Cirque Meadows by Adam Johnson


This following is an archive of articles that various members of the Weed District staff have submitted to publications over the years that may contain outdated information. Since these articles may have some outdated information, best practice is to always contact the Weed District office for the most current, cost-effective solutions to your land management needs. Again these articles are for reference only and may lack information that has since been gleaned through research and trials.

Got Weeds? We Can Help
Noxious Weeds and the Law
Leafy Spurge - A Perennial Problem
Ornamental Weeds
Mediterranean Sage in Larimer County
Myrtle Spurge - Ornamental Garden Plant or Noxious Weed?
What the Heck is a Phreatophyte?
Don't Move A Mussel.....or A Milfoil
Yellow Starthistle Saga
Canada Thistle

Got Weeds? We Can Help

You're out for a walk one evening and you notice something..."What is that?" You wonder. Soon you see more and you articulate your thoughts to your friend. "What is that? That plant with the spiny yellow flowers?"

Your friend looks at it and says "it looks like some kind of thistle but I've never seen one with yellow flowers." Curious about what it might be, you find yourself searching web pages and plant identification books to find a match. You think you've found it but still aren't sure. You ask some neighbors and they acknowledge seeing it too but no one is sure what it is. Finally, you decide to get to the bottom of it and call the Larimer County Weed District. Upon setting up a FREE site visit, you meet with a Weed Specialist who not only identifies the mystery plant as yellow starthistle but also several other weeds and native plants that are on your property. The specialist offers best management practices to control if not eradicate the noxious weeds, in the least time consuming and effective ways.

The Larimer County Weed Control District is a government agency that provides services to the taxpayers of Larimer County. We have specialists with several years of experience working in land management and weed control to offer everyone advice on best practices based on the area and type of land. We work with you to ensure that your weed issues are resolved, always keeping in mind what your ultimate goals are for the property.

At the Weed District, we encourage and promote education. The more people we can educate about noxious weeds the less environmental, wildlife, and economic damage the weeds will do. Our specialists are always willing to speak to groups such as classrooms, HOAs, clubs or neighborhood get-togethers. We want to promote our vision of a more educated public to sufficiently reduce the time and effort that goes into recovery from an infestation of noxious weeds. We can't do it alone and always encourage new ideas and public input.

Including free site visits and best management advice from experts, we also offer several services. We have cost-share reimbursements that cover noxious weed management like mowing reimbursement at a rate per acre, and discounts on herbicides. Most of all, we offer our commitment to the citizens of Larimer County to educate, exchange ideas, remain sensitive to everyone's personal beliefs, and promote safety. Education is the key to managing this daunting task and we look forward to working with you. For herbicide sales, site visits or any other information please explore the links within this site. For further information or to find out how you can help out, call us at 970-498-5768 or visit our link "Doing Your Part."

By Chad Clark, Larimer County Weed District


Noxious Weeds and the Law

Noxious weed infestations threaten wildlife habitat, agriculture, and recreational opportunities. Since 1990, the Colorado Department of Agriculture has protected the state's natural resources by enforcing regulations controlling noxious weeds.

What are noxious weeds?

The term "noxious" has a legal connotation with the Colorado Department of Agriculture - non-native and invasive. Most noxious weed species in Colorado originated in Europe and/or Asia, and were unintentionally introduced into the United States as a contaminant in crop seed or on farm machinery. Some were intentionally introduced as ornamental plants, forage, or plants used as wind breaks or for soil stabilization. These non-native plants have proliferated or have the potential to proliferate in Colorado due to:

  • The country or region of origin has a climate, soil type, and/or elevation similar to that of Colorado.
  • Introduced species lack the natural checks that kept them in balance in their native range such as:
    • Competition from plants they co-evolved with.
    • Browsing by large native herbivores they co-evolved with (Colorado's native herbivores such as deer and elk seldom feed on weedy plants).
    • Insect predation.
    • Soil nematodes, diseases, and other factors.

