Herbicidal Control

Herbicides are effective for short-term control of diffuse knapweed. Table 2 lists herbicides, rates, soil half-life, and eco-toxicity for chemicals commonly used to control diffuse knapweed. The length of control (i.e., the time the population regenerates from seeds in the soil) will depend on the soil residual activity of the herbicide, soil texture, precipitation, and the competitiveness of the plant community. Picloram applied at one pint product per acre (0.25 pounds active ingredient/acre) is the standard recommendation for herbicidal control of diffuse knapweed and can provide 90 percent population reduction for three or more years on loamy soils with a well-maintained grassland community. However, picloram is a restricted-use herbicide and cannot be applied near surface water or where there is a high water table. It is water soluble, mobile, and will leach quickly from the rooting zone in sandy soils. Picloram also breaks down in sunlight which reduces its residual activity. Because of the residual activity, timing of picloram application is not as critical as with other herbicides with less residual activity (indicated by half- life in Table 2). Spring, early summer, and fall applications result in the greatest control. Application during the hot and dry part of the summer should be avoided because uptake into the plant is limited when plants are dormant and the active ingredient breaks down rapidly in the sun.

An economical alternative to picloram is 2,4-D, a broadleaf-selective herbicide, which can be applied to environmentally-sensitive areas or where the use of picloram is prohibited. The timing of 2,4-D application is important to maximizing control because this herbicide has brief residual soil activity (see Table 2). To have the greatest reduction of diffuse knapweed populations, 2,4-D should be applied after most of the seeds have germinated and before plants flower, generally at the late bud stage but before flowers appear. This timing will target early summer rosettes, prevent the transition from rosette to flowering plant, and eliminate seed

Note: Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a violation of Federal law to use herbicides in a manner inconsistent with their labeling.

production. Repeated annual applications of 2,4-D may be necessary to maintain control of stands effectively regenerating from the soil seed bank. However, re-application will depend on the degree of seedling suppression by competitive plants.

Other herbicides available for control of diffuse knapweed in sensitive areas include products that contain dicamba, clopyralid, or triclopyr. These herbicides are more expensive than 2,4-D but clopyralid and triclopyr do not injure as many non-target forb species as 2,4-D, which makes them an attractive option where preserving the forb community is important to management objectives. These chemicals have residual activity greater than 2,4-D but less than that of picloram (see Table 2). Timing of application for these herbicides for optimum control of diffuse knapweed is similar to the timing recommended for 2,4-D.

Aminopyralid is a recently developed chemical that is effective for controlling diffuse knapweed. Its chemistry is very similar to that of picloram, clopyralid, and triclopyr. The mobility of aminopyralid in the soil is very high, however because of low application rates and short soil half-life, most of the chemical remains within the upper 12 inches of the soil profile. Aminopyralid can be used to treat ditch banks and other channels that do not carry water used for irrigation or drinking.

There are risks associated with herbicidal control of diffuse knapweed that should be balanced against the resulting benefits, and the risks of alternative management actions, including the no action alternative, when making weed management decisions. Potential risks include adverse effects on human and environmental health, and economic costs.

The risks to human and environmental health can be minimized by strict adherence to application requirements defined on the herbicide label. The relative risks of chemicals can be compared by their eco-toxicity evaluations (see Table 2). Generally, the chemicals listed pose a low risk to human and environmental health. However, the degree of risk is dependent on dose and frequency of exposure. Frequency of exposure can be minimized by integrating non-herbicidal population regulation to reduce the number of herbicide re-applications. This will also reduce economic risk. Maintaining competitive grasses can reduce the number of herbicide applications needed to suppress diffuse knapweed, thereby reducing environmental risk.

< Back to Ecology and Management of Diffuse Knapweed (Centaurea diffusa Lam.)

Last Modified: 08/21/2008