Outdoors - July 2010 Beware of planting invasive species - Beware of planting invasive species

Plants that have been moved by human activity to an area where they do not normally grow are referred to as "non-native" or "exotic" plants. Not all non-natives are bad but when they cause ecological or economic problems they are termed invasive or exotic species. Common buckthorn (Rhamnus cathartica), burning bush (Euonymus alatus), Norway maple (Acer platanoides), amur maple (Acer ginnala), Japanese barberry (Berberis thunbergii), butterfly bush (Buddleia davidii) and common tansy (Tanacetum vulgare) are examples of plants released by the horticulture industry as ornamental plants that have escaped from controlled cultivation and now survive as invasive species. You might be surprised to find some of these on the list of invasive species, but one you should be very familiar with is buckthorn and that is because in 1999 it was declared a noxious weed by the state of Minnesota.

    A noxious weed is defined as a non-native plant that is difficult to control and injurious to public health, the environment, roads, crops, livestock or property. Once a plant has been declared a noxious weed it is no longer legal to sell, transport or move the plants within the state of Minnesota and these weeds must be controlled on all public and private land. Other noxious weeds that you may have heard of recently include purple loosestrife, Eurasian milfoil and garlic mustard.

    Purple loosestrife was introduced to the east coast from Europe in the 1800s and can now be found in 40 of the 50 states and in the boarder provinces of Canada. Seeds of purple loosetrife most likely escaped from garden plants and now the plant has invaded lakeshores and marshes replacing native shoreline plants and cattails that provide natural cover, food and nesting areas for a variety of native wetland animals. Eurasian milfoil is native to Europe, Africa and Asia and was introduced into America waterways in the 1800s, quite possibly in ship ballast into Chesapeake Bay. It is an aggressive plant that forms thick dense canopies on the waters surface and is now choking Minnesota's waterways interfering with recreation and impeding water flow and navigation. Garlic mustard grows to 3 or 4 feet and is found in the forest, at the edge of wooded areas, along the roadside, near trails and where trees have been removed. It is an aggressively spreading plant that has just been identified as a threat to the wildflowers in the Eloise Butler Wildflower Garden.

    There are 11 primary plants that have been identified as noxious weeds in Minnesota, but often we encounter other invasive plants in our gardens that don't fall into this category. When we talk about invasive plants in the garden we generally mean plants that are vigorous spreaders and are moving in and displacing other plants. Sometimes they spread because they produce a large number of seeds (like dandelions) other times because they have an extensive hard to kill root system (like snow on the mountain, or lily of the valley.) Sometimes, like with crown vetch, they spread because they survive in poor soil where other plants cannot. Whatever way they spread they tend to take over the garden and crowd out all of the other desirable plants. If you choose to plant some invasives, those that spread through their vigorous root systems can sometimes be contained if you cut the bottom out of a plastic container, bury it in the ground and plant within the container. Those that spread through the production of an abundance of seeds are nearly impossible to control except by chemical or mechanical means as we see by what happened when Harriet Godfrey introduced the dandelion to Minnesota! The best way to contain invasive plants is to not plant them at all!