Noxious Weed and Invasive Species Resources

Noxious Weeds

A noxious weed is a weed that has been designated as a plant that is harmful to agricultural or horticultural crops, ecosystems, or humans or livestock. Most noxious weeds have been introduced into an ecosystem. Typically, they are plants that grow aggressively, multiply quickly without natural controls, and display adverse effects through contact or ingestion.

Impacts

Noxious weeds can have a large impact on a landscape. These plants aggressively spread and disrupt natural ecosystems by outcompeting the native plants or cultivated crops. This reduces biodiversity and reduces agricultural productivity while increasing soil erosion. Noxious weed species may also increase fire risk.

Eradication

There is no one best way to eradicate noxious weeds. Like all plants, weeds vary in how they reproduce, how they respond to herbicides, and have varying root structures. Early detection and response are important when controlling noxious weeds. See the State of Colorado's listed noxious weed species below.

List A species in Colorado are designated by the Commissioner for eradication. To view plant fact sheet, click the resources below. Examples of species include:
- Fewer than 10 known Populations in CO
  - African rue
  - Dyer’s woad
  - Elongated mustard
  - Flowering rush
  - Meadow knapweed
  - Rush skeletonweed
  - Tansy ragwort
  - Yellow starthistle
- More than 10 populations in Colorado
  - Cypress spurge
  - Giant reed
  - Knotweeds
  - Myrtle spurge
  - Purple loosetrife
  - Yellow flag iris
List B species are species for which the Commissioner, in consultation with the state noxious weed advisory committee, local governments, and other interested parties, develops and implements state noxious weed management plans designed to stop the continued spread of these species. Colorado has many List B species but some examples include:
- Absinth wormwood
- Black henbane
- Bouncingbet
- Bull thistle
- Canada thistle
- Chinese clematis
- Common tansy
- Common teasel
- Cutleaf teasel
- Dalmatian toadflax
- Dame’s rocket
- Diffuse knapweed
- Eurasian watermilfoil
- Hoary cress
- Houndstongue
- Hybrid knapweed
- Hybrid toadflax
- Jointed goatgrass
- Leafy spurge
- Mayweed chamomile
- Moth mullein
- Musk thistle
- Oxeye daisy
- Perennial pepperweed
- Plumeless thistle
- Russian knapweed
- Russian olive
- Salt cedar (tamarisk)
- Scentless chamomile
- Scotch thistle
- Spotted knapweed
- Sulfur cinquefoil
- Wild caraway
- Yellow nutsedge
- Yellow toadflax
For more information on these species, view the resources below.
List C species are species for which the Commissioner will develop and implement state noxious weed management plans designed to support the efforts of local governing bodies to facilitate more effective integrated weed management on private and public lands. The goal of such plans will not be to stop the continued spread of these species but to provide additional education, research, and biological control resources to jurisdictions that choose to require management of List C species. Examples of this include:
- Bulbous bluegrass
- Chicory
- Common burdock
- Common mullein
- Common St. Johnswort
- Cheatgrass
- Field bindweed
- Halogeton
- Johnsongrass
- Perennial sowthistle
- Poison hemlock
- Puncturevine
- Quackgrass
- Redstem filaree
- Siberian elm
- Tree of Heavan
- Velvetleaf
- Wild-proso millet
Watchlist species that have been determined to pose a potential threat to the agricultural productivity and environmental values of the lands of the state.
The Watch List is intended to serve advisory and educational purposes only. Its purpose is to encourage the identification and reporting of these species to the Commissioner in order to facilitate the collection of information to assist the Commissioner in determining which species should be designated as noxious weeds. Watch list species include:
- Baby’s breath
- Caucasian bluestem
- Common bugloss
- Common reed
- Garlic mustard
- Garden loosetrife
- Himalayan blackberry
- Hoary alyssum
- Meadow hawkweed
- Onionweed
- Perennial Sweet Pea
- Scotch broom
- Swainsonpea
- Syrian beancaper
- Tall Oatgrass
- White bryony
- Yellow bluestem
- Yellow mignonette