PO Box 419

Hotchkiss, CO 81419



fax: 720-880-3051



Noxious Weed Information


Noxious Weeds – What Are They and Where Did They Come From?

Noxious Weeds - What They Are

Noxious weeds are non-native invasive plants that displace desirable vegetation and degrade natural and agricultural lands. They threaten our drinking water supply, agricultural crops, pasture lands and native habitats.


Where They Come From

These plants have been transported accidentally from places as far away as Europe, Asia or Africa. Some have been brought here intentionally as ornamentals or forage crops but have then escaped.


Noxious weeds have an advantage in their new environment because the insects, diseases, and animals that would normally control them are not found here. The weeds are able to adapt to our varied climatic and environmental conditions.


Noxious weeds are spread by animals, humans, water, and wind. Noxious weeds can easily be introduced as seeds in soil, ornamental planting mixes, nursery stock, or hitch-hike on vehicles. Prime locations for noxious weeds to become established are on disturbed sites such as roadsides, land cleared for construction, range that is overused by animals or humans, wetlands, along riparian corridors and in lakes and streams.


Noxious weeds have well developed and specialized mechanisms to survive and can spread at alarming rates.

  • Leafy spurge seeds are expelled from their seed capsule and can fly up to 30 feet. Leafy spurge contains a sap that may cause blisters in the mouth of cattle and wildlife. The animals will eat desirable vegetation but will leave the Leafy spurge
  • Houndstongue seeds have tiny hooks that attach to fur and clothing.
  • Diffuse knapweed breaks off at the base and acts as tumbleweed. It is often found lodged in the underside of vehicles that have driven over the dried plants.
  • Eurasian watermilfoil can easily be transported on fishing equipment.
  • Purple loosestrife can produce 2-3 million seeds per plant every year
  • Orange hawkweed has developed hairy leaves that most animals will not eat
  • 75% of a Canada thistle plant is underground


Noxious weeds are moving into our valued ecosystems and displacing native plants at an alarming rate. When the native plants that wildlife use for food, shelter, or nesting are gone, wildlife leave the area.


Noxious weeds impact our valuable agricultural lands by competing for resources. When resources are not available for their desired use, it takes more land and resources to raise the same amount of crops and livestock.


The Colorado Noxious Weed Act

Since 1990 the state’s natural and agricultural resources have been protected by the Colorado Noxious Weed Act (35-5.5 CRS). More recent revisions to the Act enable county and city governments to implement management programs aimed at noxious weeds in order to reclaim infested acres and protect weed-free land. These changes included prioritizing the State's noxious weed list into three separate lists, A, B and C.

  • List A plants are designated for elimination on all County, State, Federal and Private lands.
  • List B includes plants whose continued spread should be stopped.
  • List C plants are selected for recommended control methods.

Watch List

In fall 2011 the state added a list of plants known to be invasive in areas near Colorado but are not know to occur here or whose distribution is not yet fully understood.


Management - Eradication, Control, Containment

Colorado's Noxious Weed Act (35-5.5 CRS) establishes a noxious weed list with prioritized management goals for the weeds on the A, B and C List s. Each noxious weed is required to be eradicated, contained or controlled.


Eradication - The elimination of all plant parts within the current growing season. When populations of noxious weeds that are not normally found in Colorado, or are only found in small areas, are discovered, they are required to be eradicated. By eliminating a noxious weed when its population is small, you save time, money, and much effort in the long run.


Control - If a noxious weed is found in substantial numbers in some parts of the state but not others, a "line in the sand" will be drawn to establish management areas. It may be feasible to eradicate small outlier populations however, in areas of higher density; the management goal may be suppression.


Containment - Some weeds are found in such large numbers that it is no longer realistic to think we will be able to rid the entire state of their presence. Instead we will aim to stop their spread.


As good stewards of our land, be it a 75 by 105-ft lot in town or an 87,000 acre ranch, we all must work to keep our lands free of noxious weeds.


Management Methods

Noxious weeds can be managed by using a combination of control methods including mechanical, cultural, biological, preventive and chemical. Different species of noxious weeds grow or spread differently so not all methods will be effective on all weeds. Colorado’s Noxious Weed Act requires that certain methods of control be used depending on the level of control that is mandated.

  • Mechanical control involves cutting, mowing, disking.
  • Cultural controls use materials or techniques that reduce noxious weed populations. Examples include mulching, rotational grazing, and establishing good vegetation cover.
  • Biological control uses organisms (insects, mites, diseases and grazing animals) which feed only on specific noxious weeds.
  • Since we are dealing with living things, a variety of circumstances come into play that impact the success of the establishment of the bio-control and ultimately the control of the noxious weed you are targeting. For example, an organism that works well in the plains may not work in the mountains. Although there has been some success on some noxious weeds, bio-control agents are not available for all species and are not allowed for use on species designated for eradication.
  • Prevention includes planting weed free seed, mulching with weed free material, cleaning machinery before moving between sites and controlling weeds prior to their setting seed.
  • Chemical control involves the use of herbicides

Above all, proper noxious weed identification, monitoring and consistent, diverse control methods are the most important steps to reducing or eradicating infestations. Remember, not all techniques will work in all situations. Refer to the Colorado Department of Agriculture for required controls. Consult with your local weed manager or Licensed Commercial/Professional Applicator for specific recommendations.


What You Can Do to Help

  • Familiarize yourself with the noxious weeds in your area
  • Work with other landowners to identify the extent of weed populations
  • Plan and implement control
  • Encourage local agencies to educate landowners about the threat and damage done by these aggressive invaders

As land users and managers, we need to protect our natural and agricultural resources. We must increase awareness and education about the adverse effects of noxious weeds. If we are diligent in our efforts, we will protect our lands for the enjoyment of future generations.


