Diffuse Knapweed (Acosta diffusa)

Diffuse knapweed is a profusely branched biennial or short-lived perennial. Stems are rough. Flowers are white, reddish, or lavender, and are present from June to September.

Spotted Knapweed (Acosta maculosa)

Spotted knapweed is a simple perennial that reproduces only by seed.  There are 8-15 stems per plant.  It is named after the black tipped (spotted) bracts at the base of the flowering head. It has invaded over 4 1/2 million acres of Montana.

Russian Knapweed (Acroptilon repens)

Russian knapweed is a creeping, herbaceous perennial. Shoots and leaves are covered with dense grey hairs. Flowers are pink, lavender or white. The plant produces chemicals which prohibit the growth of other plants. It is toxic to horses.

Leafy Spurge (Tithymalus esula)

Leafy spurge has bluish-green leaves and yellow-green flowers. It has an extensive root system which makes it very hard to control. While not as big a problem in Boulder County as in other parts of Colorado, it has gained ground in recent years and is considered a large threat to our wildlife habitat. 

Musk Thistle (Carduus nutans)

Musk thistle has a large purplish flowering head (often nodding) which produces numerous seeds.  It is prevalent throughout Boulder County.

Canada Thistle (Breea arvense)

Canada thistle has numerous clusters of small lavender heads. It is prevalent throughout the  state, and forms dense single species stands which have little habitat value to wildlife or livestock. It spreads very quickly via both roots and seeds.

Scotch Thistle (Onopordum acanthium)

Scotch thistle is a very large (up to 8 feet tall) plant with reddish-purple or violet colored 1" to 2" diameter flowers. Leaves may be as large as 2' long and 1' wide. The plant can form an impenetrable barrier to wildlife, livestock and people.

Bull Thistle (Cirsium vulgare)

A biennial that grows two to five feet tall and reproduces by seed only. Leaves are prickly hairy above and cottony below.

Dalmatian Toadflax (Linaria genistifolia var. dalmatica)

An escaped ornamental plant similar to snapdragon, Dalmatian Toadflax rapidly takes over infested areas. Colorado State University research has shown that in 6 years this plant expanded by 1230% at a given site.  Dalmatian Toadflax is unpalatable to both wildlife and livestock.

Yellow Toadflax / Butter and Eggs (Linaria vulgaris)  

Yellow toadflax, also known as Butter and Eggs, is a deep rooted creeping perennial that invades foothills, montane and riparian habitats.  It has been mistakenly included in posters of native Colorado wildflowers. This weed is extremely difficult to control.

Houndstongue (Cynoglossum officinale)

Houndstongue has reddish-purple flowers and spreads readily by means of burrs which attach to clothing or animal fur. It is toxic, particularly to horses and cattle, preventing liver cells from reproducing.

This biennial is controllable by handpulling. Large infestations may need to be controlled with an herbicide.

Common Teasel (Dipsacus follonum)

This biennial plant is a new invader in wetlands of Colorado. It was introduced as an ornamental for dried flower arrangements and for carding wool. 

Small infestations may be controlled by handpulling. Large infestations may need to be controlled with an herbicide.

Tamarisk (Tamarix ramosissima, Tamarix parviflora)

Also known as saltcedar, tamarisk is a large shrub up to 20 feet tall. This ornamental plant has escaped cultivation and become an enormous problem in riparian areas throughout the west. It is capable of drying up an area because of the amount of water that evaporates from the plant.