MaryAnne Spinner, Chicago Master Gardener

Most gardeners in our area can name the most nefarious plants to have been deemed "invasive" in our community: Purple loosestrife (Lythrum salicaria), Garlic mustard (Alliaria petiolata), Common buckthorn (Rhamnus cathartica), and Tree-of heaven (Ailanthus altissima). But there are many more plants, some of which have pride of place in our gardens, with the potential to be invasive. These are being watched carefully–and nervously–by plant conservationists.

What is an invasive plant?

According to the U.S. Department of Agriculture, an invasive plant is one that is non-native (also known as alien, non-indigenous, or exotic) to the ecosystem under consideration, and one whose introduction is likely to cause economic, environmental, or physical harm. Note that not all non-native plants are considered to be invasive; it is estimated that less than 10% of the 4,000 exotic plant species in the country are potentially harmful. Invasive plants have typically been deliberately introduced into a new habitat by humans, and have learned how to thrive; the introduction of kudzu (Pueraria ?>Montana v. lobata) in the Southeast is a prime example. Invasives are typically aggressive, highly adaptive, and reproduce at a very high rate, and because they are new to the environment, they usually have no natural enemies (like insects, frogs, and birds) to keep them under control.

Some invasive plants are worse than others. The U.S. National Arboretum points out that many invasive plants continue to be admired by gardeners who may not be aware of their weedy nature. Others are recognized as weeds, but property owners still fail to do their part in preventing their spread. Some do not even become invasive until they are neglected for a long time. Invasive plants are not all equally threatening; some only colonize small areas and do not do so very aggressively, while others may spread and dominate large areas in just a few years.

Invasive plants disrupt many natural habitats, especially closed ecosystems such as wetlands and sand dunes, where rare native plants are often found. Invasive plant species thrive on disturbed sites like construction areas and road cuts. Even foot traffic can create a temporary void that is quickly invaded–some national parks have restricted the areas where visitors are allowed to walk with the warning, "we can watch purple loosestrife grow from people's footsteps."

Why should we care?

According to the Illinois Native Plant Society, American farmers and ranchers spend some $5 billion dollars each year to control invasive species, and related crop losses and loss of productivity total another $7 billion annually. Just one plant, that notorious purple loosestrife, costs $45 million each year to control in the 36 states–including Illinois–in which it grows. Approximately 100 million acres in the United States are dominated by invasives, and the average annual increase in acreage is estimated to be 14%. In absolute terms, every day various invasive species become the dominant vegetation on 4,600 acres of public land; the accumulated annual increase is some three million acres annually, or the land equivalent of the state of Connecticut.

Financial considerations aside, invasive plants disrupt our local ecology. In some situations these non-native species can cause serious ecological imbalances. In the worst case, invasive plants begin to choke out other plant life, including rare and endangered species. This threatens not only native plants, but also the whole local population of insects, birds, amphibians, and mammals that rely on this plant material for their food and shelter. Even if this were not a potential ecological disaster to the food chain, at the very least it would reduce the diversity of our native habitats, which is a regrettable loss to the community.

Bad Guys in Illinois

In addition to the plants already mentioned, there is a slew of invasive, and potentially invasive plants in Illinois, some of which I have blissfully grown in my own garden for many years. Most horticulturalists are on the same page with certain plants–like Purple loosestrife, Tree-of-heaven, Japanese honeysuckle (Lonicera japonica), Japanese rose (Rosa multiflora), and Lesser Burdock (Arctium minus)–which definitely represent a threat to our environment. You only need to know that a single burdock plant is capable of producing 17,000 seeds in one season, to know that it's not something you'd welcome in your backyard.

