Controlling Invasive Plants

Master Gardeners regularly receive numerous questions about several favorite topics. Close to the top of the list is :How do I get rid of ___? Bugs and weeds are always first; here are some ways to manage invasive weeds.

Our environment is always in a state of change; one of the changes that gardeners and landowners have noticed is the proliferation of invasive weeds in the last decade. There is no single cause but among the leading ones are drier and warmer summers, more disturbed places, less effort at weed control, and increased transport of forage, plants, gravel, and soil.

Controlling weeds in your own garden, flower bed or land should start with a management plan. In planning you need to think through how to prevent the spread of noxious weeds and how to limit the number by reducing the plant's ability to regenerate.

Weed control takes three forms. First is biological control. An example that is showing promise is the use of beetles to control purple loosestrife and spotted knapweed. Another is a fungus called pseudomonas that infects Canada thistle. Biological control has to be specific to the plant being controlled.

Cultural control is the oldest and most familiar. Cultivating, mowing, cutting, pulling, and burning work very well for certain plants. I have tried burning thistle and knapweed with a hand-held torch the last few years with very good success. This would be difficult on a large scale though. Mulching and planting cover crops have proven very successful in other situations.

Chemical control is the third method. Herbicides may have effect on a very narrow range of species or cam be nonselective. Safety and environmental impact should be a first concern in selecting this method. Timing, dose and temperature all have an effect on herbicide success. For example, a lot of money is wasted and needless contamination of turf grass occurs by applying crabgrass control at the wrong time. Also remember that children, pets, fish, birds, bees, and other beneficial organisms are exposed to whatever chemical you may choose to use.

It is very important to identify the plants you wish to control. Each has its own unique control.

An excellent booklet is Noxious Weeds of Minnesota by the late Carol Estes Mortenson.

An excellent online source is the noxious weeds list at www.extension.umn.edu. Simply search noxious weeds. Another excellent source is the invasive species page at www.dnr.state.mn.us/invasives. All th\e above references give specific control methods and chemical recommendations.

Regardless of the method you choose, diligence pays big rewards in weed control. Knowing that weeds like thistle, tansy, or leafy spurge will require at least three years worth of effort to make a marked difference. There is no quick fix. All it takes is one bloom of Canada thistle to spread seeds for hundreds of yards! However, mulching your garden year after year will result in nearly a weed free garden in the future If you buy forage, manure, gravel, or soil from somewhere else, insist that it is weed free. Eventually the suppliers will get the message. Good Gardening!