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Learn About Invasive Species

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Invasive Plants


Invasive plants are non-native trees, shrubs, and herbaceous plants that are spread by global trade, human and animal transport and escaping from gardens. They invade forests and block out native plants from growing, which in turn decreases the available habitat for native wildlife. Many invasive plants cannot be used by wildlife for food which puts grazing pressures on the few native plants that remain. They also pose threats to agricultural fields, due to their ability to spread quickly, outcompete crop and forest plants, and deteriorate soil quality. The thick spread of invasive plants makes them costly and time consuming to remove once they have taken hold.  
Giant Hogweed

Giant hogweed is a member of the carrot family and it’s resemblance to Queen Anne’s lace caused it to become a garden ornamental. However, giant hogweed spreads easily and can establish along roadsides, ditches, and streams. Giant hogweed has a thick (3-8 cm in diameter) bright green stem with dark reddish purple spots and coarse white hairs at the base of the leaf stock. The plant can be 2-5.5 metres tall with broad leaves that are deeply lobed and serrated. It produces a large upside-down umbrella-shaped head up to 80 centimetres across with clusters of tiny white flowers from late spring to mid-summer. Giant hogweed has a phototoxic sap, that when exposed to light can cause severe burns if on the skin and has been reported to cause blindness. Removing hogweed can be dangerous because of the sap; it should also not be burned or composted for this reason. The easiest way to remove it is to pull it when it is still very young and small and store all plant components in sealed black garbage bags until the plant is dried and seeds are no longer viable. Do not plant giant hogweed in gardens and report any sightings.

Fact Sheet


Common buckthorn is native to Europe and is also known as European buckthorn. In Canada, it is found from Nova Scotia to Saskatchewan. Buckthorn is shade and drought tolerant and grows in a wide range of habitats, spreading rapidly along road sides, fence lines, woodland edges, and in pastures abandoned fields. Invasions can harm the environment as it out-competes native plants, reduces biodiversity, degrades the quality of wildlife habitat, and impacts a wide range of industries. Buckthorn is a woody plants that ranges in size from a shrub to a small tree; reaching heights of 6-7 m. When soil is moist, small plants up to 1 m can be pulled. Larger plants can be dug out, or pulled out suing a weed wrench tool. Common buckthorn is listed as a noxious weed in Ontario's Weed Act Control 

Japanese Knotweed

Japanese knotweed is native to eastern Asia and was introduced to North America as a horticultural plant in the late 19th century. Japanese knotweed grows in a wide range of habitats including riparian areas, wetlands, roadsides, ditches, and fence lines. It forms dense thickets of bamboo like vegetation that aggressively outcompetes native plants, and negatively impacts wetland and riparian areas. Japanese knotweed has hallow, smooth, purple to green coloured stems up to 2.5cm in diameter.

Fact Sheet


Phragmites is an aggressive plant that spreads quickly and outcompetes native species for water and nutrients. Toxins are released from its roots into the soil to hinder the growth of surrounding plants. Identifying invasive phragmites can be difficult due to the existence of native subspecies. Generally, invasive phragmites reach heights of up to 5 metres, and has stems that are tan in colour with blue-green leaves and large, dense seed heads. Invasive phragmites can grow so densely that it crowds out other species, while native phragmites are not as dense and allow biodiversity.

Fact Sheet

Garlic Mustard

Since its introduction, garlic mustard has spread throughout southern Ontario, becoming a serious invader and threat to Ontario’s biodiversity. Garlic mustard grows in a wide range of habitats and spread quickly along roadsides, trails, and fence lines. Seeds fall close to the parent plants, and rarely dispersed by wind or water. The main pathway for seed spread over long distances is through humans, and pets. Within 5-7 years, garlic mustard can enter, establish itself and become the dominant plant in the forest understory. This is achieved by dispersing chemicals within the soil that prevent the growth of other plants and grasses.

Fact Sheet

Himalayan Balsam

Himalayan balsam is an invasive herbaceous plant that was initially introduced to North America as a garden ornamental. Himalayan balsam is widely distributed across Ontario and can be found as far north as Thunder Bay and Dryden. The annual reproduction occurs in the summer, when the flowers are pollinated by insects. The insects may transfer pollen between flowers of conspecifics or from the same plant. It typically grows to 1 to 2 m (3.3 to 6.6 ft) high, with a soft green or red-tinged stem, and lanceolate leaves 5 to 23 cm long. The crushed foliage has a strong musty smell. Below the leaf stems the plant has glands that produce a sticky, sweet-smelling, nectar. The flowers are pink, with a hooded shape, 3 to 4 cm tall and 2 cm broad; the flower shape has been compared to a policeman’s helmet. Himalayan Balsam creates dense and tall stands that prevent native plants from establishing and reduces biodiversity within the land. This plant is a prolific nectar producer and produces about 800 seeds per plant. This annual species can aggressively replace native perennial plants along river banks, leading to soil erosion.

Purple Loosestrife

Purple loosestrife has spread rapidly across North America and is present in nearly every Canadian province and almost every U.S. state. This plant has the ability to produce as many as two million seeds in a growing season, causing dense stands of purple loosestrife to outcompete native pants for habitat. Resulting in changes to ecosystem functions such as reductions in nesting sites, shelter and food for birds, as well as an overall decline in biodiversity. The plant mass grows on average to be 60 to 120 cm tall, as well as averaging 1 to 15 flowering stems.

Fact Sheet

Dog-strangling vine

Dog-strangling vine is found in parts of Ontario, southern Quebec and several American states. This plant grows aggressively by wrapping itself around tress and other plants, and can grow up to two metres high. This forms dense stands that overwhelm and crowd out native plants and young trees, preventing forest regeneration. The plant produces bean-shaped seed pods four to seven centimetres long and pink to dark purple star-shaped flowers.

Wild Parsnip

Wild parsnip is a member of the carrot/parsley family, like giant hogweed, is produces sap containing chemicals that can cause effects to human skin. In North America, scattered wild parsnip population are found from BC to California, and from Ontario to Florida, while being reported in all provinces and territories of Canada expect Nunavut. The plant forms dense stands that outcompete native plants, reducing biodiversity, also reducing the quality of agricultural forage crops such as hay, oats, and alfalfa. Wild parsnip can grow up to 1.5 metres tall with compound leaves arranged in pairs, with sharply toothed leaflets that are shaped like a mitten. Small clusters can be removed with proper protective clothing.