The Colorado Noxious Weed Act states that noxious weed management is the responsibility of local governing agencies: incorporated municipalities, counties, and lands owned by state and federal agencies. The Act states that the board of county commissioners of each county in the state shall adopt a noxious weed management plan for all the unincorporated lands within the county. The Larimer County Noxious Weed Management Plan was approved by the Board of County Commissioners on March 6, 2008. Enforcement procedures stipulated by the management plan are the responsibility of the Land Stewardship Manager under the County's Department of Natural Resources. Larimer County, in cooperation with the Colorado Department of Agriculture, Colorado State University Cooperative Extension, and other agencies, actively promotes compliance of the Noxious Weed Act. The County will enforce with the Act's provisions, on noncompliant landowners.

  • The Larimer County Noxious Weed Management Plan was adopted in 2008 to conform to the Colorado State Noxious Weed Act, which was most recently revised on May 3, 2004.
  • The basic change that occurred in the 2004 revision of the State Weed Act was the categorization of noxious weed species into 3 lists: A, B, and C.
    • List A species require mandatory eradication by local governing agencies.
    • List B species are mandated for eradication in some parts of the state, and recommended for suppression or containment in other areas depending on distribution and densities around the state.
    • List C species are widespread and well established.
    • Watch List are species which are on this list to educate and encourage identification and reporting. They are determined by: 1. Not known to occur in the state but their noxious characteristics are found in the region. 2. They have been found to display noxious characteristics in plant communities similar to those in Colorado. 3. They have noxious characteristics as determined by the plant assessment process used by the state, yet their distribution and effect on lands in the state is still unknown.
  • The Larimer County Noxious Weed Management Plan requires eradication of all List A species and infestations of certain List B species identified as regionally scarce by the state, and containment and suppression measures for List B species identified as significantly troublesome in Larimer County. County outreach programs will emphasize identification and management of 16 List B species but reserves the right to enforce on any state-listed noxious weed if the infestation size and density deems it necessary.
  • The County Weed Plan cannot be enforced on private or public property without first applying the same measures to any land or rights-of-way owned or administered by the County that are adjacent to such properties.

The Larimer County Weed District emphasizes identification and management of the following 16 List B species, but reserves the right to enforce on any state-listed noxious weed if the infestation size and density deems it necessary. As mandated by the State Noxious Weed Act, the Larimer County Weed District must eradicate all List A species.

The following is a list of the troublesome weeds of Larimer County that the Weed District is frequently asked about, though these are not currently on the County Weed List.

List A species in Colorado that are designated by the Commissioner for eradication:

List B weed 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:

  • Absinth wormwood (Artemisia absinthiumState info
  • Black henbane (Hyoscyamus nigerState info
  • Bouncingbet (Saponaria officinalisState info
  • Bull thistle (Cirsium vulgareState info
  • Canada thistle (Cirsium arvenseState info
  • Chamomile (Chamomile spp.State info
  • Chinese clematis (Clematis orientalisState info
  • Common tansy (Tanacetum vulgareState info
  • Common teasel (Dipsacus fullonumState info
  • Dalmatian toadflax, broad-leaved (Linaria dalmaticaState info
  • Dalmatian toadflax, narrow-leaved (Linaria genistifoliaState info
  • Dame's rocket (Hesperis matronalisState info
  • Diffuse knapweed (Centaurea diffusaState info
  • Eurasian water milfoil (Myriophyllum spicatumState info
  • Hoary cress (Cardaria drabaState info
  • Houndstongue (Cynoglossum officinaleState info
  • Jointed goatgrass (Aegilops cylindricaState info
  • Leafy spurge (Euphorbia esulaState info
  • Moth mullein (Verbascum blattariaState info
  • Musk thistle (Carduus nutansState info
  • Oxeye daisy (Chrysanthemum leucanthemumState info
  • Perennial pepperweed (Lepidium latifoliumState info
  • Plumeless thistle (Carduus acanthoidesState info
  • Russian knapweed (Acroptilon repensState info
  • Russian-olive (Elaeagnus angustifoliaState info
  • Salt cedar (Tamarix chinensis, T.parviflora, and T. ramosissimaState info
  • Scotch thistle (Onopordum acanthiumState info
  • Spotted knapweed (Centaurea maculosaState info
  • Sulfur cinquefoil (Potentilla rectaState info
  • Wild caraway (Carum carviState info
  • Yellow nutsedge (Cyperus esculentusState info
  • Yellow toadflax (Linaria vulgarisState info

List C weed species are species for which the Commissioner, in consultation with the state noxious weed advisory committee, local governments, and other interested parties, 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.