This threat to Colorado's environment and economy has been recognized and addressed through the Colorado Noxious Weed Act. This Act, among other things, requires all local governing agencies to develop weed management plans for their jurisdictions. The Act has listed 71 weed species as noxious in Colorado. The most recent revision of the Act went into effect May 3, 2004. The basic change that occurred was categorization of 71 noxious weed species into 3 lists:

  • List A - 18 species not well established in Colorado.
  • List B - 39 species common enough in parts of Colorado that eradication is not feasible.
  • List C - 14 species widespread and well established in Colorado.

List A species are potentially a large problem to this state, and require mandatory eradication by local governing agencies. Of the 18 List A species, only 9 are known to currently exist in Colorado and infestations are small enough that eradication is feasible. The other 9 species are problems in neighboring states and introduction into Colorado is likely. To prevent establishment, the Act's required management objective is eradication, meaning prevention of production of seed or other propagules (reproductive vegetative parts). Prescribed techniques for management of List A species are hand pulling, digging, or herbicide application. Mowing, grazing, and insect bio-control are not acceptable forms of management for these species.

List B species are common enough in parts of the state that eradication is not feasible, though the species are still recommended for eradication, suppression, or containment depending on distribution and densities around the state.

List C species are widespread and well established. Control of List C species is recommended but not required by the state, although local governing bodies may require management.

All local governing agencies (cities and counties) are required to have a weed management plan, and that plan must have, at a minimum, the List A species. Listing of B and C species is discretionary, depending on local distribution and density. Enforcement on List B and C species can only occur if the species is listed on the weed plan of a local agency or included in the Rules and Administration of the State Weed Law. The required management objective for List B species is eradication, containment and/or suppression, meaning prevention of dispersal of seed or other plant propagules. In most cases this is accomplished by mowing; though hand pulling, mechanical tillage, grazing, or herbicide application can also be effective. The goal of management plans for List C species is not to stop the continued spread but to provide additional education, research, and biological control resources to jurisdictions that choose to manage to require management.

General information on the State's Noxious Weed Management Program can be accessed at:

By Kelly Uhing, State Weed Coordinator
Colorado Department of Agriculture
700 Kipling St., Suite 4000
Lakewood, CO 80215


Leafy Spurge - A Perennial Problem

Leafy spurge (Euphorbia esula) is likely the worst noxious weed problem in Ft. Collins and much of Larimer County. This perennial invader has taken over many acres along the Poudre River from the middle section of the Poudre Canyon to the Weld County line and beyond. It is commonly found along the North Fork of the Poudre from Livermore to Seaman Reservoir, and is a widespread problem from Bellvue through Rist Canyon, and on a multitude of pastures in and around LaPorte and north of Ft. Collins.

Leafy spurge was first reported in Larimer County in a pasture near LaPorte in the early 1960's. Unfortunately it was not considered a problem at the time, was not controlled, and has since spread to thousands of surrounding acres. Leafy spurge reproduces by spreading roots, and seed spread by animals and floating along irrigation ditches and rivers. The plant displays bright yellow bracts and flowers from April through June, and can easily be identified by the milky latex found when breaking a stem or leaf.

Native to Europe and western Asia, leafy spurge was introduced into the western U.S as a seed contaminate in the late 1800's. This noxious weed now infests more than 5 million acres in the U.S. and Canada drastically reducing rangeland productivity, native plant diversity, wildlife habitat and land values. Leafy spurge is on the State and Larimer County noxious weed lists, meaning residents are obligated to control this invasive species. Control measures in Larimer County, at a minimum, require mowing to prevent seed dispersal. Other control measures include sheep or goat grazing, insect bio-control, and herbicide application. The best way to suppress leafy spurge, or any weed, is to properly manage range or pasture. Disturbances, particularly excessive grazing, reduce the competitive ability of grasses and open the door for weed invasion.

How do we get a handle on controlling this invasive species? Education is a good start. Residents need to learn how to identify noxious weed species and realize how costly they can be. Weed seed moves in many different ways - wind, water, vehicles, bird droppings, in the fur and manure of livestock and wildlife, and in hay grown in infested pastures. When residents understand their weed problem becomes their neighbor's weed problem and threatens our natural areas, they will more likely take responsibility for managing their weeds.