But I was quite surprised at the breadth of a list compiled by the Chicago Botanic Garden, naming both plants that we all agree should be removed immediately, as well as those that are being watched and evaluated for their potential invasiveness. This roster includes several plants that I have grown in my garden, or have seen for sale at virtually every local garden center. It includes Baby's breath (Gypsophila paniculata), Purpletop verbena (Verbena bonariensis), Chameleon plant (Houttuynia cordata), Yellow flag iris (Iris pseudacorus), Beefsteak plant (Perilla frutescens), Porcelainberry vine (Ampelopsis brevipedunculata), Common periwinkle (Vinca minor), Japanese barberry (Berberis thunbergii), Common privet (Ligustrum vulgare), European cranberry bush viburnum (Virburnum opulus), and Crab apple (Malus spp.).

Actually, I have to admit that, from personal experience, I wasn't entirely surprised about the inclusion of a couple of these plants. Verbena bonariensis self-seeds wantonly in my garden, but hadn't worried me because it is both very pretty and fairly easy to rip out. But in just a few short years, our porcelainberry vine has turned into a thug that has taken over the arbor on which it was originally planted, as well as the adjacent (and invasive!) barberry hedge, the power lines to our garage, and one entire side of our house, and is working its way to the roof. We had already decided that the vine was going to become this year's taming project, but now it somehow seems more urgent.

What Can We Do?

In 2001 and 2002, the Center for Plant Conservation gathered a group of representatives from a number of horticultural organizations into workshops in St. Louis and Chicago, where they discussed initiatives designed to prevent the spread of invasive plants. One outcome of these meetings was the establishment of several Voluntary Codes of Conduct–one each for Government, Nursery Professionals, the Gardening Public, Landscape Architects, and Botanical Gardens and Arboreta–that set forth guidelines designed to curb the use and distribution of invasive plant species.

Following is the Voluntary Code of Conduct for the Gardening Public, which provides us, as gardeners, a multi-faceted strategy for fighting the spread of invasive plants:

• Ask for only non-invasive species when you acquire plants. Plant only environmentally safe species in your gardens. Work towards and promote new landscape design that is friendly to regional ecosystems.
• Seek information on which species are invasive in your area. Sources could include botanical gardens, horticulturists, conservationists, and government agencies. Remove invasive species from your land and replace them with non-invasive species suited to your site and needs.
• Do not trade plants with other gardeners if you know they are species with invasive characteristics.
• Request that botanical gardens and nurseries promote, display and sell only non-invasive species.
• Help educate your community and other gardeners in your area through personal contact and in such settings as garden clubs and other civic groups.
• Ask garden writers and other media to emphasize the problem of invasive species and provide information. Request that garden writers promote only non-invasive species.
• Invite speakers knowledgeable on the invasive species issue to speak to garden clubs, master gardeners, schools and other community groups.
• Seek the best information on control of invasive plant species and organize neighborhood work groups to remove invasive plant species under the guidance of knowledgeable professionals.
• Volunteer at botanical gardens and natural areas to assist ongoing efforts to diminish the threat of invasive plants.
• Participate in early warning systems by reporting invasive species you observe in your area. Determine which group or agency should be responsible for reports emanating from your area. If no 800 number exists for such reporting, request that one be established, citing the need for a clearinghouse with an 800 number and website links to information about invasive plant species.
• Assist garden clubs to create policies regarding the use of invasive species not only in horticulture, but in activities such as flower shows. Urge florists and others to eliminate the use of invasive plant material.

For more information on plants considered to be invasive or potentially invasive in Illinois, as well as lists of invasive plants in other states, please visit the following links.

The Chicago Botanic Garden's list of Plants Invasive in the Chicago region: http://www.chicagobotanic.org/research/conservation/invasive_chicago.html

The Center for Plant Conservation's website on invasive plants and Voluntary Codes of Conduct: http://centerforplantconservation.org/invasives/

The U.S. Department of Agriculture's National Invasive Species Information Center: http://www.invasivespeciesinfo.gov/whatis.shtml

Do you have a gardening question? Email the Extension's Electronic Plant Clinic at rwolford@uiuc.edu, and our Master Gardeners will be glad to assist you.