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Larimer County Weed List

The following 16 weed species are categorized as List B weeds with Colorado Department of Agriculture, and are considered significantly troublesome in Larimer County. These weeds are emphasized in identification and management outreach programs and require management by landowners in the county. For the most part, compliance with county weed law requires prevention of the dispersal of seed, or at a minimum, mowing. The exceptions on the Larimer County list are bull thistle, Scotch thistle and spotted knapweed which are uncommon enough that eradication is possible. County Weed Law requires management methods that prevent seed production of these weeds: pulling, digging, or herbicide application. For effective management of any noxious weed remember:

  • Weeds are typically opportunistic and readily invade disturbed sites.
  • Cultural control, the establishment of desirable and competitive vegetation, prevents or slows down invasion by non-native species and is an essential component of successful noxious weed management.
  • Prevention of new infestations through identification and on-the-spot eradication (early detection and rapid response) saves substantial time and expense.

Descriptions, management recommendations and methods of control are listed below.

Canada thistle (Cirsium arvense)

photo of weed
Canada thistle flower

Canada thistle is a deep-rooted perennial that reproduces by seed or underground rootstalks. Seed can remain viable in soil for up to 20 years. Canada thistle originated in Eurasia, was introduced to the U.S. 300 years ago, and has become the most common weed problem in the Western United States.

photo of weed
Canada thistle rosettes

The most effective herbicides for control of Canada thistle are Perspective, Milestone, Curtail, Redeem, Transline, Tordon, and Telar. Fall applications, prior to a hard frost, are rated best for control but any of these listed herbicides applied in the spring or summer provides good control. Because of the extensive underground root system, Canada thistle is very resilient and eradication requires several seasons of management.

Insect agents are available that feed on Canada thistle but have not been proven effective.

photo of weed
A small stand of Canada thistle
Canada thistle is palatable to livestock, and grazing can provide suppression through spring and summer.

Shallow tillage (disk, sweep) on a deep-rooted perennial can be counter-productive, creating a denser, more uniform stand. Deeper tillage such as moldboard plowing, if feasible, can provide 1-2 years control.

Mowing effectively suppresses Canada thistle, and in combination with a fall herbicide application provides best control. To read more about Canada thistle see the article "Canada Thistle."

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Musk thistle (Carduus nutans)

photo of weed
A close-up of the musk thistle flower

Musk thistle is a tap-rooted biennial that reproduces only by seed. Plants typically germinate in late summer or early fall, over-winter as a rosette, bolt and flower in the spring or early summer, disperse seed in mid-summer. Seed longevity is variable but can last up to 10 years. Mature plants are 2-8 feet tall. Musk thistle is fairly common in Larimer County.

photo of weed
Musk thistle rosettes

Since musk thistle reproduces only by seed, management methods should be applied prior to seed set.

The most effective herbicides for control of musk thistle are Perspective, Milestone, Curtail, Redeem, Transline, Tordon, and Telar. Apply anytime from rosette stage to early flower. Applications after the early flower stage are not beneficial since the plant can produce viable seed while dying down.

A seed head weevil, Rhinocyllus conicus, released for control of musk thistle, is well dispersed throughout Colorado and further releases are unnecessary. The impact from this weevil has been cyclic, some years very effective, other years not. Other insect predators are available, but have only been marginally effective.

Because population levels are low in Larimer County, required management methods are those that result in eradication like herbicide application, digging or pulling. Grazing, mowing and insect biocontrol are not acceptable forms of treatment in Larimer County. When digging or hand pulling it is essential to dig up at least 3-4 inches of the tap-root to prevent re-growth.

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Bull thistle (Cirsium vulgare)

photo of weed
Bull thistle flower

Bull thistle is a tap-rooted biennial that reproduces only by seed. Plants typically germinate in late summer or early fall, over-winter as a rosette, bolt and flower in the spring or early summer, disperse seed in mid-summer. Seed longevity is variable but can last up to 10 years. Mature plants are 1.5 - 6 feet tall. Bull thistle is not common in Larimer County but appears to be spreading.

photo of weed
Bull thistle rosette

Since bull thistle reproduces only by seed, management methods should be applied prior to seed set.

The most effective herbicides for control of bull thistle are Perspective, Milestone, Curtail, Redeem, Transline, Tordon, and Telar. Apply anytime from rosette stage to early flowering. Applications after the early flower stage are not beneficial since the plant can produce viable seed while dying down.

No effective insect biocontrol agents are available for bull thistle.

Because population levels are low in Larimer County, required management methods are those that result in eradication like herbicide application, digging or pulling. Grazing, mowing and insect biocontrol are not acceptable forms of treatment in Larimer County. When digging or hand pulling it is essential to dig up at least 3-4 inches of the tap-root to prevent re-growth.