Listed below are the Watch List species which are on this list to educate and encourage identification and reporting. They are determined by: 1. Not known to occur in the state but their noxious characteristics are found in the region. 2. They have been found to display noxious characteristics in plant communities similar to those in Colorado. 3. They have noxious characteristics as determined by the plant assessment process used by the state, yet their distribution and effect on lands in the state is still unknown.

Access the entire Watch List for photos and detailed descriptions of each species on the list. It is currently comprised of the following 20 species:

  • Asian mustard (Brassica tournefortiiState info
  • Baby's breath (Gypsophila paniculataState info
  • Bathurst burr/Spiney cocklebur (Xanthium spinosumState info
  • Brazilian eqeria (Egeria densaState info
  • Common bugloss (Anchusa officinalisState info
  • Common reed (Phragmites australisState info
  • Garden Loosestrife (Lysimachia vulgarisState info
  • Garlic mustard (Alliaria petiolataState info
  • Himalayan blackberry (Ribus armeniacusState info
  • Japanese blood grass/Cogongrass (Imperata cylindricalState info
  • Meadow hawkweed (Hieracium caespitosumState info
  • Onionweed (Asphodelus fistulosusState info
  • Purple pampasgrass (Cortideria jubataState info
  • Scotch broom (Cytisus scopariusState info
  • Sericea lespedeze (Lespedeza cuneataState info
  • Swainsonpea (Sphaerophysa salsulaState info
  • Syrian beancaper (Zygophyllum fabagoState info
  • Water hyacinth (Eichhomia crassipesState info
  • Water lettuce (Pistia stratiotesState info
  • White bryony (Bryonia albaState info
  • Woolly distaff thistle (Carthamus lanatusState info
  • Yellow flag iris (Iris pseudacorusState info
  • Yellow floating heart (Nymphoides peltataState info

The following is a list of plants that the Weed District is asked about quite often for management recommendations or other advice. Some of the plants listed here are native and some are not. The information that follows each is based on our experience thus far, however; through collaborative research efforts and education, we hope to provide more information on each in the future.

Black medic (Medicago lupulina) – Black medic is a non-native member of the legume family with an annual or short-lived perennial life cycle. The plants are very low-growing with small yellow flowers. In areas where control is desired black medic can be controlled by Garlon, Milestone, Quicksilver, Transline, or Vista.

Broadleaf cattail (Typha latifolia) – Broadleaf cattail is a native perennial herb that thrives in marshy areas throughout Larimer County. Cattails reproduce by seed and rhizomes, and grow to 10 feet tall. Cattails can present a problem along ditches and roadsides. Selective control can be provided by applications of Clearcast. Non-selective herbicides that provide control are Rodeo and Habitat.

Common cocklebur (Xanthium strumarium) – Common cocklebur is an annual of the sunflower family and native to the United States. Cocklebur grows to 4 feet tall, and by late summer produces spiny burs that can be a nuisance to livestock, pets and hikers. Cocklebur is a problem in farmland, run-down pastures, ditchbanks and abandoned fields. The most effective herbicides for control are Curtail, Milestone, Redeem, or dicamba + 2,4-D - product names Outlaw, Rangestar, Veteran 720, Weedmaster and others.

