Management of invasive species is a necessary, but costly and often time-consuming part of restoration work. Given the opportunity, invasive plants can cover large swaths of land, outcompeting native plants and reducing biodiversity.
The more complex a habitat is, the more species it can support, the more resistant it is to stressors, like climate change, and better it can provide ecosystem services. The term ecosystem services describes services a natural environment can provide to people, like how wetlands are valuable for flood protection and water filtration.
Our newly-planted restoration site, where trees and shrubs are still small, is especially prone to invasive plants. I regularly visit the site to remove invaders to ensure that the native species we planted survive.
Here are some of the plants that I work to manage at our restoration site or am keeping my eye out for.
You probably expected to see this one! Himalayan blackberry is the most common invasive plant I manage. It grows rapidly, spreads readily, and crops up nearly everywhere in the lower mainland and beyond. Because it is so widespread, it can readily be reintroduced to the site by bird or animal droppings. Getting rid of it completely is unrealistic, so we work to minimize the impact it has on native plants. The benefit of removing blackberry from the streambank and replacing it with species like big-leaf maple is erosion control and stream shading. Blackberry roots do not penetrate the ground as deep as native tree species, meaning the streambanks are more prone to erosion and collapse. Stream shading helps keep water temperature cooler for aquatic species like salmon.
While some would argue that Himalayan blackberry provides benefits including food for birds and other species, we have native species that fill that same role, such as thimbleberry and salmonberry. These plants are often pushed out by the hearty foreign berry bush.
If you have blackberry growing on your property, at the least, pick and eat the berries to limit the spread. If you want to remove it, digging up the roots will give you the best chance of getting rid of it. Repeatedly cutting the above-ground growth can also be effective as it reduces the root’s energy reserves and provides an opportunity for disease to enter the plant through the cut canes.
Reed canarygrass. Photo credit: Ryan Hodnett (license)
Tracing the origin of invasive reed canarygrass is a bit complicated. We think one native variety was present in B.C. but hybridization with introduced varieties resulted in the invasive plant we see today. Canarygrass can survive being submerged and has the potential to infill waterways, increasing flood risk. As it can grow to 2 m tall and grows quite densely, it can prevent the growth of trees and shrubs that would do a better job of preventing erosion and stream shading. At the restoration site we are working to keep it under control until the native plants are tall enough to shade it out—it doesn’t thrive in low-light conditions.
Yellowflag iris. Photo credit: Jonathan Billinger (license)
This species has not been found at our restoration site yet, but is present along the Katzie Slough and could spread to our site. An attractive plant with bright yellow flowers, yellowflag iris grows in moist sites along streams and in wetlands. Once established, yellowflag iris roots spread outwards, creating a dense mat that prevents growth of native wetland species. The iris can also spread easily by seed. Each plant releases a large number of small, hockey puck looking seeds that can float downstream. For a drainage channel such as Katzie Slough, this plant can be devastating, limiting water flow and increasing flood risk if the plant spreads to narrow, shallow sections of the waterway.
sEvergreen blackberry looks similar to Himalayan blackberry but its leaves are much more jagged and delicate looking (another name for the plant is cutleaf blackberry as the leaves look cut up) At the restoration site, we have not found much of it, but have seen it growing along the slough. We manage it with repeated cutting and root removal.
Japanese knotweed growing over infrastructure
If you remember only one plant from this article, make it this one. Japanese knotweed (or any of it’s similar looking relatives: Bohemian, giant and Himalayan knotweed) is a nasty plant, and if you find it on your property, you are legally responsible to control it. The roots have the potential to damage structures, including foundations. If you find knotweed on your property, contact your local government or the Invasive Species Council of BC for advice on how best to remove and dispose of the plant. Depending on how established the plant is, management approaches will vary, but manual removal is not advised for most situations, and mowing it facilitates its spread.
Knotweed has not yet been observed at our restoration site, but several patches have been observed downstream. The plant spreads out from horizontal root systems, creating a dense stand that outcompetes native vegetation. It also can spread by fragments which, if adjacent to moving water, can result in downstream spread of the plant.
Removing invasives is ongoing
With our initial restoration site entering its fourth year of growth, the plants are getting nice and tall and are much less susceptible to being shaded out, or out-competed by invasive plants. Our second site, however, which has only been planted for a year, will need ongoing maintenance to ensure that the invasive plants don’t take over.
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Check out the websites of the Invasive Species Council of BC or Metro Vancouver if you want to learn more about invasive plants, including tips on how to identify invasive species, and suggestions of what to plant in your yard instead.