Noxious weeds, as defined in the Colorado Noxious Weed Act, are alien (non-native) plants that aggressively invade; are detrimental to economic crops or native plant communities; are poisonous to livestock; are carriers of detrimental insects, diseases or parasites; or have direct or indirect effects that are detrimental to the environmentally sound management of a natural or agricultural ecosystem.  Grand County has 26 of the 80+ state-listed noxious weeds.

When it comes to noxious weed management, integrating more than one method of control can increase rates of success. This holistic approach, called Integrated Weed Management, often includes a combination of chemical, biological, mechanical, preventative and cultural control.  Even when multiple management techniques are used, however, noxious weed management is not a one-time deal. Landowners must exercise patience, persistence, and perseverance when treating noxious weeds due to the evolutionary adaptations, such as high seed count and lengthy seed viability periods, that give them a competitive edge over native vegetation. 

When comparing the various means of control, not all options are suitable for all weed species due to differing life cycles and root structures. Understanding and acknowledging these differences is critical to effective noxious weed control.  Using the wrong treatment can be counterproductive and have damaging effects. Regarding life cycles, there are annuals (plants that only live one year), biennials (plants that live two years), and perennials (plants that live three or more years).  Chamomile daisy is an example of an annual; common mullein is a biennial; hoary cress is a perennial.  Regarding root structures, there are taproots, fibrous roots, and rhizomes.  Plants with fibrous roots (like cheatgrass) are easy to pull whereas taproots (like houndstongue) are difficult to pull and may need to be dug out. Rhizomes are underground stems that connect aboveground plants and produce new roots and shoots.  Rhizomes should not be pulled or dug out as damage to rhizomes can spur new growth.  Canada thistle is a prime example of a rhizomatous plant.  The photo above depicts the extent of rhizomes under a Canada thistle colony.

Chemical Control (Herbicides)

Chemical control is the use of herbicides or plant growth regulators to disrupt the growth of noxious weeds. Concerns are frequently raised regarding the potential effects of herbicides on human and environmental health; however, herbicides vary considerably in their chemistry, selectivity, application requirements and environmental characteristics. The type of herbicide as well as the timing and mode of application can impact (and minimize) health concerns. For example, two herbicides commonly used to treat noxious weeds in Grand County are Milestone (aminopyralid) and Telar (chlorsulfuron).  Both herbicides are non-restricted use; thus, when applied according to label requirements, they can be used by the general public without the need for a pesticide applicators license.  Both herbicides are also selective, meaning they primarily target specific broadleaf plants and have little effect on most grasses.  Lastly, when selected appropriately and applied according to label requirements, both herbicides are “practically nontoxic” to birds, fish, honeybees, earthworms and aquatic invertebrates. Due to their specificity and combined effectiveness on nearly every noxious weed we have in the county, a mix of Milestone and Telar is what Grand County Natural Resources provides to the public at their weekly herbicide giveaways. 

When selected appropriately and applied according to label requirements, the use of herbicides has many benefits.

·       They are highly effective at killing the whole plant, including the roots.

·       You can spot spray specific plants without impacting nearby vegetation.

·       You can spray whenever and wherever needed and can selectively respray as new growth emerges.

* Note: Organic herbicides (a mix of herbicidal vinegar, salt, and soap) are not selective and can have mixed success as well as undesirable changes in soil chemistry.  Furthermore, some herbicidal vinegars have higher toxicity levels than some common inorganic herbicides (including aminopyralid and chlorsulfuron).  Read this article for more information.

Biological Control (Insects/Fungi & Grazing)

Biological control is the use of organisms to disrupt the growth of noxious weeds. Biological agents limit the spread and density of target weed species by feeding on leaves, stems, roots and/or seed heads. 

Insects, Mites, Pathogens

The Colorado Department of Agriculture’s Palisade Insectary imports, raises, researches, and distributes biological controls (insects, mites or pathogens) for the state of Colorado. The Insectary’s Request-A-Bug service involves collection and distribution of biological controls (insects, mites or pathogens) to suppress noxious weeds or insect pests. Biocontrols can be requested by private landowners in Colorado or other government agencies. Supplies can be limited and vary from one year to the next. The Insectary has agents for purchase that target Canada Thistle, Dalmatian Toadflax, Diffuse Knapweed, Field Bindweed, Leafy Spurge, Musk Thistle, Russian Knapweed, Spotted Knapweed, Tamarisk/Salt Cedar, and Yellow Toadflax.  It is worth noting, however, that biological control via insects, mites, and pathogens has a limited success in our area due to our harsh winter conditions and lack of landscape-level infestations that support continued biocontrol populations.

Grazing

Grazing is both a biological and a mechanical means of control to emulate mowing.  It can be effective but is not a catchall “natural” alternative to be used in every situation.  Compared with targeted herbicide applications, prescribed grazing introduces additional management variables that must be considered.

MPCD does not advocate for or against grazing as a category of weed control. We advocate for selecting management practices based upon the biology of the target weed species, site conditions, measurable objectives, and long-term monitoring. When initiating grazing operations, one must consider the type of livestock used, the targeted weed species to be controlled, the other species of vegetation present, timing and duration of grazing, and other environmental factors/stressors.  If grazing is administered incorrectly, it can have long-term detrimental impacts.

Type of Livestock Used

Goats tend to be the best species of livestock for noxious weed control. They are primarily browsers and will eat a wider variety of vegetation than either cattle or sheep.  Keep in mind, as browsers, they are particularly fond of deciduous trees and shrubs, such as aspen and willows.