Dandelion (Taraxacum officinale) – Dandelion is a non-native perennial plant of the sunflower family. Dandelions produce a bright yellow flower that matures into a cottony seedhead that disperses windborne seed. Reproduction also occurs by way of shoots sprouting from the root crown. Dandelions are not typically considered a weed problem outside of turf areas, but can become a dominant plant in run-down pastures. Though palatable to livestock, dense infestations inhibit the growth of perennial grasses and other desirable species. Control of dandelions in range and pasture is best achieved by establishment of competitive grasses and applications of Curtail, Escort, Milestone, Redeem, Telar, Transline, Vista, or dicamba + 2,4-D pre-mixes of Outlaw, Rangestar, Veteran 720, Weedmaster, and others.

Foxtail barley (Hordeum jubatum) – Foxtail barley is a native perennial grass that reproduces by seed. Plants grow 1-2 feet tall and produce 2 inch long awns at maturity. Foxtail barley is tolerant of alkaline soils, and is an indicator plant of wet, poorly drained pasture areas. Such swales are often dominated by this grass which is palatable to livestock early in the season, but the awns of mature plants can be injurious to the mouth, throat, nose and eyes of livestock. There are currently no selective herbicides for controlling foxtail barley in pasture. Glyphosate (Roundup and others) can kill the plants, and would take out any desirable grasses as well. Tillage can be used to non-selectively take out infestations of foxtail barley. But even after re-seeding such areas with desirable pasture grasses, foxtail barley will eventually come back in if the soil remains poorly drained and saline conditions favor foxtail barley over other grasses.

Milkweed (Asclepias spp) – Showy milkweed is a native perennial frequently found in pastures and on roadsides in Larimer County. Plants grow to 5 feet tall, contain a milky latex, and produce large, spiny seedpods in late summer. Seed is dispersed in windborne clusters of cotton-like pappus. The densest infestations of milkweed typically occur on poorly drained sites due to the plant's tolerance of alkaline soils. Though not palatable, milkweed is toxic to livestock. In areas where control is desired, applications of Tordon or dicamba (Vanquish, Banvel, Clarity) can provide selective control.

Pigweed (Amaranthus spp) – Pigweed species most common in Larimer County are redroot, tumble and prostrate pigweed. These species are non-native annuals that germinate in May and set seed by August. Pigweed is a problem primarily in crop production, but can be found in abandoned fields and run-down pastures. The most effective herbicides for control are Milestone, Telar, Vista or dicamba + 2,4-D - product names Outlaw, Rangestar, Veteran 720, Weedmaster and others.

Poison ivy (Toxicodendron rydbergii) – Poison ivy is a native perennial vine-shrub that can be found in a variety of habitats in Larimer County. The leaflets are arranged in characteristic groups of 3 and often have reddish coloration and tend to droop. Because of the toxic residue that causes severe skin rash, poison ivy is seldom a desirable plant. Best products for control are dicamba (Banvel, Clarity, Vanquish), Garlon, Milestone, and Redeem.

Prickly lettuce (Lactuca serriola) – Prickly lettuce is a non-native biennial plant that can grow to 5 feet tall. Prickly lettuce grows from a taproot, producing small yellow flowers in mid-summer. The stems contain a milky juice. At maturity the seeds disperse in puffballs, much like a dandelion. Prickly lettuce is an invader of abandoned fields, roadsides and run-down pastures. Best control provided by applications of Escort, dicamba (Banvel, Clarity, Vanquish), Garlon, Milestone, Redeem, Vista, Telar, or Transline.

Rabbitbrush (Chrysothamnus nauseosus) – Rabbitbrush is a common name for 12 species in the western United States. The species most common in Larimer County is gray or rubber rabbitbrush. This native perennial shrub can grow to 4 feet tall, thriving on dry sites. Unpalatable to horses and cattle, rabbitbrush can dominate over-utilized pastures. Controlling rabbitbrush is difficult. Spot-spraying individual plants with a non-selective herbicide, such as Arsenal, is most effective.

Ragweed (Ambrosia artemisiifolia) – Ragweed species common in Colorado are western and giant ragweed. Western ragweed is most common in Larimer County. These plants are native annuals that produce a pollen causing hay fever. Ragweed is not highly competitive or invasive. In areas where control is desired, applications of Curtail, Milestone, Redeem, Vista, or dicamba + 2,4-D - product names Outlaw, Rangestar, Veteran 720, Weedmaster and others.

