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Learn About Invasive Species

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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. It is not currently illegal to sell invasive ornamental plants, so it is very important that buyers learn what is and isn’t invasive.
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

Purple Loosestrife

Purple loosestrife is an invasive flowering plant that spreads rapidly and densely in ditches, wetlands, and heavily disturbed areas. A single underground root system can spring up 30 to 50 individual stocks, each up to 2.5 metres high; the stocks are topped with flower spikes that are covered in small five to seven petaled, pink-purple flowers. Purple loosestrife forms thick root mats that can spread over large areas and crowds out native plants, which reduces biodiversity and habitat for native wildlife. This invasive plant was used by gardeners and is still being sold in some nurseries however it spreads out of gardens very quickly and can take over. Populations of purple loosestrife have been successfully reduced by the use of Europeans leaf-eating beetles as biocontrol.

Fact Sheet

Dog Strangling Vine

Dog strangling vine refers to two invasive species of swallowwort (pale and black swallowwort). It can climb up trees and tall plants up to two metres and can form dense mats that run along the ground, strangling out other vegetation. Dog strangling vine has oval leaves with pointed tips that are usually between 7 to 12 centimetres long. The flowers are small, pink to dark purple and star shaped. Native wildlife will not eat dog strangling vine which increases grazing pressure on the diminishing native vegetation. The thick mats that run along the ground prevent native plants from growing and make it extremely difficult to walk through or manage the forest. It is difficult to completely remove dog strangling vine because of the extensive root system; if the roots are not excavated it will re-sprout. Frequent cutting back will prevent the spread of seeds and stop the seed bank from being replenished. After removal of dog strangling vine, native plants should be planted to prevent reinvasions and new invasive species from taking hold.

Fact Sheet


Phragmites australis or European common reed refers to the species of invasive phragmites that invades ditches, streams, and wetlands. It grows up to 5 metres tall with light tan stalks, long upright blue-green leaves, and dense fringed seed heads. Invasive phragmites grows extremely densely (with up to 200 stocks per square metre) preventing any other vegetation from growing. It also releases a toxin from its roots that kills any vegetation that may be competing for resources. The fast growing plant sucks up large amount of water which drops the water level in that area. Stands of phragmites also consist of many dry dead plants which pose a fire hazard. It decreases biodiversity of native wildlife because it is not a suitable habitat or food sources. Due to the dense seed head it is very easy to spread the plant, so whenever there is any contact with phragmites equipment, clothing, shoes, and pets should be cleaned off before going to a new area. While it is shown that herbicides can effective control phragmites it must be done carefully and by the proper authority as there can be many undesirable consequences on the surrounding ecosystem. 

Fact Sheet

Common Buckthorn

Once planted as an ornamental shrub, common (or European) buckthorn, spread quickly and aggressively. It usually grows to be around two to three metres tall but can reach up to six metres tall, looking more like a tree than a shrub. They have oval serrated leaves that have veins curving down towards the tip and small yellow/green flowers with four petals. In summer it produces green berries that turn dark red then to black in late summer/early fall. Each stem ends in a long sharp spine. Common buckthorn grows quickly, dominates forest understories, and shades out any native plants. Like other invasive plants, common buckthorn alters the soil composition to create better conditions for its own growth and the detriment of other plants. It also does not provide the same quality of habitat and protection for birds that native shrubs do. To control common buckthorn the female (fruit bearing) plants should be initially targeted as they are the only ones that carry seeds. Young saplings can be pulled or treated with herbicide (depending on area regulations). Once the population is at a manageable level, all the male trees should then be removed.

Fact Sheet

Garlic Mustard

Garlic mustard is an invasive plant that was originally brought here to be used as a medicinal herb. It is high in vitamins A and C and has a distinctive garlic taste and smell. In the first year of growth, garlic mustard forms a basal rosette which consists of kidney-shaped leaves with scalloped edges and deep ‘winkles’. In the second year stalks form and the leaves grow off of it alternating sides; they are more triangular and have deeply toothed edges. It also grows clusters of small white flowers, each with four petals and a yellow centre. Garlic mustard invasions have the ability to blanket forest understories which can reduce the growth of native plants including sugar maples and trilliums. By reducing the native plant populations the wildlife populations will also decrease, many native butterflies rely on the forest plants to survive. The soil ecosystem is also damaged due to reduced fungi quantities, this prevents native plants from being able to re-establish. Garlic mustard can be removed fairly easily by hand pulling, it is important that the plant is pulled from the base of the plant and that the majority of the root system is pulled out with it. Once the plant is removed it must be bagged to ensure that no seeds escape and disposed of in accordance to local waste management procedures.

Fact Sheet

Japanese Knotweed

Japanese knotweed is a 1 to 3 metre tall, bamboo-like perennial that can invade both forests and wetlands. The woody stem is light green with red-purple markings and distinctive node lines along it. It has large broad triangular leaves reaching 3-6 inches long and 2-5 inches wide, which alternate along the stem. In late summer and fall Japanese knotweed has clusters of small white flowers (each with 5 petals). The fruit of the plant is a small, white, three-winged key surrounding a single brown seed, this allows them to travel in the wind and spread more easily. Japanese knotweed has a very large root system that has been known to break asphalt, and concrete; this leads to road damage, and cracks in housing foundations. The dense thickets that are formed crowd out native vegetation and decreases available habitat for native wildlife. Because new plants can spring up from very small root particles, excavation is not feasible for large populations of Japanese knotweed. Continuous cutting for several years to prevent the growth of photosynthetic material can get rid of populations.

Fact Sheet

3 C's are the Key 

"The love of gardening is a seed once sown that never dies" - Gertrude Jekyll

Protect what you love with the 3 C's of conscious gardening! Your garden is at great risk from invasive species which out-compete native species and alter local environmental conditions. By compromising native plant populations, invaders in-turn affect wildlife populations by decreasing available habitat and food source. This Spring as you tend to your garden, be conscious of invasive species and use these tips from the Ministry of Natural Resources and Forestry's 'Gardener Action Plan'.

1. Choose Native Plants: The fundamental rule of gardening is to choose plants that are native to your region. The Grow Me Instead Guides for Northern and Southern Ontario highlight excellent alternative species that provide similar function and appearance to the invasive plants you may appreciate having in your garden. 

2. Compost Carefully: Even after being uprooted from your garden, invasive plants can sprout new roots and propagate nearby. Before adding uprooted invasives to your compost pile. leave them in a black garbage bag in the sun for one week. This will ensure they don't continue to grow unchecked in nearby areas. 

3. Check the Pond: Invasive plants can spread in and out of your ornamental pond as well. Aquatic invasive plants can be just as threatening for their ability to grow quickly and deprive aquatic ecosystems of sunlight and oxygen. Keep your eyes peeled for the common aquatic invaders, but take extra care not to dump pond water and fish into other water bodies.