With a cooperative effort from private landowners, public land managers, ditch companies and others we can start to control leafy spurge where it exists and prevent further spread in Larimer County. For information on noxious weed identification and management, site visits, educational presentations, and enforcement procedures contact the Larimer County Weed District @ 970-498-5768.

By Tim D'Amato


Ornamental Weeds

For those who enjoy learning new oxymoron terms, here's one for you - "ornamental weeds". The term describes noxious weed species that originated as garden plants then jumped fences to invade forests, river systems, wetlands, pastures, rangeland, and roadsides. Many of the weed problems confronting Colorado land managers originated from backyard flower gardens. How did this come about? Over the course of decades, the plant nursery industry has selected ornamental plants from all over the world for various characteristics. A showy, attractive flower obviously tops the list, but other characteristics desirable in the western U.S. were also selected for: drought tolerance, soil stabilization, and food and cover for wildlife. In most cases this has not been a problem. Landowners are encouraged to plant xeric species to reduce water needs, and to provide food and cover for wildlife.

Unfortunately many of the ornamental plants introduced to the U.S. from other parts of the world have adapted too well. Without environmental checks from native lands such as predatory insects, soil nematodes, disease and other factors, some plant species have become invasive and detrimental to our natural areas. A few examples are:

  • Yellow toadflax, better known as butter-and-eggs. This perennial of the snap dragon family has invaded many natural areas near mountain communities. In the Flat Tops Wilderness near Meeker, Colorado, yellow toadflax is a major weed infesting thousands of acres.
  • Purple loosestrife, a perennial plant that thrives in riparian areas displacing native plants and wildlife. Purple loosestrife dominates millions of acres of marshland in eastern and Midwestern states, but in Colorado, is uncommon enough that diligent management efforts could eventually result in eradication.
  • Russian olive, a thorny tree that readily spreads by seed. Russian olives were at one time promoted for wind breaks by government agencies and were widely planted as ornamental trees. Many river bottoms and other moist sites are now dominated by dense thickets of Russian olives.

These plants and others determined to be invasive are now on the prohibited sales list for plant nurseries in Colorado. Many ornamental plants are on the State Noxious Weed list categorized as List A species that require eradication: myrtle spurge, cypress spurge, dyer's woad, orange hawkweed, and purple loosestrife. Others categorized as List B species may require management depending on local weed ordinances: absinth wormwood, bouncingbet, chamomile, Chinese clematis, dalmatian toadflax, dames rocket, oxeye daisy, Russian olive, saltcedar, and yellow toadflax.

Be a responsible gardener and remove any of these species if you have them on your property. For further information contact the Larimer County Weed District office at 970-498-5768 or Colorado State University Extension office at 970-498-6003.

By Tim D'Amato


Mediterranean Sage in Larimer County

Mediterranean sage (Salvia aethiopis L.), a plant native to the Mediterranean region, Southeastern Europe and North Africa, was introduced into this country as an ornamental garden plant. Like many other exotic garden plants, Mediterranean sage has jumped borders and become an aggressive weed that has crowded out other plants and taken over pastures and rangeland. This biennial (lives two seasons) of the mint family emerges in the fall or spring, and forms a rosette (flattened wide leaves). At this early growth stage Mediterranean sage is often confused with common mullein, but it's very hairy leaves emit a strong sage-like smell when pinched, a good identification characteristic. In the late spring a central stem grows out from the rosette, becomes branched and produces white flowers that become seed heads by late June. The central stem breaks from the plant by mid-summer and becomes a tumble-weed that disperses seed as it blows across pastures. Mediterranean sage is unpalatable to grazing animals and has become an invasive rangeland weed that infests over 1.5 million acres in California, Idaho, Oregon, and Washington.

