Pike & San Isabel National Forests
Cimarron & Comanche National Grasslands

USDA Forest Service
Pike & San Isabel
National Forests
Cimarron & Comanche
National Grasslands
2840 Kachina Drive
Pueblo, CO 81008
719-553-1400

Telephone for the
Hearing Impaired
719-553-1404

Noxious weeds are undesirable plants that are particularly threatening to public health, agriculture, recreation, wildlife or any public or private property. Most important, however, are those invasive species introduced from other continents that are outside of their natural habitats. Predators and plants normally competitive with these species are not present to control their spread. Noxious weeds readily invade disturbed land because disturbance compromises the native ecosystem and gives aggressive weed species a competitive advantage.

Yellow Starthistle is an annual and grows up to 2 to 3 feet tall. This species was introduced from Europe and grows along roadsides, rangelands and waste areas. Yellow starthistle is a rigid plant covered with a cottony fiber, and sharp thorns up to 1 inch long are located directly below the yellow flowers. Flowering occurs throughout the summer beginning as early as June. “Chewing disease” can result when horses eat this plant.

Whitetop is a deep-rooted perennial mustard reproducing by roots and seeds. This species grows up to 2 feet tall, has blue-green leaves and produces thousands of seeds. Whitetop is common on alkaline soils and disturbed areas and becomes very competitive with other species. This weed emerges early spring and flowers in April and May. Numerous, dense flowering branches give the plant a white-topped appearance.

Houndstongue is a biennial forming a rosette the first year and flowering the second year.  This species was introduced from Europe and spreads by seed.  Houndstongue grows in pastures, along roadsides and in disturbed areas.  The leaves resemble a hound’s tongue and are rough to the touch.  Flowers are reddish-purple and rarely white.  The seeds (nutlets) cling to animals and clothing and are easily transported.  Houndstongue contains toxic alkaloids, causing liver cells to stop reproducing.  Horses and cattle are more susceptible to the toxins than are sheep.  

Diffuse Knapweed is an annual, biennial or short-lived perennial originating in the Mediterranean region. This species is a diffusely branching, greyish-green, highly competitive weed representing a threat to rangelands, pastures and roadsides. Flowers are usually white and sometimes pink, rose or lavender. The flowering heads are narrow and numerous, and flowering occurs from June to September.

Russian Knapweed is a particularly invasive and insidious weed because it produces seeds that are long-lived in the soil. Knapweed develops roots that spread horizontally and vertically and produces a toxin that discourages the growth of other plants. Russian knapweed on rangeland is of particular concern to ranchers because it is toxic to horses. Once established, the weed is extremely difficult to control.

Everyone can help discourage noxious weed invasion. Learn to identify those weed species prevalent in the area and avoid spreading weed seeds by driving or walking through infested patches. Report suspected infestations to your county, state, local or federal weed managers. Be assured that these beautiful, natural western landscapes will thrive because of your diligence and be forever grateful.