Rye (Secale cereale) – Rye is a cereal crop that has escaped into natural areas, pastures and roadsides and become a weedy species. Referred to as feral rye or volunteer rye, this grass is highly competitive and difficult to control. The life cycle of feral rye is the same as cheatgrass, winter wheat, and other species known as 'winter annuals'. This cycle begins with germination in late summer or early fall (some spring germination can occur as well), over-wintering as a green grass, and resumption of growth in the spring. Winter annuals set seed and complete their life cycle by mid-summer. Feral rye has grazing value in the spring, but the invasive character of this grass over-rides such value. If left unchecked, feral rye can dominate the plant community and is a wildfire hazard. Most effective control is Landmark or Oust (read label carefully for application within appropriate areas) or an application of glyphosate (Roundup and other brand names) at a rate and timing that is not injurious to desirable perennial grasses. Other selective herbicides are being investigated by Colorado State University and the Larimer County Weed District. Recommendations are available yet best practice would be to set up a site visit with a Weed Specialist to insure best management.

Smooth brome (Bromus inermis) – Smooth brome is a non-native perennial grass commonly utilized for hay and forage production in Larimer County. Smooth brome is a sod-forming plant that, once established, is extremely competitive and is often found in pure stands. Smooth brome's competitive ability and invasiveness has made this grass undesirable in areas where establishment of a native plant community is desired. Control is difficult and may require 2 or more applications of glyphosate (Roundup and others). In some cases, moldboard plowing, or complete turnover of the soil to disrupt the extensive underground root system is necessary, in addition to glyphosate applications.

Sunflower (Helianthus spp) – Sunflower species common in Larimer County are prairie sunflower and common sunflower. These native annuals can grow to 10 feet tall under favorable conditions. In situations where dense stands of sunflower are undesirable, control can be attained by applications of dicamba (Banvel, Clarity, Vanquish), Escort, Milestone, Redeem, Telar, or Vista.

Western salsify (Tragopogon dubius) – Western salsify is a non-native biennial of the sunflower family. The plant has long, narrow leaves and a milky juice. Western salsify produces bright yellow flowers early to mid-summer, followed by mature puffball seed heads similar in appearance to dandelions. This forb grows 1-2 feet tall and is most common on disturbed sites and neglected pastures. A vigorous stand of grass will keep this plant in check. In situations where western salsify populations are dense enough to be a problem, control can be achieved with an herbicide application of dicamba + 2,4-D - product names Outlaw, Rangestar, Veteran 720, Weedmaster and others.

White locoweed (Oxytropis sericea) – White locoweed also known as silky crazyweed, is a native perennial plant of the legume family and is common on western rangeland. White locoweed produces white flowers and contains a compound called swainsonine, which is toxic to all livestock. Locoweed can be controlled by Escort, Tordon, and Transline.

Yellow sweetclover (Melilotus officinalis) – Yellow sweetclover is a non-native plant of the legume family that grows to 6 feet tall. This alfalfa-like plant has an annual or biennial life cycle, producing a profusion of yellow flowers by mid-summer. Yellow sweetclover was introduced into Larimer County for roadside restoration, but is no longer utilized for that purpose. Occurrence appears to be affected by moisture patterns, some years sweetclover seems to be everywhere, other years not much can be found. Yellow sweetclover is not often a desirable species due to a 'stemy' growth habit, invasiveness in certain situations, and potential to cause bloat in cattle. Landowners wishing to control sweetclover should apply a tank mix of Milestone + 2,4-D, or Milestone + Escort (brand name - Opensight).

Yucca (Yucca glauca) – Yucca is a native perennial plant found on dry, sandy sites. Also referred to as soapweed yucca, these succulent plants grow to 4 feet tall with long, stiff pointed leaves. The flower pods are highly palatable to wildlife and livestock. On run-down rangeland yucca may become dense enough to be undesirable. Herbicides labeled for control is Velpar.