Invasive Species

By Sharleen Pratt, Master Gardener

According to the Ontario Invasive Plant Council, an Invasive Species is an alien species whose introduction or spread negatively impact native biodiversity, the economy and/or society, including human health.

Therefore, an invasive plant species is often a plant that has been brought into Ontario from another country, possibly for medicinal reasons or as an addition to one’s garden. For various reasons, it becomes aggressive, spreads quickly and often displaces native plants.

Here is a detailed description of five invasive species that could show up in your garden. They are all Category 1 Invasive as designated by the Credit Valley Conservation described as species that exclude all other species and dominate sites indefinitely. Plants in this category are a threat to natural areas wherever they occur because they tend to disperse widely (for example, through transport by birds or water). They are the top priority for control but control may be difficult.

Rhamnus cathartica
Common Name: Common Buckthorn, European Buckthorn Common-Buckthorn

Height: Up to 10m tall
Type of Plant: Deciduous Shrub or small tree that is fast growing and short lived
Leaves: Smooth, dark green leaves with slightly serrated leaf margins, somewhat elliptical and arranged in opposite to sub-opposite pairs along the stem. A sharp thorn can be found on the end of most branches.
Flowers: Flowers occur in the spring. They are yellowish/green, with four petals in clusters of 2 to 6 near the base of the petioles. They are small and inconspicuous.
Fruit: Produces clusters of berry-like globose black fruit in late summer and fall; although it’s mildly poisonous, birds and other wildlife eat the fruit and disperse the seeds.
Culture: Can thrive in a wide range of soil and light conditions. It is shade tolerant.
Invasion Pathway: Introduced from Eurasia to North America in the 1880s for ornamental landscaping. It was widely planted for fencerows and windbreaks in agricultural fields. The large number of seeds are spread by birds and animals.
Impacts: Habitat destruction and because it leafs out early, it is a danger to native species. It also alters the nitrogen levels in the soil. The soybean aphid, an insect that damages Ontario soybean crops, can use buckthorn as a host plant to survive the winter.
Control Measures: Physical removal, herbicides, fire, girdling

Alliaria petiolata
Common Name: Garlic Mustard

Height: 30 – 100cm tall
Type of Plant: Biennial Herb in Mustard Family
Leaves: In First Year: Leaves are dark green, cordate shaped with crenate margin edges. In Second Year: Leaves are alternate on a larger stem with somewhat doubly serrated edges. The lower leaves on the stem are broad, cordate shaped and up to 10cm across. The upper leaves on the stem start to narrow.
Flowers: In Second Year: Four white petals appear, arranged in cross shape.
Fruit: In Second Year: The fruit is erect, slender, 4-sided pod, green, maturing pale grey-brown, two rows of small shiny black seeds. Hundreds of seeds can be produced from a single plant.
Culture: Alliaria can grow in a wide range of sunny and fully shaded habitats, including undisturbed forest, forest edges, riverbanks and roadsides.
Invasion Pathway: Introduction for perceived medicinal value as a disinfectant, a diuretic and sometimes being used to treat gangrene and ulcers. It was also planted as a form of erosion control. European settlers also used it as a garlic type flavouring. Seeds can remain in the soil for several years and still be able to germinate. Hundreds of seeds produced from one plant.
Impacts: Alliaria forms dense stands, replacing native plants and has been implicated as partial cause for endangered status of our native wood poppy (Stylophorum diphyllum), American ginseng (Panax quinquefolius) and our provincial Trillium (Trillium cernuum). It is toxic to larvae of certain butterfly species that lay eggs on the plant.
Control Measures: Removal by hand, mowing or burning in early spring before flowering. Should always be bagged and burned.

Cynanchum louiseae and C. rossicum
Common Name: Dog Strangling Vine

Height: 2m high
Type of Plant: Twining Vine
Leaves: Oval with a pointed tip and grow opposite
Flowers: Pink to dark purple star-shaped flowers have five petals
Fruit: Produces bean-shaped seed pods that open to release feathery white seeds in late summer
Culture: Prefers open sunny areas but can handle some shade. More dominant in meadows or woodland edges.
Invasion Pathway: Introduced in the U.S. in the mid 1800s for use in gardens. Produces 28,000 seeds per square metre. Seeds spread by wind and new plants also can grow from root fragments
Impacts: Forms dense stands that overwhelm and crowd out native plants and young trees, preventing forest regeneration. Invading ravines, hillsides, stream banks and utility corridors. Leaves and roots may be toxic to livestock.
Control Measures: Digging is most effective. Hand pulling is not recommended as the plant will send up multiple shoots.

Vinca minor
Common Name: Periwinkle

Height: Up to 15 cm tall
Type of Plant: Evergreen herb that exhibits a trailing mat with a medium growth rate.
Leaves: Lance shaped, shiny, evergreen with a subtle white mid vein. They are opposite along stem.
Flowers: Showy blue/purple with 5 fused pin-wheel like petals and a short tubular throat that bloom in late spring.
Culture: Various soil types. Found in forests and along streams, roads and wetlands. Typically associated with residential gardens.
Invasion Pathway: Introduced as a garden ornamental and medicinal herb. It spreads by means of arching stolons, which root at the tips. Grows most vigorously in moist soil with only partial sun, but it can grow in the deepest shade and even in poor soil.
Impacts: Still sold as a groundcover which is a major concern. It spreads quickly and is a threat to native biodiversity.
Control Measures: It can be pulled, raked, or dug up, though re-sprouting will likely occur. It can also be cut or mowed in spring during its rapid growth stage.

Aegopodium podagraria
Common Name: Goutweed

Height: 2m tall
Type of Plant: Herb
Leaves: Compound leaf with serrated edges, can be non-variegated or variegated green and white, alternate
Flowers: Flat topped ‘umbrella like’ flower head with many small white flowers in late spring held above the foliage on leafy stems (which look similar to Queen Anne’s Lace).
Culture: Various habitat. Full sun to part shade. An escapee from residential gardens into forested areas.
Invasion Pathway: Goutweed seeds require recently disturbed soil and a sunny location to survive after germination. For this reason, Goutweed does not have much success reproducing by seed in forest ecosystems. However, even one established plant can create a large colony by spreading through its aggressive rhizomes.
Impacts: Forms dense patches that displace native plants.
Control Measures: Because it has limited reproductive success by seed, small patches of Goutweed can be easily controlled by digging up the plant (with careful attention given to removing the entire rhizome) or covering with a tarp or weed barrier for at least one growing season.