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Scotch thistle (Onopordum acanthium, Onopordum tauricum)

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A mature Scotch thistle plant
basking in the sun
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A dense Scotch thistle stand showing
their height compared to a grown male

Scotch thistle is a tap-rooted biennial that reproduces only by seed. Plants typically germinate in late summer or early fall, over-winter as a rosette, bolt and flower in the spring or early summer, disperse seed in mid-summer. Seed longevity is variable but can last up to 10 years. Mature plants are 2-10 feet tall and extremely thorny. Scotch thistle is not common in Larimer County but appears to be spreading. Because population levels are low in Larimer County, required management methods are those that result in eradication like herbicide application, digging or pulling. Grazing, mowing and insect biocontrol are not acceptable forms of treatment in Larimer County. When digging or hand pulling it is essential to dig up at least 3-4 inches of the tap-root to prevent re-growth.

The most effective herbicides for control of Scotch thistle are Perspective, Milestone, Curtail, Redeem, Transline, Tordon, and Telar. Apply anytime from rosette stage to early flowering. Applications after the early flower stage are not beneficial since the plant can produce viable seed while dying down.

No effective insect biocontrol agents are available for Scotch thistle.

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Hoary alyssum (Berteroa incana)

photo of weed
Hoary alyssum invades a field near Estes Park

Hoary alyssum is an annual, biennial or short-lived perennial plant that grows to 2 feet tall and produces white flowers. This member of the mustard family is commonly found along roadsides and disturbed areas of range and pasture. Hoary alyssum is a common weed in the Estes Valley, Red Feather Lakes and Loveland areas of Larimer County and is toxic to horses.

The most effective herbicides for control of hoary alyssum are Escort and Telar.

No insect biocontrol agents are available for hoary alyssum.

Digging and hand pulling can provide effective control if persistent.

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Hoary cress (Cardaria draba) & Perennial pepperweed (Lepidium latifolium)


Photo taken in Utah demonstrates how important weed issues are
in this case the infestation is hoary cress.
photo of weed
Perennial pepperweed

Hoary Cress and perennial pepperweed are perennial plants of the mustard family. These species are problem weeds in rangeland, pasture and roadsides. Perennial pepperweed is generally more common on moist sites. Hoary cress greens up and blooms quite early.

The most effective herbicides for controlling these perennial mustards are Escort, Perspective, Plateau/Panoramic, and Telar. Herbicide application needs to occur at flowering for most effective results. Mustard species are frequently referenced in the herbicide guide because of this plant family's unique tolerance to numerous herbicides used for managing noxious weeds.

No insect biocontrol agents are available for control of these species. Mustards are generally not palatable to livestock.

Hand pulling and digging are generally not effective for control of these perennial weeds unless persistent. Mowing can provide suppression and reduced seed production.

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Dalmatian toadflax (Linaria dalmatica, Linaria genistifolia)

photo of weed
Dalmatian toadflax flower

A member of the snapdragon family that was introduced to this country from Eurasia as an ornamental plant. Dalmatian toadflax is termed an 'escaped ornamental', a former garden species that has become an invasive weed problem in rangeland and natural areas.

photo of weed
Dalmatian toadflax spreading up and over a hill
Dalmatian toadflax is particularly abundant on the shallow, rocky soils along Colorado's front-range. This perennial plant reproduces from seed and vegetative root buds. Seeds can remain viable in the soil for at least 10 years. Plants grow to 3 feet tall and produce yellow flowers throughout the summer.

The most effective herbicides for control of Dalmatian toadflax are Telar, Tordon and Plateau. Fall applications provide best control.

The most effective insect for suppression of dalmatian toadflax is Mecinus janthinus, a stem-boring weevil (not effective on yellow toadflax). Other toadflax-feeding insects are available, though only marginally effective. For more detailed information on infestation size and density to warrant an insect release, and for obtaining insects go to Colorado Biological Pest Control Program .

Mowing, hand pulling, or digging can reduce seed production and stress plants, but this perennial will readily grow back.

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Yellow toadflax (Linaria vulgaris)

photo of weed
Yellow toadflax flowers

Better known as butter-and-eggs, this close relative to Dalmatian toadflax has become a significant problem in mountain communities. Yellow toadflax is an escaped ornamental plant that is generally more common than Dalmatian toadflax at higher elevations and cooler temperatures.

photo of weed

Once used as an ornamental, yellow toadflax is
aesthetically pleasing but multiplies quickly
and can crowd out native vegetation.

This perennial plant reproduces from seed and vegetative root buds. Seeds can remain viable in the soil for at least 10 years. Plants grow 1-2 feet tall and produce yellow flowers with an orange center. Prior to flowering, the narrow leaves and overall size of yellow toadflax is similar to leafy spurge.

