problematic in wetlands, where their fast growth causes water levels to drop. Stands of phragmites are also poor habitat and food for wildlife.

One challenge with invasive phragmites is that there is also a species of native phragmites that is closely related, making identification of the invasive species difficult. Generally, invasive phragmites are taller than the native variety, reaching heights of up to five metres. Native phragmites are also not as aggressive, existing in stands with more biodiversity than invasive phragmites. Native phragmites have reddish-brown stems, while invasive phragmites have tan or beige stems with blue-green leaves.

If you have invasive phragmites on your property, a combination of management measures is usually needed to eradicate them, including herbicide application, mechanical excavation, flooding and prescribed burning. Simply cutting them or pulling them out won’t work, and may actually make the situation worse. It’s best to call an exterminator if invasive phragmites are a problem on your property.

As with other invasive plants, it’s important not to compost invasive phragmites in a backyard composter, as both the plant’s seeds and rhizomes can survive in compost. If you have municipal composting in your area, check to make sure the facility reaches high enough temperatures to kill off the plant’s means of reproducing.

Garlic mustard

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Easily identified by the strong garlicky, oniony smell, its triangular upper leaves and small white flowers, garlic mustard is both edible and high in vitamin A and C, but that doesn’t make it any less invasive, nor is it a good food source for native wildlife. Stands of garlic mustard can double every four years, with plants producing hundreds of seeds starting in their second year of growth. Seeds are super long-lived, surviving in soil for up to 30 years before sprouting.

Garlic mustard, which has spread from gardens into forests, can displace native plants like trilliums and trout lily. Its aggressive spread is especially problematic for several native plant species at risk, like American ginseng, wild hyacinth, false rue-anemone and wood poppy. It also carries diseases that affect garden plants.

Humans and animals spread garlic mustard seeds on their boots, fur, paws or clothing, so it’s important to carefully clean off any debris after hiking or taking a walk along recreational trails.

If you’ve got a relatively small area of infestation, garlic mustard can be managed by pulling, mowing or cutting, clipping the flower heads or by applying a herbicide. (Keep in mind that some herbicides have to be applied by a pro.) Make sure to pull plants in the early spring before they’ve set seed. If you’re going to mow, do it just after the plants have flowered, but before they produce seed, usually in May. In badly infested areas, a controlled burn may be necessary, but that’s a method best left up to professionals as well.

Giant hogweed

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Giant hogweed isn’t as common as some of the other invasive plants on this list, but it’s much more dangerous. That’s because giant hogweed, which resembles Queen Anne’s lace and is a member of the carrot family, can cause severe burns if its sap gets on your skin and your skin is then exposed to sunlight. Contact with the sap and sunlight can cause painful blisters that result in scars that may last several years. Blindness from eye contact with the sap has also been anecdotally reported.

Giant hogweed grows along roadsides, ditches and streams, and can grow up to 5.5 metres tall. Its flower heads can measure almost one metre wide. The sap is found in stiff hairs that cover the plant’s stem, and may also be in the base of the stem.

Because the sap can be dangerous, you should have a professional remove any giant hogweed on your property. If you need to be working near a giant hogweed plant, make sure to wear protective clothing, including a long-sleeved shirt, long pants, waterproof gloves and eye protection. When you take off your clothing, make sure to do so carefully to avoid getting any sap on your skin.

If you get sap on your skin, wash it immediately with soap and water and stay out of the sun. If sap gets in your eyes, rinse them thoroughly with water and see a doctor. Also see a doctor if your skin turns red after exposure.

If you do choose to remove giant hogweed yourself, use a shovel to remove as much of the root as possible. If the flower heads are green, this means that seeds are being produced, which makes removal without spreading seeds very difficult. Repeated removal will probably be required. Don’t compost or burn the plant matter, instead, place flower heads in black plastic bags and leave them in direct sunlight to kill the seeds. Let stems and roots dry out thoroughly, then send bags to a landfill.

Dog strangling vine

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Dog strangling vine doesn’t actually strangle dogs. Instead, it wraps itself around trees and plants to get as much sun exposure as possible. The name “dog strangling vine” refers to two highly similar Eurasian invasive plants, the black swallowwort and the pale swallowwort, both of which look like milkweed.

Dog strangling vine produces thousands of seed per square metre, and can also grow from root fragments. It grows on prairies, hillsides, ravines, plains and in forests. It grows so aggressively that it crowds out other plants, and can form dense mats that are difficult to walk through. Because of its resemblance to milkweed, monarch butterflies lay their eggs on its leaves, which are then are not able to get enough food to complete their metamorphosis. Look for pink or dark purple star-shaped flowers with five petals and oval leaves with a pointed tip. The plant will likely be twining around another plant, or growing along the ground. Like milkweed, dog strangling vine produces downy fluffs that carry seeds away from the plant on the air.  

While dog strangling vine isn’t currently regulated like other invasive species, it’s a good idea to reduce its numbers if you find it on your property. While applying herbicide is an effective way of controlling the vine, you can also try digging it up, removing the seed pods or clipping it. Tarping may be necessary in target areas of medium to dense infestation.

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