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Regulated Weeds

Leafy Spurge (Euphorbia esula) is a perennial that spreads by roots and seeds. The roots will grow to a depth of 15 or more feet. The seed are frequently propelled 15 feet or more from parent plant when the three celled capsule matures.
Canada Thistle (Cirsium arvense) is a perennial that spreads by roots and seeds. Canada thistle has an extensive, creeping root system. Root buds will come from depths of 3 feet or more to emerge and produce more plants for seed production. The leaves of Canada thistle are many lobed with spines on the ends of the lobes. Spines are absent on the stem.
Musk Thistle (Carduus nutans) is a biennial that can grow up to 6 feet tall and is very prolific. It spreads by seed only. Leaves are dark green, deeply lobed, and have spiny or prickly margins. Sometimes called "Nodding thistle".
Diffuse Knapweed (Centaurea diffusa) is one of the "tumble weeds". When mature the flower petals form a plug over the seed head and as it tumbles this plug comes out and the seed is dispersed.
Russian Knapweed (Acroptilon repens) is the perennial of the knapweeds. Deep rooted with numerous root buds make it very difficult to control.
Spotted Knapweed (Centaurea maculosa) Gets its name from the spots on the seed head. This plant is an aggressive invader if not kept in check.
Dalmatian Toadflax (Linaria genistifolia) This is a perennial plant that spreads by seed and root stalks. The extensive root system with waxy leaves make it hard to control.
Yellow Toadflax (Linaria vulgaris) Commonly known as "Butter and Eggs". It is an aggressive perennial. There are no natural enemies of the toadflaxes here in the US, so they tend to spread quickly.
Tamarisk spp. (Tamarix ramosissima, parviflora) Also known as saltcedar. This deep rooted perennial shrub-like tree invades riparian habitat in the Western U.S. It has small scale-like leaves and small feathery pink flowers. It establishes quickly, outcompeting native water loving plants like plains cottonwood and willow.

Larimer County's "To Watch" List

The following weeds have not been declared noxious, but are troublesome.

Perennial Pepperweed or Tall Whitetop (Lepidium latifolium) A perennial in the mustard family commonly found in riparian areas, this plant has deep-seated rootstocks that make it difficult to control.
Hoary Cress (Cardaria draba) Hoary cress is a deep-rooted perennial also in the mustard family.