If in doubt, remember yellow toadflax lacks the milky latex present on leafy spurge stems and leaves found upon breaking or tearing these plant parts.

The most effective herbicides for control of yellow toadflax are Telar, Tordon, a tank mix of Telar & Tordon, or a tank mix of Tordon & Overdrive.

There are no recommended insect biocontrol agents for control of yellow toadflax.

Mowing, hand pulling, or digging can reduce seed production and stress plants, but this perennial will readily grow back.

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Diffuse knapweed (Centaurea diffusa)

photo of weed
A close-up of the diffuse knapweed flower

Diffuse knapweed is a biennial plant and sometimes a short-lived perennial that germinates in fall or early spring, over-winters as a rosette, and bolts and flowers in the summer. Flowers are typically white, sometimes pink, with spiny bracts beneath. After flowering, diffuse knapweed sets seed in late summer or fall, after which the plant breaks off at the base and becomes a tumbleweed spreading seed as far as the wind will carry the plant. Diffuse knapweed reproduces only by seed, and the seed can remain viable in the soil for 15 years.

photo of weed
Diffuse knapweed rosettes

The most effective herbicides for controlling diffuse knapweed are Tordon, Transline, Curtail and Redeem. Milestone is effective for controlling plants at the rosette stage.

Numerous insects are available for biological control of diffuse knapweed and can provide fair to good results. The most effective have been a seed head weevil Larinus minutus and root weevils Cyphocleonus achates and Sphenoptera jugoslavica. For more detailed information on infestation size and density to warrant an insect release, and for obtaining insects go to Colorado Biological Pest Control Program .

Mowing and grazing can suppress populations, but plants will still flower and set seed from a reduced height. Hand pulling and digging, when feasible, is very effective. If hand pulling or digging plants that have flowered, be sure to bag and dispose the plants to prevent seed dispersal.

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Spotted knapweed (Centaurea maculosa)

photo of weed
A close-up of the spotted knapweed flower

Spotted knapweed is a short-lived perennial plant that reproduces only by seed. Plants grow to 3 feet tall and produce pink to purplish flowers with prominent black spots on the bracts beneath. Spotted knapweed is closely related to diffuse knapweed but is more prevalent at higher elevations. This noxious weed is not common in Larimer County but has the potential to thrive here much as it has in other western states and Canadian provinces. Montana alone reports infestations covering an estimated 5 million acres.

photo of weed
Spotted knapweed

Because spotted knapweed is highly invasive and population levels are currently low in Larimer County, required management methods are those that can result in eradication. Insect biocontrol, mowing, and grazing are not acceptable forms of management. Required methods are herbicide application, digging or hand pulling.

The most effective herbicides for controlling spotted knapweed are Tordon, Transline, Curtail and Redeem. Milestone is effective for controlling plants at the rosette stage.

Spotted knapweed reproduces only by seed, so mechanical control methods should be conducted prior to seed production. Hand pulling and digging are effective if the top 2-4 inches of the tap-root is removed. If hand pulling or digging plants that have flowered, be sure to bag and dispose the plants to prevent seed dispersal.

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Russian knapweed (Acroptilon repens)

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Russian knapweed flowers

Russian knapweed is a perennial that reproduces from seed and vegetative root buds. Russian knapweed grows to 3 feet tall and produces pink to lavender flowers with papery bracts beneath.

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Russian knapweed plants
beginning to take over this field.
Seed remains viable in the soil for 3 years. Russian knapweed eliminates competing vegetation by exuding an allelopathic chemical that inhibits growth of other plants and can result in solid stands. This noxious weed is toxic to horses and unpalatable to other livestock.

The most effective herbicides for controlling Russian knapweed are Tordon, Transline, Milestone, Curtail, Redeem, Telar, and Plateau.

There are currently no insect biocontrol agents available for control of Russian knapweed.

Mowing can reduce seed production and stress the plants, and combined with an herbicide application in the fall, provides excellent control.

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Leafy spurge (Euphorbia esula)

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A close-up of leafy spurge flowers

Leafy spurge is a deep-rooted perennial that reproduces by rootstalks and seed. Leafy spurge emerges in early spring, flowers in April/May. Plants grow to 3 feet tall with yellow-green flowers that, when mature, can propel seeds 15 feet. Seeds can remain viable for 8 years. The entire plant contains a milky latex that can be toxic to livestock and irritating to humans. Leafy spurge is the most difficult-to-control weed in Larimer County.

photo of weed
Leafy spurge spreading as far as the eye can see...

