Exotic Invasives

In spring we see a multitude of wildflowers in the woods, but we can also observe the greatest impact of invasive plants.  There are sixteen species that are regulated by the Illinois Department of Natural Resources which are illegal to buy, sell, or distribute without a special license.  All of these plants present a threat to natural areas and the health of native species.

Woodland habitats are characterized by stands of trees forming a closed canopy that provides a shady forest floor.  Native woodland wildflowers compete for sunlight by blooming in early spring before the trees leaf out.  Other understory plants are shade-adapted and able to grow in the reduced light under the canopy.  Invasive plants grow aggressively and spread outside their native range.  Exotic or non-native plants have few issues with disease and the foliage is not eaten by native animals.  The combination of these factors make exotic invasive species a serious threat to the sustained health of our native woods.  The following three examples are species appearing now in our area woodlands. 

Garlic mustard, Alliaria petiolata, was imported from Eurasia in the 1800s as a pot herb.  It is a biennial, developing flowers and seeds in its second year.  Growing 1′-3′ tall, it has medium green leaves with upper stems terminating in a group of small white flowers that bloom for 6-8 weeks in late spring.  The taproot has a characteristic “S” shape near the top, but plants are easy to pull out.  If the entire root is not pulled out, the plant will quickly send up new shoots.  If flowers are mature, seeds will continue to develop and disperse even after the plant is pulled out of the ground and composted.  When removal is done in late spring, plants need to be bagged and totally removed from the woodland. 

Common buckthorn, Rhamnus cathartica, was imported from Eurasia as a hedge and landscape plant.  It develops as a large shrub or small tree, growing to 25′ tall.  Buckthorn has glossy leaves and rough textured gray to brown bark covered in white lenticels.  If the bark is scratched or peeled away, it is bright yellow underneath.  A plant can be either male or female.  After flowering, female plants develop dark blue berries, which turn to black by autumn.  The berries are eaten by robins, starlings, and other birds, but seeds run through the digestive track undamaged and are thus spread about.  Buckthorn releases a chemical called emodin, produced in leaves, berries, bark and roots that exhibits negative effects on other plants in its immediate vicinity.  This chemical is toxic to amphibian embryos, disrupting development and preventing successful hatching.  Cutting and herbicide application to the stump is an effective treatment.

Amur honeysuckle, Lonicera maackii, was imported as early as the 1700s from Eurasia as an ornamental species.  It is a multi-stemmed shrub, growing up to 20′ tall.  Honeysuckle has medium green leaves and gray, woody branches with narrow vertical grooves.  Plants bloom in late spring for about 3 weeks, then develop Juicy, bright red berries containing 2-3 seeds each, which are eaten by a variety of songbirds.  Honeysuckle is one of the earliest plants to leaf out in spring, effectively preventing other understory plants around it from growing.  Cutting the plant down promotes aggressive regrowth, with additional multiple stems and foliage.  An herbicide treatment of the stump or foliage is an effective management method. 

Control of exotic invasive species is quite difficult.  In many cases, cutting simply promotes aggressive regrowth, and studies show that burning does not have much of an impact.  The best control methods include preventing establishment of these species or removing the entire plant including the root system.  This is not always a practical method.  Herbicide has been the next best treatment option, especially in conjunction with cutting before flowers or seeds are produced.  Note that the best treatments may be different for each species, so be sure to research different approaches.  Several sources of information are available (click on names to link to each) from United States Department of Agriculture, University of Illinois Extension, the Illinois Department of Agriculture, and The Nature Conservancy.  Consider the following documents as possible sources to start your research:

• From Illinois Invasive Species:  Invasive Species of Concern
• From Northwest Illinois Forestry Association: Exotic, Invasive Plants in Illinois Habitats
• From Northwest Illinois Forestry Association: TNC Weed Control Methods Handbook
• From University of Illinois Extension: Illinois Exotic Weed Act

If you have these species in your yard or know of them in the area, consider getting involved in their eradication.  Your other plants and animals will be better off.