Targeted Weed Species to be Controlled

Not all noxious weed species are palatable to livestock.  Furthermore, some species, like houndstongue and field bindweed, are poisonous to livestock.  Palatability can also depend on the maturity of the target species.  For example, thistles are more palatable when they are young and spines are soft.  When spines mature and harden, animals are unlikely to eat the spiny leaves and stems. 

Other Species of Vegetation Present

If grazing an area that contains both weeds and desired species (grasses, forbs, shrubs, and deciduous trees), the animals may graze the desired vegetation first due to palatability.  Only once there is no ‘tasty’ vegetation left will they shift to the less palatable species (commonly the noxious weeds). Even then, they may not eat the targeted noxious weeds at all.

Timing and Duration of Grazing

When considering the timing of grazing, it is important to coordinate grazing with the right stage of plant growth and maturity.  Ideally, grazers should be introduced when they will have the greatest impact on target species but the least impact on non-target species. This also includes considerations for repeat grazing (grazing a given area again after it has rested and recovered).  Unlike some herbicides that kill the whole plant, grazing only removes the top growth and may suppress flowering or seed production in certain species under certain conditions without killing the plant. Thus, a single grazing event should not be represented as eliminating the infestation. To effectively manage noxious weeds with grazing alone, the stand of weeds must be grazed repeatedly.  If a pasture has more than noxious weeds in it, one must also account for the impact of repeat grazing on the desired vegetation.

Managing the duration of grazing is vital to preventing overgrazing.  Overgrazing is the act of excessively grazing an area to the point of vegetation and land damage.  According to Emi Kimura, agronomist with Texas A&M AgriLife Extension, “Overgrazing is the sin of all sins.” Overgrazing results in lower forage production and persistence, increased soil erosion and water runoff, depressed animal performance, and higher incidence of weed encroachment.

While high intensity mob grazing – grazing a large number of animals in a small area for a short period, followed by a long rest period for the pasture – is a commonly used approved practice in grazing systems, it must be carefully planned and managed.  A livestock owner should not ignore a pasture’s carrying capacity or the proper grazing principle of “take half leave half” just because they are practicing mob grazing. 

Other Environmental Factors/Stressors

As we all know, Mother Nature is unpredictable and unforgiving.  We can plan all we want, but at the end of the day, Mother Nature can (and will) do as she wishes.  It is for this reason that grazing must be adaptive.  Livestock managers must be willing to throw all plans away and adapt on the fly to do what is best for the land.  Take this year’s drought for example.  Not only have our lakes and rivers suffered from the lack of moisture, our native vegetation is extremely stressed as well. The following map, produced on August 9, 2026, shows the Vegetation Drought Response Index (VegDRI), a weekly depiction of drought's effects on vegetation stress across the contiguous United States.  Stressed vegetation does not recover as quickly or robustly as healthy vegetation. Thus, a livestock manager must be prepared to remove animals sooner than expected, forego grazing all together, and/or destock (sell some of their animals due to lack of sufficient grazing conditions).  

August 9, 2026, map of VegDRI, a weekly index produced by the National Drought Mitigation Center.

Mechanical Control

Mechanical control is the management of weeds through physical disruption. It may include tilling, mowing, burning, flooding, hand-pulling, hoeing, and grazing.  As discussed in the introduction, mechanical control can work for some species. For other species (ones that reproduce by rhizomes), some forms of mechanical control can have inverse effects by triggering new growth and expansion of infestations.  Species identification is essential if choosing to use mechanical control.

Preventative and Cultural Control

An ounce of prevention is worth a pound of cure!  Preventative control is about taking actions to reduce the incidence of noxious weed establishment before it happens.  The first, and most important, step to preventative control is awareness and knowledge.  One must be educated on what noxious weeds are, how to identify them, how they establish, and knowing where to turn when you do not know the answer to your questions.  At the end of this brief, we have information on the local resources for noxious weed management.  Actions one may take to prevent noxious weed establishment may include properly manage your land to prevent overgrazing, maintaining desired native/adapted species, monitoring roadways and disturbed areas for noxious weeds, detecting and identifying unknown species early on,  and aggressively managing all infestations when they are small.

Cultural control is similar to preventative control, but cultural control is more about increasing the native vegetation’s competitiveness and resistance to noxious weed establishment. It may include maintaining optimal soil health, planting density, spatial arrangement, and species selection as well as using mulch and weed barrier.  Establishing competitive and desired vegetation through seeding is one of the most effective means of cultural control.  When choosing to seed desirable species, there are many factors to consider. The Middle Park Conservation District has created two guides to help landowners increase chances of a successful seeding: Keys to Successful Seeding and Why Did My Grass Not Grow? 

Conclusion

There are a variety of strategies that one might use to mitigate noxious weed encroachment. Nevertheless, not all strategies suit all situations. As such, landowners seeking to implement a new noxious weed management plan should carefully consider the targeted weed species and the management options available.  For advice on noxious weeds and their management, Grand County Natural Resources, Summit County Weed Control Department, and Middle Park Conservation District are three local resources available in Grand and Summit Counties, Colorado. 

Printed copies of the Grand County Noxious Weed Guide can be picked up at Grand County Noxious Resources (469 E. Topaz, Granby, CO 80446). 

Printed copies of the Summit County Noxious Weed Guide can by picked up at Summit County Weed Control (37 Peak One Dr, Frisco, CO 80443 – lower floor).