Colorado's Noxious Weed List

The Colorado Noxious Weed Act and supporting Rules establish a prioritized list of noxious weeds. For detailed information visit the State's site.

A List

List A species are invasive weeds that are either not known to occur in Colorado or are of very limited distribution and are required to be eradicated (completely eliminated).

African rue (Peganum harmala)
Camelthorn (Alhagi pseudalhagi)

Common crupina(Crupina vulgaris)

Cypress spurge (Euphorbia cyparissias)

Dyer's woad (Isatis tinctoria)

Elongated mustard (Brassica elongata)

Flowering rush (Butomus umbellatus)

Giant reed (Arundo donax)

Giant salvinia (Salvinia molesta)

Hairy willow-herb (Epilobium hirsutum)

Hydrilla (Hydrilla verticillata)

Japaneese, Giant and Bohemian knotweed (Polygonum cuspidatum, P. sachalinense and P.x bohemicum)

Meadow knapweed (Centaurea pratensis)
Mediterranean sage
(Salvia aethiopis)
(Taeniatherum caput-medusae)
Myrtle spurge (Euphorbia myrsinites)
Orange hawkweed (Hieracium aurantiacum)

Parrotfeather Myriophyllum aquaticum)
Purple loosestrife (Lythrum salicaria)
Rush skeletonweed (Chondrilla juncea)
Squarrose knapweed (Centaurea virgata)
Tansy ragwort (Senecio jacobaea)
Yellow starthistle (Centaurea solstitialis)



B List

List B species are invasive weeds with populations of varying distribution and densities within the state. The level of mandated control is based on local conditions. These weeds may require eradication within certain areas of the state.

Absinth wormwood (Artemisia absinthium)

Black henbane (Hyoscyamus niger)
Bouncingbet (Saponaria officinalis)
Bull thistle (Cirsium vulgare)
Canada thistle (Cirsium arvense)
Chinese clematis (Clematis orientalis)
Common tansy (Tanacetum vulgare)
Common teasel (Dipsacus fullonum)
Corn chamomile (Anthemis arvensis)
Cutleaf teasel (Dipsacus laciniatus)
Dalmatian toadflax, broad-leaved (Linaria dalmatica)
Dalmatian toadflax, narrow-leaved (Linaria genistifolia)
Dame's rocket (Hesperis matronalis)
Diffuse knapweed (Centaurea diffusa)
Eurasian watermilfoil (Myriophyllum spicatum)
Hoary cress (Cardaria draba)
Houndstongue (Cynoglossum officinale)
Jointed goatgrass (Aegilops cylindrica)
Leafy spurge (Euphorbia esula)
Mayweed chamomile (Anthemis cotula)
Moth mullein (Verbascum blattaria)
Musk thistle (Carduus nutans)
Oxeye daisy (Leucanthemum vulgare)
Perennial pepperweed (Lepidium latifolium)
Plumeless thistle (Carduus acanthoides)
Russian knapweed (Acroptilon repens)
Russian-olive (Elaeagnus angustifolia)
Salt cedar (Tamarix chinensis, T.parviflora, and T. ramosissima)
Scentless chamomile (Tripleurospermum perforatum)
Scotch thistle (Onopordum acanthium)
Scotch thistle (Onopordum tauricum)
Spotted knapweed (Centaurea maculosa)
Sulfur cinquefoil (Potentilla recta)
Wild caraway (Carum carvi)
Yellow nutsedge (Cyperus esculentus)
Yellow toadflax (Linaria vulgaris)


C List

List C species are widespread and common within the state. They may pose a risk to agricultural lands and may be required to be controlled.

Bulbous bluegrass (Poa bulbosa)
Chicory (Cichorium intybus)
Common burdock (Arctium minus)
Common mullein (Verbascum thapsus)
Common St. Johnswort (Hypericum perforatum)
Downy brome (Bromus tectorum)
Field bindweed (Convolvulus arvensis)
Halogeton (Halogeton glomeratus)
Johnsongrass (Sorghum halepense)
Perennial sowthistle (Sonchus arvensis)
Poison hemlock (Conium maculatum)
Puncturevine (Tribulus terrestris)

Quackgrass (Elymus repens)

Redstem filaree (Erodium cicutarium)
Velvetleaf (Abutilon theophrasti)
Wild proso millet (Panicum miliaceum)


Watch List

Watch list species are plants known to be invasive in areas near Colorado but are not know to occur here or whose distribution is not yet fully understood.



Asian mustard (Brassica tournefortii)
Baby's breath (Gypsophila paniculata)
Bathurst burr, Spiny cocklebur (Xanthium spinosum)
Brazilian egeria, Brazilian elodea (Egeria densa)
Common bugloss (Anchusa officinalis)
Common reed (Phragmites australis)
Garden Loosestrife (Lysimachia vulgaris)
Garlic mustard (Alliaria petiolata)
Himalayan blackberry (Rubus armeniacus)
Japanese blood grass/cogongrass (Imperata cylindrica)
Meadow hawkweed (Hieracium caespitosum)
Onionweed (Asphodelus fistulosus)
Purple pampas grass (Cortaderia jubata)
Scotch broom (Cytisus scoparius)
Sericea lespedeza (Lespedeza cuneata)
Swainsonpea (Sphaerophysa salsula)
Syrian beancaper (Zygophyllum fabago)
Water hyacinth (Eichhornia crassipes)
Water lettuce (Pistia stratiotes)
White bryony (Bryonia alba)
Woolly distaff thistle (Carthamus lanatus)
Yellow flag iris (Iris pseudacorus)
Yellow floatingheart (Nymphoides peltata)
Yellowtuft (Alyssum murale, Alyssum corsicum)