In Colorado, Mediterranean sage is found only in Boulder, Garfield and Larimer Counties. Because of the plant's potential to take over large areas, the Colorado Department of Agriculture has listed the species for eradication through state designation as a 'List A' weed (for detailed information contact the Colorado Department of Agriculture, Conservation Services Division, Local government agencies are directed by the state to carry out eradication efforts. Effective management of Mediterranean sage consists of digging or pulling (removing at least the top 3 inches of taproot) or applying an herbicide before seed is produced. The Larimer County Weed District, with grant money made available by the state, provides herbicide applications free of charge.

Larimer County has an estimated 250 acres of privately owned pasture land west of Berthoud infested with Mediterranean sage. Land owners say the weed has been around that part of the county for at least 15 years, probably introduced into the area with contaminated hay. This area is bounded by County Roads 4 and 8 on the south and north, and County Roads 23 and 27 on the east and west. Aggressive management efforts in Larimer County over that past several years have contained and suppressed infestations of Mediterranean sage, but this plant persists due to seed longevity and wide range dispersal of seed by way of the tumbleweed nature of plants at maturity. If you think your property contains Mediterranean sage and would like to request a site visit and/or herbicide application, contact the Larimer County Weed District at 970-498-5768.


Myrtle Spurge - Ornamental Garden Plant or Noxious Weed?

Myrtle spurge (Euphorbia myrsinites) is a garden plant long used as a xeriscape (drought tolerant) species in flower beds and rock gardens throughout Colorado and other western states. Myrtle spurge produces bright yellow, flower-like bracts very early in the spring, an attractive aspect of this showy ornamental. The blue-green leaves are very fleshy, aligned in an alternate arrangement along trailing stems, which gives the plant its other name, donkey-tail spurge.

So what's the problem with this showy, drought-tolerant garden plant? Myrtle spurge has escaped from city flower beds to become an invasive weed on many open space areas in the state, particularly in mountain counties. Like other escaped ornamentals which are now on the state weed list such as butter-and-eggs and purple loosestrife, this highly competitive plant has the potential to become an expensive, tough-to-control weed in Colorado's back country. Additionally, myrtle spurge produces a caustic, milky latex within its stems and leaves that, when exposed to the skin, can cause severe irritation, or temporary blindness if in contact with an eye.

The Colorado Department of Agriculture placed myrtle spurge on the state's List A category, meaning this plant is prohibited from being sold at nurseries and garden shops, and is designated for eradication. Eradication of a weedy species requires pulling, digging, or applying an herbicide to completely kill the plant, as opposed to suppression methods such as mowing. For complete information on the State Noxious Weed Act, noxious weed lists, and fact sheets, contact the Colorado Department of Agriculture:

Because myrtle spurge has become so well established in city and suburban flower gardens, enforcement of state and local weed laws has been somewhat lenient. Hopefully education on the invasiveness and impacts of myrtle spurge will encourage landowners to comply with local weed law. Most management activity has been hand pulling. Myrtle spurge is a perennial that produces a simple taproot, unlike perennials such as Canada thistle that produce an extensive underground root system. So pulling most of the root from the ground will effectively control the plant. Keep in mind the sap from the plant is caustic and precautions must be taken to protect skin and eyes. If seed has been produced it will be necessary to bag the pulled plants to prevent seed dispersal. A number of herbicides are effective for controlling myrtle spurge. For further information on identification and management, or to set up a site visit, please call 970-498-5768.


What the Heck is a Phreatophyte?

If you aren't a crossword puzzle aficionado or a botanical geek, chances are you haven't heard the term 'phreatophyte'. The term is defined as a deep-rooted plant that obtains water from a permanent ground supply or from the water table. In most cases phreatophytes are beneficial, providing wildlife habitat and stabilizing riverbanks.

Locally, cottonwood trees, boxelders and willows serve that function and are desirable species in riparian ecosystems.

