Invasive Species
The term “invasive species” can mean a non-indigenous species that has a negative impact on its new habitat (like the Asian long-horned beetle). Or it can mean a species, indigenous or non-indigenous, that proliferates in a given area (like the mint plant). Often the two go hand in hand, because when a plant is non-native, it is more likely to take over (like the dandelion, originating in Eurasia). Invasive plant species threaten the biodiversity of native plant life, upsetting the delicate, slowly-attained balance of a natural ecosystem.
According to the Canadian Botanical Conservation Network, the invasive species overtaking our native plants generally have several things in common, making them aggressive competitors:
- Have rapid growth rates in diverse environments
- Produce seeds prolifically
- Are effective at seed dispersal, germination, and colonization
- Often can perform vegetative propagation (a form of asexual reproduction which can be very efficient)
- Are not plagued by pests or natural pathogens
- Can out-compete native plants
- Are too expensive to combat
The following plants are species prevalent in Ontario, with the highest invasiveness risk for the province. Most invasive plant species have their origin in Europe or Eurasia. We know that many of them have been introduced quite recently – in the past century or more.
Originating in Europe
- Garlic mustard (Alliaria petiolata) – competes with the wood poppy (Stylophorum diphyllum), which is endangered, and the white wood aster (Aster divaricatus), which is threatened. Introduced to North America in 1879.
- Dog-strangling vine, Black swallowwort (Cynanchum nigrum). A tough, twining vine that grows in waste places, in abandoned pastures and along fencerows.
- Leafy spurge, Wolf’s milk, Faitour’s grass (Euphorbia esula). Grows on fields, pastures and roadsides. This weed is poisonous to livestock. It propagates vegetatively.
- European frog-bit (Hydrocharis morsus-ranae). Grows in the wet, in areas such as ponds, ditches and canals. Introduced in 1932, the European frog-bit displaces native flora. Propagates vegetatively.
- European buckthorn, Common buckthorn (Rhamnus cathartica). This variety of buckthorn easily outcompetes native seedlings.
- Glossy buckthorn, black buckthorn (Rhamnus frangula). This buckthorn also displaces native species. Both buckthorn species were introduced to North America before 1913.
Originating in Eurasia
- Purple loosestrife, Swamp loosestrife (Lythrum salicaria). The loosestrife has clusters of vivid purple flowers on terminal spikes. It spreads in gardens, ditches and wetlands.
- Eurasian water milfoil, Spiked water milfoil (Myriophyllum spicatum). Thrives in stagnant or running water. Since its introduction in 1960, the milfoil displaces all other species with which it comes in contact. It propagates vegetatively.
Originating in Asia
- Japanese knotweed, Mexican bamboo (Polygonum cuspidatum). Japanese knotweed is sometimes called “Mexican bamboo” or “false bamboo” because of its hollow stems and horizontal joints (nodes). It can have ornamental usage.