A variety of tools are needed to control spotted knapweed depending on the size and density of the infestation as well as additional environmental or economic factors that are site specific.
The first step is to assess the knapweed infestations on the land that you manage. Survey the knapweed and determine the location, approximate size, and approximate density of each infestation. Make general notes of the surrounding vegetation and area including the proximity of bodies of water to the infestation. This information will guide which control method is best suited to the situation.
This is the ideal time to control knapweed. If the infestation is very small, consider hand pulling (wear gloves) or spot spray with a herbicide. Contact your University of Minnesota Regional Extension Educator for management recommendations. Both hand pulling and spraying require follow up for at least 3 years because seedlings continue to emerge from the seedbank.
Consider integrating control methods for medium to large infestations.
Biological control using insects
Biological control is a cost-effective, long-term, sustainable choice for medium to large infestations, although it takes many years before an acceptable level of control is achieved. Minnesota has an active biological control program. The prime time to implement biological control in Minnesota is from early July through mid-August. More information about spotted knapweed.
Goats and sheep grazing
Goats and sheep can consume spotted knapweed. Many factors such as grazing frequency and timing, breed, and herd management affect the success of grazing. Williams and Prather (2006) determined that goat grazing of spotted knapweed at the bud to bloom stage over a three year period reduced plant cover, plant density, and seedhead production. Spotted knapweed seedling, rosette, and mature plant densities decreased with repeated summer sheep grazing of 1-7 days in June, July, and September for 3 years (Olson et al. 1997). Goats are consuming invasive forbs including spotted knapweed at Minnesota's Itasca State Park while helping to grind recently spread native grass seed into the ground (T. Robertson 2007). For more information on using targeted grazing to control spotted knapweed and other invasive plants, go to Weed Control Methods - Grazing or Targeted Grazing - An Interactive Guide for Land Managers
Herbicides can be a valuable tool for knapweed management. Spraying large infestations may not be cost effective, but using herbicides to contain large infestations and prevent them from spreading could be very effective. Some general information on herbicide use for knapweed control is listed below. Contact your University of Minnesota Regional Extension Educator or County Agricultural Inspector for specific recommendations.
Timing the herbicide application is very important. Often knapweed plants are recognized and treated after flowering which is too late in the season. The plants may produce seed before they die from the herbicide. Sheley et al. (2000) determined that spraying knapweed in the bolt (rosette sends up stalk) and bud (flower buds formed, but not opened) stage was the most effective. The general time period for these target growth stages is June in Minnesota. Hahn and Stachowski (2006) found that at 140 days after treatment with aminopyralid applied at a rate of 4 fl oz/acre, spotted knapweed was 50% controlled in a grass pasture. In contrast, Holden et al. (2007) demonstrated greater than 97% control with 2,4-D ester at 4 pt/acre, clopyralid plus 2,4-D at 2 pt/acre, aminopyralid at 5 oz/acre, and aminopyralid plus 2,4-D at 2 pt/acre. Concern for non-target broadleaf plants and environmental sensitivity (such as proximity to water) of the site may influence herbicide choice.
Herbicide treatments will require years of follow up. Knapweed seeds germinate throughout the growing season so seedlings may emerge after herbicides dissipate or leach into the soil (Jacobs and Sheley 1998). MacDonald et al. (2007) found no long-term decrease in knapweed density, biomass, or dominance with a single herbicide application.
Prescribed burning is a management technique of deliberately using controlled ignition techniques to burn a specific area as opposed to a non-contained wildfire mentioned previously in the "Habitat" section of this document. In the northern Midwest, annual prescribed burning can reduce knapweed infestations. Burn timing and frequency are important considerations. Abella and MacDonald (2000) suggest that fire can reduce seed germination and MacDonald et al. (2001) that spring burns could decrease seedling recruitment. At a site in Michigan, Emery and Gross (2005) found that annual summer burns for 3 years were consistently effective at reducing the spotted knapweed growth rate. In this study, spring burns were effective one year at reducing the percentage of flowering knapweed plants, but were not consistently effective. Fall burns were not effective at reducing knapweed (Emery and Gross 2005). In contrast, MacDonald et al. (2007) found annual spring burns for 3 years reduced knapweed populations and increased the growth of native warm-season grasses at a site in Michigan.
The advantages of integrating burning with other control methods depend upon the specific integrated practices. Biological control can be integrated with spring burning. At 6 sites in Minnesota and Wisconsin, populations of seedhead weevils, Larinus minutus and L. obtusus, root weevils, Cyphocleonus achates, and root boring moths, Agapeta zoegana, did not decrease after a spring burn (W. Oehmichen, personal communication, 2008). Preliminary results of this study indicate that one species of seedhead fly, Urophora affinis, survived burning, but U. quadrifasciata may have been negatively impacted by burning. Integrating summer burning with biocontrol has not been evaluated, but would be expected to negatively impact biological control populations. Rice (2005) describes a Montana study by Carpenter that compared treating knapweed infested grasslands with a backing fire followed by herbicide application to treating with herbicide alone. Knapweed control was not improved with the combined fire and herbicide treatment compared to herbicide alone.
Plant Protection Division
Minnesota Department of Agriculture (MDA), 625 Robert Street N, St. Paul, MN 55155-2538, email@example.com