Noxious weeds are non-native plants, usually from Europe or Asia, that have no natural predators or diseases to keep them in check.They are invasive, capable of displacing native vegetation and turning a productive ecosystem into a monoculture of weeds.

In 1990, the Colorado General Assembly passed the Colorado Noxious Weed Act that requires each county to adopt a weed management plan.
Larimer County has designated noxious weeds that must be controlled in GVM. Each property owner has the responsibility to help eliminate these weeds. They are listed and described below. Please note that if you click on a picture, then a slightly larger picture will open up in another window.


click for larger picture This is a biennial that is rapidly invading disturbed areas of GVM."Biennial" means that during the first year of growth, the plant appears as a low growing rosette of leaves.During the second year, Diffuse Knapweed grows into a 2 to 3 feet tall plant with many branches, blooming from June to September. Small white to purplish flowers are found at the end of the branches and prickly spines surround each flower.These spines can cause injuries or obstructions in the digestive tract of livestock. It only reproduces by producing seeds, so control is through the cutting of the plant before the seeds are dispersed, or the rosettes can be sprayed with a systemic herbicide before they develop and produce seeds the second year. It has been discovered that the roots of this plant release a chemical which depresses growth of any other plant in its vicinity leading to an invasive monoculture of Diffuse Knapweed.


click for larger picture This perennial is very difficult to control. It spreads both by seeds, which are thrown 20' by exploding seed capsules, and by deep, creeping roots. The yellowish green flowering tops must be cut before the flowers set seed, followed by spraying with a systemic herbicide early in October. This entire plant contains a milky juice that is damaging to eyes and sensitive skin. Do not handle this plant without gloves.


click for larger picture This perennial has flowers that have the appearance of a yellow and orange snapdragon flower.It is extremely difficult to control as it reproduces both by a heavy production of seed and spr It can be weakened and ultimately controlled by vigilant pulling of seedlings and cutting stalks before seeds develop, followed by the application of a systemic herbicide after the first frost.


click for larger picture This perennial is heavily distributed throughout GVM. This thistle has the ability to out-compete just about all of our native plants. It grows 3' tall and is covered with 1/2" to 3/4" purple flowers. The leaves are spiked but not the stems. It reproduces both by seed and rootstock. Roots radiate as much as 16' horizontally and up to 22' deep. This weed must be mowed before going to seed 2 or 3 times during the growing season. In early October it should then be sprayed with a systemic herbicide. It will take a number of years of this treatment to fully eradicate this noxious weed.


click for larger picture This is also known as Nodding Thistle because the 1 to 3" purple flowers found at the end of stems, droop when mature.  Both the leaves and the stems have spines. This weed is a biennial, which means that it appears only as a rosette the first year and as a flowering plant the second year. Consequently, it does not have a massive root system and multiplies only by seed. It is fairly easy to control by pulling it up when it first appears. It can also be sprayed with a systemic herbicide in the spring while in the rosette stage. This thistle can be confused with the native Colorado Thistle, which has white flowers and blue-green foliage. This Colorado Thistle is NOT considered to be a noxious weed and need not be eradicated.


click for larger picture Mistletoe is a brownish green parasite that grows in our pine trees. It gets its water and nutrients from the host tree, eventually killing it from the top down. It can't be killed by spraying with an herbicide. It must be removed from the tree or, if infestation is severe, the entire tree must be cut down. Once removed from the tree, the mistletoe will die. From August to mid-September, the sticky mistletoe seeds explosively discharge and travel as far as sixty feet. If they land on a tree branch, the seed will penetrate the bark and grow, infesting another tree.