Other phreatophytes have been introduced into the western United States that have become invasive plant problems. Two such species are tamarisk, or saltcedar (Tamarix spp.), and Russian olive trees (Elaeagnus angustifolia). These non-native species were brought into the western U.S. to serve as windbreaks, stabilize streambanks, and for ornamental plantings. Unfortunately, tamarisk and Russian olives have invaded river systems, displacing native flora and in many cases forming solid stands. Less plant diversity means less wildlife diversity and denser stands of trees utilize greater quantities of water.

Because of the cost to Colorado's environment and economy, the Colorado Department of Agriculture has placed both species on the state noxious weed list. These trees cannot be sold by plant nurseries in the state and citizens are encouraged or required to remove them from their property. In the case of tamarisk, most local governing agencies have listed the species as one requiring removal. Tamarisk is on the Larimer County noxious weed list which means landowners with tamarisk are sent notification letters from the Weed District informing them of their obligation to remove the tree(s). Tamarisk has become a major problem on the Arkansas and Colorado River systems. Fortunately on the South Platte River drainage, including the Poudre River, tamarisk is not common enough to be an insurmountable problem, and with landowner cooperation, could be eradicated in the foreseeable future. One only has to take a drive along the Arkansas River near Rocky Ford, Colorado and view the vast, dense thickets of tamarisk to realize what could happen along the Poudre River over time if no management action is taken.

Because of the ubiquitous occurrence of Russian olives in cities and rural areas, no governing agencies are requiring removal. The best management strategy is education and awareness. Landowners need to realize that birds can transport seed several miles to riparian areas where these trees flourish. River bottoms dominated by Russian olives support fewer native species, and exotic birds like starlings are favored. After becoming aware of the detrimental effects of Russian olives, property owners need do the responsible thing by removing these trees from their landscapes.

For more information on phreatophytes, or identification and management of any noxious weed species, call 970-498-5768.

By Tim D'Amato
Land Stewardship Manager
Larimer County Dept. of Natural Resources


Don't Move A Mussel.....or A Milfoil

Aquatic nuisance species (ANS), non-native invasive plants and animals threatening our aquatic ecosystems, are potentially a huge threat to the ecology and economy of many western states. Eurasian watermilfoil and zebra/quagga mussels, well established in other states, are just beginning to be identified in Colorado. These invasive species:

  • degrade ecosystems by altering habitat and diminishing food supplies that desirable plants and animals need to survive.
  • entangle swimmers and boats in dense mats
  • ruin boat engines and jam steering equipment
  • clog water distribution systems of drinking water plants, power plants, canals, and industrial processes increasing the cost of maintenance and ultimately increasing the end user fees
  • affect human health by accumulating toxins.

Eurasian watermilfoil is a feathery submerged aquatic weed on the Colorado Department of Agriculture Noxious Weed Act List B category. Though not widespread in Colorado yet, infestations have been identified in several Front Range counties. Contact the Colorado Department of Agriculture, Conservation Services Division, Noxious Weed Management Program for more information (

Zebra mussels are invasive freshwater bivalve mollusks, with light and dark stripes, and are the only freshwater mollusks that will attach to hard surfaces in North America. For years they have been held east of the 100th Meridian. Lake Powell's surveys turned up a Quagga mussel population in 2006, and a lake in southern California discovered zebra mussels earlier this year. Zebra/Quagga mussels have now hitchhiked their way into Pueblo Reservoir, Colorado, primarily by attaching themselves to boats like yours. For more information on Zebra/Quagga mussels and how to prevent their spread, please visit or

Once aquatic nuisance species are in your waters, they are nearly impossible to get rid of. Colorado Division of Wildlife has written an Aquatic Nuisance Species Management Plan and a Rapid Response Plan. State and County reservoir parks will be inspecting high risk vessels.

