Invasive Non-Native Plants

As gardeners, we manipulate our environment by introducing plants that don’t naturally grow here.  We do this because the plant has some value to us.  It may produce edible fruit or beautiful flowers.  It might be showy by itself, or perhaps is just distinctive and unique.  This doesn’t usually cause a problem, but occasionally one of these cultivars “escapes from captivity” and becomes a nuisance and, in at least one case, a hazard.  This installment of the gardening column will mention a few of the culprits that can be found locally.

The first order of business is the Giant Hogweed, Heracleum mantegazzianum.  Many local outbreaks of this attractive but dangerous noxious plant have been found.  Beware this plant. All parts of the plant can cause severe skin irritation in susceptible people, causing painful burning blisters that can leave large disfiguring purple to brown scars that may last several years.  Giant Hogweed can grow up to fourteen feet tall with huge leaves that can get to be five feet across.  It has a hollow ridged stem with purple blotches and white hairs.  It develops large white flat-topped flower clusters that are like giant versions of the Queen Ann’s Lace  you can see growing along most roadways and in many fields.  If you suspect that you have this plant growing on your property, do NOT touch it or attempt to cut it down.  Contact Joyce Price at the Extension Office of M.S.U. (906) 932-3992 or Lynn Adams at the University of WI Extension Office (715) 561-2695 for a positive identification and more information.

Thankfully the rest of the local invasive plants are not physically dangerous.  The Big Three invasive plants, according to the Invasive Plants Association of Wisconsin (http://www.IPAW.org), are Purple Loosestrife, Buckthorn, and Garlic Mustard.  All three of these can be found in our area.  Another Internet information source is the Wisconsin DNR website (http://www/dnr.state.wi.us/org/land/er/invasive/).  Surprisingly, many of the plants considered to be invasive are not on lists of banned species and can be obtained at nurseries, even within Wisconsin.

Purple Loosestrife, Lythrum salicaria, is a beautiful flowering plant.  It was sold as a garden plant and seeds can still be obtained from a few sources. By law, Purple Loosestrife is a nuisance species in Wisconsin. It is illegal to sell, distribute, or cultivate the plants or seeds, including any of its cultivars. It forms a showy stalk filled with small purple to pink flowers.  It also produces over a quarter-million seeds per plant.  Purple Loosestrife will form a dense mat in wetland areas, completely choking out native vegetation.  It will intrude into shallow marsh areas, choking out open water, destroying wildlife nesting areas and fish breeding areas.  It provides little to no food for wildlife.  One large patch in Ironwood that was successfully eradicated was on Vanderhagen Road near the Northern Natural Gas pipeline north of US2.  Loosestrife is also prevalent in marshy areas alongside Highway 13 between Washburn and Bayfield. It can be controlled by physical removal, by using glyphosate (Active ingredient in Roundup®), or by the introduction of one of several forms of predator insects.

Ornamental grasses can become a problem.  Two listed varieties that local gardeners might plant in landscape beds are Phragmites, Phragmites australis and reed canary grass, Phalaris arundinacea.  Phragmites, has become a major problem in the Green Bay area where it has completely replaced hundreds of acres of cattails.  It reproduces from seed and from rhizomes (root structures such as found in quack grass).  Reed canary grass will also invade wetlands and form large stands that allow few other plant species so is of little use to wildlife. Once established, reed canary grass dominates an area by building up a tremendous seed bank that can eventually germinate and re-colonize treated sites.

Among landscape plantings which can escape captivity are honeysuckle, Lonicera family, and Japanese Barberry, Berberis thunbergii.  I have found both of these shrubs growing in my woodlot far from any deliberate plantings. Their vigorous growth inhibits development of native shrub and ground layer species.  Eventually they may entirely replace native plants by shading and depleting the soil of moisture and nutrients. The early leafing of these species is particularly injurious to early spring flowers which have evolved to bloom before trees and shrubs have leafed out. Both shrubs are attractive to birds, which spread the seeds far and wide.

While there are a lot of invasives listed on the Wisconsin DNR web site (over 100), we are interested in only those that are a problem in this area and are connected with gardening or landscaping.  For example, Bird’s Foot Trefoil is listed as an invasive species because it invades prairies in Southwestern Wisconsin, yet here in the North farmers plant it as a desirable forage crop.  As gardeners, we must be aware of the potential consequences of our behavior and act to conserve our environment.