Noxious Weeds – What Are They and How Did They Get Here?

Most of our state listed noxious weeds came from Europe or Asia accidentally, or were garden pants that escaped cultivation. A few species, such as poison ivy, are native to Minnesota and are listed as noxious weeds due to harmful effects that they pose directly to human health or agricultural systems. Noxious weeds are problematic because they are highly adaptable to disturbed areas, are able to spread at alarming rates, cause economic damage, and outcompete native vegetation that is important to our unique landscapes throughout Minnesota. Noxious weeds also create problems with soil erosion, diminished forage production on lands used by domestic and wild animals and impact the public in many negative ways. The main differences between a common weed and a noxious weed are that the latter have an enormous capacity for destruction and are very difficult to control. An estimated 5,000 invasive non-native weeds now occur in U.S. natural areas.

Examples:

• Purple loosestrife can produce 2-3 million seeds per plant every year.
• Canada thistle has an extensive perennial root system that can produce new plant shoots in addition to its high annual seed output.
• Leafy spurge contains a milky sap that may cause blisters in the mouths of cattle and wildlife.
• Garlic mustard outcompetes highly valuable forest understory plants, creating large monocultures that have little or no value for forest ecosystems.

Noxious Weeds and Our Natural Areas

Noxious weeds are spreading and moving into our valued ecosystems and displacing our native plants at a disturbing rate. When the native plants that wildlife need for food, shelter, or nesting have disappeared, the wildlife move out and seek new areas for survival. Noxious weeds play an important role in lessening our natural plant and animal biodiversity statewide.

Noxious Weeds and Our Agricultural Lands

Noxious weeds also impact our important agricultural lands. The more invasive and unnecessary weeds that are on the land diminish available resources; this in turn results in more land required to raise the same number of cattle or bushels of corn.

How Noxious Weeds Move

Noxious weeds are spread by humans, animals, water, and wind and if left unchecked, spread at an astounding rate of up to 14% per year. Primary locations for noxious weeds to become established are disturbed sites such as roadsides, trails, sites cleared for construction, gravel pits, areas that are overused by animals or humans, old agricultural fields, wetlands, and river corridors. Noxious weeds also hitchhike on vehicles, equipment, and in shipments of goods and products. Many weeds produce copious amounts of seeds with barbs, hooks or other attaching devices that easily adhere to people, animals or equipment. Some weeds have been unintentionally introduced through planting of contaminated crop seeds, the feeding of forage contaminated by weed seed to livestock, or on boats and machinery.

Examples:

• Leafy spurge seeds are expelled from their seed capsule and can fly up to 30 feet.
• Buckthorn seeds are spread by birds and may remain viable in the soil for 20 years.
• Garlic mustard can produce hundreds of seeds, which can scatter several meters from the parent plant and may remain in the soil up to 11 years.
• Purple loosestrife seeds can become imbedded in the boots of outdoor enthusiasts and moved to new wetland areas.

The Economic Cost of Noxious Weeds

Weeds compete with crops and diminish the quality of food, feed and other commodities. Natural weed populations in most fields are high enough to cause significant yield losses if left uncontrolled. Loss figures of 50-90% are common for crops grown in weed infested lands. In 46 major crops including pasture, hay, range, and animal health losses; control costs were estimated to be more than $15 billion per year. In non-crop areas including turf, ornamentals, golf, highway rights-of-way, industrial sites, aquatic sites, forestry and other sites, losses and control costs totaled about $5 billion per year.

Herbicide use is important for weed management and control programs. It is estimated that losses in the agricultural sector would increase by approximately 500% without the use of herbicides. Since introduced species account for 65% of the total weed flora in the United Sates, their total economic impact on the U.S. economy equals or exceeds $13 billion per year. (From “Invasive Plants – Changing the Landscape of America” fact book, Federal Interagency Committee for the Management of Noxious and Exotic Weeds, 1998)

Weed Eradication, Control, Containment

• Eradication - When populations of invasive weeds not normally found in Minnesota, or only found in small areas are discovered, they need to quickly be eradicated. Eliminating these weeds when the population is small saves time, money, and resources. Eradication means destroying the above and below ground propagating parts of the weed.
• Control - If a noxious weed is found in sizeable numbers in limited areas of the state, a cooperative plan is developed where county agricultural inspectors, state staff and the landowner may work together to manage the weed. Control means that the plant is managed to prevent further reproduction so that the plant population is managed and reduced to a suitable level.
• Containment - Some noxious weeds are already well established in Minnesota and found in such large numbers that it is no longer realistic to eradicate the population. Instead we work at the county and municipal level to stop or slow their spread.

Control Methods

Each species of noxious weed grows and spreads differently requiring different methods of control. Not all techniques are suitable and not all will work in every circumstance.

Some techniques that may be used include:

• Biological control which uses other organisms to control noxious weeds. For example, in Minnesota, there are three existing biological control programs for leafy spurge, purple loosestrife, and spotted knapweed. These programs use host-specific natural enemies of the weed from its native range, i.e., Europe or Asia, to control its populations in Minnesota. All biological control agents undergo extensive host-specificity testing before being approved for release in the United States. Several other biological control programs are currently being evaluated in Minnesota for garlic mustard, common tansy, and European buckthorn.
• Chemical control which uses herbicides to control noxious weeds. All herbicides must be used in accordance with the registered label. For more information on recommended herbicides and rates, contact your local extension office.
• Cultural control uses materials or techniques that reduce noxious weed populations. Examples include mulching, rotational grazing, and establishing good vegetation cover.
• Mechanical control uses process like cutting, mowing, and disking.

What Can You Do?

• Familiarize yourself with Minnesota’s noxious weeds and learn about the threat and damage done by these destructive invaders.
• Work with other landowners to identify the scope of weed populations and control them on land you own or manage.
• Support private and governmental efforts to manage noxious weed populations.
• Enhance awareness, knowledge and education efforts about the adverse effects of noxious weeds.

As land users and managers, we need to protect our natural, urban and agricultural resources. If we are conscientious in our efforts, we will protect our resources for the enjoyment of future generations.