Noxious Invasive Weeds

By Jan Cashman


Why should I care about noxious weeds?

Driving through our beautiful state, especially in Western Montana, you can easily see the impact of noxious weeds.

There are large stands of the deceivingly pretty pinkish-purple flowers of spotted knapweed. All of these noxious, invasive weeds choke out native plants, reduce the natural variety of plants in that area, make areas more susceptible to fire, and reduce productivity in agriculture by invading range and cropland. The expense to us and our government to control these noxious weeds runs to the billions of dollars.

What is a noxious weed?

A noxious weed is a plant that is “competitive, persistent and harmful.” Most invasive noxious plants have been introduced—are not native to the area; most come from Europe and Asia. Therefore, they do not have natural enemies to limit their spread.

Categories of noxious weeds:

The state of Montana has ranked noxious weeds according to their prevalence in our state. Those ranked 1A and 1B have limited or no presence here. The 2A category weeds are present only in certain areas of the state. Those ranked 2B pose a serious threat, containing familiar, dreaded names such as leafy spurge, Canadian thistle, and spotted knapweed. There is another category, “3”, whose plants, such as Russian olive trees, have the potential for negative impact.


Canadian Thistle

Field Bindweed

Leafy Spurge


Russian Knapweed

Spotted Knapweed

Are some noxious weeds worse than others?

Difficult to control spotted knapweed is probably the most widespread and invasive weed in the state, especially in Western Montana. In 1920 knapweed was found in one county in Montana; today it is found in every county and infests about 4.5 million acres. Each knapweed plant produces thousands of seeds that live over 8 years after landing on the ground. Its long taproot makes it difficult to pull.

What can I do to control noxious weeds?

The best way to control a noxious weed is to limit weed seed dispersal by vehicles, clothing and animals. If the weed is found on your property, remove it before it becomes established. Then, if weeds have become established, the following methods of control are recommended:

  1. Herbicide-use the proper herbicide. Multiple sprays may be necessary.
  2. Mechanical means such as simply pulling works for small areas. Be aware that some weeds are strengthened by pulling if some of the root is left in the ground so be sure to get all the taproot. Wear gloves.
  3. Biological control– introducing insects that are a natural predator of the weed. Biological control can take a long time to eradicate the weed—sometimes years.
  4. Reseeding with desirable plant species, usually grasses, that will compete with the weed.
  5. Grazing, most often with sheep or goats.
  6. There are other controls for noxious weeds such as mowing (before flowering), tilling, or burning. However, not all weeds respond to these methods.


It is against the law to permit a noxious weed to exist on your property. But whether it is a law or not, it is to everyone’s benefit if landowners control the noxious weeds on their property, whether big or small infestations.