What you can do to prevent the spread of Eurasian watermilfoil and zebra/quagga mussels:

  • INSPECT boats, trailers, boots, gear, waders, and equipment.
  • REMOVE any mud, plants, fish or animals.
  • DRAIN water from boat, motor, bilge, live wells, and bait containers before leaving the water access area.
  • CLEAN and DRY anything that comes in contact with the water.
    • Zebra Mussel Hotline: 1-877-STOPANS
    • Colorado Department of Wildlife, ANS coordinator: 303-291-7362



Houndstongue (Cynoglossum officinale) is a biennial forb of the Boraginaceae family. This non-native plant is thought to have been accidentally introduced into the United States from Eurasia as a contaminant in seed. Houndstongue presents a problem

photo of weed
Houndstongue seeds stuck on a shoe
in rangeland and pasture areas due to it's toxicity to livestock. The plant contains alkaloids that can be lethal, especially to horses. The seeds are a nuisance as well, with Velcro-like protrusions, houndstongue seed clings aggressively to clothing and fur of pets, livestock and wildlife.

Houndstongue has been designated a noxious weed species by the Colorado State Noxious Weed Act and is categorized as a List B species, defined as "common enough in parts of the state that eradication is not feasible, though still recommended for eradication, suppression, or containment depending on distribution and density". This species is under proposal to be added to the Larimer County weed list, which would mean property owners will be required to prevent the dispersal of seed, or at a minimum keep this weed mowed.

Typical of biennial plants, houndstongue emerges as a rosette in the first season then sends up a flowering stalk the second season. Mature plants can be up to four feet tall with reddish purple flowers. Reproduction is by seed only. Management methods include:

  • Mechanical removal as long as 2-3 inches of the taproot is severed.
  • Herbicide applications of dicamba, 2,4-D, Escort, Telar, Plateau, or Tordon.
  • An established, competitive stand of desirable vegetation minimizes invasion by houndstongue or any other noxious weed species.

Targeted grazing for management of houndstongue is not an option due to toxicity. There are currently no insect biocontrol agents approved for management of houndstongue in the United States.

For further information on houndstongue or any other noxious weed species, call the Larimer County Land Stewardship office to schedule a site visit for identification and management recommendations - 970-498-5768.


Yellow Starthistle Saga

Out west of Berthoud grows a plant with sharp spine and winged stems, that will make a horse sick (chewing disease) and Californians cry (20 million acres infested)... Brought in with alfalfa seed from Chile in the 1850s to California, yellow starthistle has spread to 40 of the 48 contiguous states. In the state of Colorado there are 48+ acres, found in 7 counties. Heavy equipment or hay transported it west of Berthoud and now invades about 5 acres.

Yellow starthistle is potentially Colorado's worst weed. Though less than 100 acres in the state are currently infested, this invasive species could rapidly spread to rangeland and natural areas if aggressive steps are not taken. One only has to see the thousands of acres of solid yellow starthistle infestation in the Hell's Canyon area on the Idaho/Oregon border to see how invasive this plant is in areas with climatic conditions similar to Colorado. Recognizing this major threat to the environment and economy of Colorado, the Colorado Department of Agriculture's Noxious Weed Act categorized yellow starthistle as a List A species in 2003. The designation requires eradication of the species, meaning no plants can produce seed for up to 15 years. Larimer County Weed District, in collaboration with the Colorado Department of Agriculture and effected landowners, has been actively managing it with an integrated approach since 2004.

photo of weed
Yellow starthistle plant near Berthoud

Yellow starthistle is an annual in the sunflower family. In June and after every rainstorm the plants emerge into a rosette with deeply lobed leaves. The plants produce branched stems about 2 feet tall, that are winged and covered with cottony hairs. It has a disk only flower that is bright yellow, with 1-2 inches straw-colored spines below, creating a star-like appearance on the end of short branches. Under dry conditions it will grow a couple inches tall with a solitary flower. Within weeks the plant is producing seed. After the seeds have been dispersed cotton tufts remain over winter.

During the growing season, crews wielding shovels scout the 5 acre area removing plants and bagging them if a flower is present. If an area has higher concentrations then crews spot spray the plants with herbicides. To prevent yellow starthistle from spreading, landowners need to be educated on proper identification and manage rangelands and pastures to be vigorous and competitive against invasive plants.

For more information on yellow starthistle or any noxious weed, management, site visits or educational presentations, call (970)498-5768 or visit If you suspect this plant is growing on your property, call Larimer County Weed District immediately at (970)498-5768.