Paramount, tank mixed with Overdrive provides effective control of leafy spurge without injuring grass or trees. Other effective herbicides for control of leafy spurge are Perspective, Tordon, Tordon/2,4-D, dicamba, and Plateau (fall applied). Tordon, Perspective and Plateau can be very effective, but the higher application rates necessary for good control can cause injury to grasses or trees.

Several species of flea beetle (Apthona spp.) can be effective in significantly reducing stands of leafy spurge. Results are often site-specific, varying according to soil type, moisture conditions, and other factors. Leafy spurge is not palatable to horses or cattle and the latex in the plant can cause mouth blistering. Sheep and goats are immune to the blistering and can be trained to browse leafy spurge and provide suppression through the growing season.

Flea beetles feed on the leaves of leafy spurge in the summer, but most plant damage occurs after the insects have laid eggs and the resulting larvae are underground feeding on the roots. Mowing, grazing, and herbicide application are only compatible with insect bio-control if these management efforts occur in the late summer or fall after the insects have laid their eggs and newly hatched larvae are underground.

Mowing and grazing are effective tools for depleting root reserves. Hand pulling or digging can reduce seed production and stress plants, but this perennial will readily grow back. For more on leafy spurge see the article "Leafy Spurge - A Perennial Problem."

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Houndstongue (Cynoglossum officinale)

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A mature houndstongue plant

Houndstongue is a tap-rooted biennial that reproduces by seed only. Houndstongue germinates in summer/fall, over-winters as a rosette, bolts and flowers in spring/summer, sets seed in the fall. Plants grow to 4 feet tall with reddish-purple flowers. The velcro-like seeds are a nuisance, attaching tightly to clothing and animals. Houndstongue contains alkaloid compounds that are toxic to livestock.

photo of weed
Houndstongue seeds ready to stick
to anything that passes by

The most effective herbicides for controlling houndstongue are dicamba, Escort, Telar, 2,4-D, Plateau, and Tordon.

No insect biocontrol agents are currently available in Colorado.

Digging or hand pulling houndstongue is effective if before seed set. Flowering plants should be bagged and disposed. Be sure to remove the top 2-3 inches of tap-root to prevent re-growth. For more information about houndstongue see the article "Houndstongue."

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Common teasel (Dipsacus fullonum)

photo of weed
Common teasel flowers

Common teasel is a tap-rooted biennial that reproduces only by seed. Plants typically germinate in late summer or early fall, over-winter as a rosette, bolt and flower in the spring or early summer. Common teasel thrives on moist sites and can dominate wetlands. Though not yet common in Larimer County, common teasel is a significant weed problem on riparian areas of Boulder and Jefferson Counties.

Numerous herbicides are available that effectively control teasel: Milestone, Curtail, Redeem, 2,4-D, dicamba, Escort, Telar, Plateau, Tordon, and Transline. Check label restrictions for applications near water.

No insect biocontrol agents are currently available for control of common teasel.

Mowing can reduce seed production but plants generally re-flower and set seed from a reduced height. Hand pulling or digging is very effective prior to seed set.

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Tamarisk or saltcedar (Tamarix ramosissima)

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A tamarisk flower

Tamarisk is a deciduous shrub or small tree introduced into this country for erosion control, windbreaks and as an ornamental. Tamarisk is a phreatophyte, (see the article "What the Heck is a Phreatophyte?") or moisture dependant plant that is invasive in riparian areas. This noxious species eliminates competition by exuding salts and creating a saline soil unfavorable to other vegetation. Tamarisk reproduces by seed, and is a prolific seed producer, but seed longevity is just a few months.

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Tamarisk along a local reservior

Due to this plants proximity to water, appropriate herbicide choices are limited to those that have an aquatic label or can be applied to water's edge. Arsenal is the most commonly used herbicide for foliar application, if overspray has potential to go into water use Habitat - same active ingredient with an aquatic label. Larger trees can be controlled by cut-stump treatments - cutting the tree at the base and applying an herbicide over the stump to prevent re-growth. Garlon or Habitat are frequently used for stump application.

A leaf-feeding beetle, Diorhabda elongata, was released in Colorado in 2005, with promising results. Diorhabda beetles have significantly impacted the dense infestations of tamarisk in western Colorado and Utah. In Larimer County tamarisk is found in scattered stands near reservoirs and ditches, in densities too small to support diorhabda beetle life cycle and movement, making releases in this area impractical. Insect biocontrol is not an appropriate tool for tamarisk management when infestation levels are low enough that eradication is possible by other means.

Mechanical treatments such as cutting, bulldozing, and fire are only temporarily effective. Sprouting is a problem and an herbicide application is necessary to prevent subsequent re-growth.

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Background Image: Rocky Mountain National Park by Sue Burke. All rights reserved.