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Methods of Weed Control

Weed management decisions vary according to plant life cycles, infestation size, environmental parameters and management objectives. Hand-pulling a perennial weed species such as Canada thistle or leafy spurge is a futile effort, but very effective for control of a biennial plant such as diffuse knapweed. Releasing biocontrol insects for control of dalmatian toadflax on a 500 acre property is wise, but relying on insects for controlling small patches on a 40 acre pasture is inefficient. Successful weed management requires proper plant identification, selection of effective management methods and monitoring the effects over time.

Preventive Control

Prevention is the most essential aspect of weed management. Once a noxious weed infestation becomes established, any increase in size and density creates increasingly more expensive management efforts. Awareness of weed seed sources and plant identification is a must. Feel free to call the Weed District office for help with identification or to set up a site visit, and recognize:

  • Weed seed can be spread from neighboring properties, adjacent road rights-of-way and trails. Direct sources are often livestock, manure, seed, hay, vehicles and equipment.
  • Disturbed ground is most vulnerable to weed invasion; new roads, pipelines and other sites where competitive vegetation has been removed. With no restoration (see cultural control) weeds will likely appear.
  • Early detection and rapid response saves time and money. Aggressive management action on small, newly established infestations can result in eradication. "An ounce of prevention is worth a pound of cure."

Cultural Control

Cultural control, the establishment of competitive and desired vegetation, prevents or slows down invasion by weedy species and is a key component of successful weed management. Weeds are typically opportunistic and readily invade disturbed sites. Impacts from road construction, intensive livestock grazing, densely populated prairie dog colonies and other disturbances that damage or remove desirable and competitive vegetation create sites for noxious weed invasion. Controlling weeds on such sites can be futile without vegetative restoration, as weeds will readily re-invade the disturbed area. Establishment of grassland or pasture can be challenging. Success often depends on proper species selection suitable for a particular soil type, moisture regime and growing season. Other factors such as soil compaction, seeding depth, time of year, and weed control during establishment can be critical to success. Whether seeding to native plants or pasture grasses, it's important to consult with NRCS, a commercial seeder, CSU Extension or other experts prior to investing time and money.

Chemical Control

Spray truck

Herbicide application can provide the most effective and time-efficient method of managing weeds. Numerous herbicides are available that provide effective weed control and are selective in that grasses are not injured. Along with herbicide use is user responsibility and compliance with all product label requirements for herbicide handling, use, and cleanup. Always read the label and keep in mind the label is legally binding. When using herbicides be mindful of proximity to water, trees, shrubs and other desirable vegetation. The Herbicide Reference Guide lists products commonly used for control of noxious weeds on small acreage properties. The guide provides information necessary to help decide which herbicide is most appropriate for a particular weed in a particular setting, but is not the complete label. View complete label information.

Herbicides are applied by spot spraying — single nozzle application targeting individual plants, or broadcast spraying — multiple nozzles covering an entire area. Whatever method is used, calibration of spray equipment (gallons per acre spray output) is essential for accurate delivery and mixing calculations. Estimating or guessing sprayer output can lead to misapplication which either injures non-target plants or results in failure to control the target weed species, see Sprayer Calibration. For professional application see the applicator list in this website.

Biological Control

The goal of biological control is not eradication, but the use of living agents to suppress vigor and spread of weeds. Such agents can be insects, bacteria, fungi, or grazing animals such as sheep, goats, cattle or horses. Grazing produces results similar to mowing, and bacteria and fungi are seldom available for noxious weed management. Biological control is most commonly thought of as 'insect biocontrol'.

A natural predator

USDA-APHIS (United States Department of Agriculture-Animal and Plant Health Inspection Service), is the federal agency responsible for authorizing the screening and importation of biocontrol insects. APHIS conducts intensive multi-year screening programs assuring an insect agent's host specificity (feeds only on target weed species, not other plants) prior to approval for release.

Biological weed control through insect/plant interactions is an important component of the County's weed management program. Insect agents, proven to be effective, are utilized in cases where eradication is impractical due to the vastness or inaccessibility of an infestation, and where other methods of management are not feasible. Insect agents typically require 3-5 years for establishment and can limit the spread and density of target weed species by feeding on leaves, stems, roots and/or seed heads. One must realize that eradication of a weed cannot be attained through insect biocontrol. The most effective scenario is a weed infestation reduced to a 'tolerable level', a level where the insect agents are significantly limiting distribution and abundance of the target weed species and the weed density is no longer considered detrimental to the desired plant community. Some biocontrol insects proven to be successful in Larimer County are:

  • Bindweed mites – field bindweed
  • Flea beetles – leafy spurge
  • Mecinus janthinus – dalmatian toadflax
  • Larinus minutus and Cyphocleonus achates – diffuse knapweed

For more information on insect biocontrol and to obtain insects, see Colorado Biological Pest Control Program or call the Colorado Department of Agriculture Insectory at Palisade, Colorado (866) 324-2963.

Mechanical Control

Mowing

Mechanical control consists of methods that kill or suppress weeds through physical disruption. Such methods include pulling, digging, disking, plowing and mowing. Success of various mechanical control methods is dependant on the life cycle of the target weed species.

  • Hand pulling and digging are effective on annual and biennial species such as kochia, musk thistle, and diffuse knapweed. It is important to remove the upper 2-3 inches of taproot to prevent re-growth. Hand pulling or digging a perennial weed such as leafy spurge can be a futile effort unless one has the time necessary to diligently dig or pull re-growth over several seasons.
  • Shallow tillage with a disk or sweep is effective for controlling annual species such as cheatgrass or kochia, but can actually be counter productive if trying to control perennial weeds such as Canada thistle, field bindweed, leafy spurge or Russian knapweed. Perennial root systems often have meristematic buds that can set roots and produce a new plant from root segments deposited on the soil surface. Shallow tillage of perennial weeds can result in a larger, denser and more uniform infestation than the initial patch.
  • Moldboard plowing (complete turnover of the top 10-12 inches of soil) disrupts underground root systems and buries seed from the surface to a depth too deep to germinate. This type of tillage is seldom feasible to practice on a regular basis.
  • Mowing is a suppression measure that can prevent or decrease seed head production. Mowed weeds will re-grow and set seed from a reduced height so a combined control method is necessary to be effective. Mowing causes perennial plants to weaken when forced to send up carbohydrates from underground root reserves to nourish re-growth. So mowing a perennial weed such as Canada thistle a couple of times during the summer can significantly weaken the plants, and when combined with a fall herbicide application, provides excellent control.
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