Maxine Guill, Weed Specialist


Canada Thistle

photo of weed
Figure 1: Canada thistle root box used
to demonstrate the effects 1 plant can have.

Canada thistle is the most common noxious weed problem in Colorado and most states in the Rocky Mountain region. This exotic species was first introduced from Eurasia to the United States during Colonial times, 400 years ago, and has since spread to 42 U.S. states and the Canadian provinces. It is a detriment to the environment and economy of Colorado, negatively affecting agriculture and natural areas. See CSU Extension for comprehensive information.

photo of weed
Figure 2: The extensive root system of one
Canada thistle plant grown over approximately
14 months in a root box.

Canada thistle is a common problem on roadsides, rangeland, pastures, farmland and natural areas in Larimer County. This aggressive perennial can develop an incredibly extensive root system in a short amount of time (figure 2.). The underground rhizomes aggressively compete for moisture and nutrients feeding above ground growth that can out-compete many plants for sunlight. Canada thistle is an expensive weed problem in corn, barley, wheat and other local crops, significantly reducing farm production.

Extensive stands of Canada thistle in natural areas reduce native plant abundance and diversity, and in doing so reduce wildlife habitat. Aesthetically, this noxious weed is a detriment to the beauty of our natural areas, often seen as the dominant plant along drainages.

Canada thistle can be controlled by diligent land managers and private landowners but persistence is required. A combination of management efforts can remove the majority of a Canada thistle infestation in 2-3 years, but subsequent follow-up spot treatments may be necessary for several years. Best management recommendations:

photo of weed
Canada thistle buds
  • Cultural control - establishment of competitive desirable vegetation is the bottom line for management of any noxious weed. Weeds do best when invading disturbed sites, so by maintaining a vigorous stand of competing vegetation, weed problems are minimized. Grasses provide best competition because of their tolerance to mowing and most herbicides used for control of Canada thistle. If grasses are not present in a problem area, then seeding is necessary. If grasses are present at all, the densities should increase as Canada thistle decreases.
  • Mowing/grazing - removal of Canada thistle above-ground plant tissue will stimulate re-growth which reduces carbohydrate reserves within the root system and weakens the plant. Canada thistle is palatable to livestock until the plants begin to dry down. Grazing needs to be monitored for adverse effects on desirable grasses, otherwise this management tool becomes counter-productive. Mowing Canada thistle at the early flower stage maximizes stress on the plant's carbohydrate reserves. Mowing/grazing alone will not control Canada thistle, but used in conjunction with a fall herbicide application, provides excellent control.
  • Herbicide application - provides the most effective control of any perennial weed, particularly when applied in the fall following summer mowing/grazing. The most beneficial time to apply is September or October, prior to a hard frost. The most effective herbicides for controlling Canada thistle:
    • Milestone - provides excellent control, has low toxicity, and can be applied to water's edge.
    • Transline and pre-mixes with Transline - Redeem and Curtail. These products have all shown excellent results for Canada thistle control in University field trials.
    • photo of weed
      Canada thistle flowers
    • Tordon is very effective for control of Canada thistle but is a restricted use product requiring certification for purchase and use.
    • Telar alone provides fair control, but when tank-mixed with any of the above products produces excellent results.
    • Do not use Roundup or any other glyphosate products unless treating individual Canada thistle plants. Glyphosate is non-selective and can kill or injure surrounding grass or other desirable vegetation.

Other management methods:

  • Hand pulling/clipping - not effective for controlling Canada thistle infestations unless one is extremely persistent (see root box photos). Above ground growth is the "tip of the iceberg" and hand pulling, at least on larger areas, is seldom feasible.
  • Insect bio-control - not effective. Experts with Colorado Department of Agriculture, Colorado State University and USDA do not recommend insect releases for control of Canada thistle even though insect agents are available.

photo of weed
Figure 6: The Canada thistle infestation in this barley field is about to get worse.


Background Image: Cirque Meadows by Adam Johnson. All rights reserved.