Illinois' exotic plants, say Jean Mangun and Kristin Floress and SIU-Carbondale's Department of Forestry, are non-native plants that did not grow in Illinois before the arrival of European settlers. Many of them have escaped cultivation to become invasive, threatening Illinois' native species and depriving wildlife of shelter and food. Between 20 and 33 percent of the plant species now growing in Illinois are exotic plants. They range from single-season annuals to 15-foot shrubs.
Canada thistle (Cirsium arvense) is a perennial that's native to Europe, according to Illinois' Institute of Natural Resource Sustainability. Standing 2 to 5 feet high, it's common in the northern half of the state. Its straight, hairy stems have lance-like, prickly leaves. White or purple appear from late June to August. Canada thistle spreads by feathery seeds carried on the wind and by its own roots. Large colonies of it will crowd out native plants. Canada thistle, says the Illinois Institute of Natural Resources Sustainability (IINRS), flourishes in disturbed locations like roadsides, old pastures and fence rows, but is not successful invading woodlands and well-maintained pastures.
Multiflora rose (Rosa multiflora), introduced to Illinois in the 1950s as a source of wildlife food and shelter, is native to Asia. A thorny, spreading shrub that can grow 15 feet high, it has oval dark green leaves and profuse clusters of white blooms up to 1 1/2 inches across in late spring. Its bright red hips stay on the shrub all winter, feeding birds and mammals. Multiflora rose is common in adjoining fields, along roadsides and in pastures. Even dense woods can't discourage this exotic plant's spread. Wildlife may drop seeds that survive for up to 20 years before germinating, according to the IINRS. If unchecked, it will form dense thickets that crowd out all other plant species.
Garlic mustard (Alliaria petiolata) has a garlicky odor. Standing anywhere from 2 inches to 4 feet high, it has basal clumps of kidney-shaped, deep green foliage and stems with toothed triangular leaves. A cluster of tiny, four-petaled white flowers tops each stem in May. Long, thin seedpods follow the blooms. Garlic mustard, says the IINRS, is an exotic plant native to Europe that now grows in 41 Illinois counties. While plants die after producing seed, they produce enough of it that garlic mustard spreads easily. Garlic mustard is especially invasive in Illinois' shaded areas and open woodlands, and along roadsides.