Begin management now for improved Knapweed control

Kari Lewis   

             The purple flowers of spotted knapweed may not be showing quite yet, but most knapweed plants throughout the county are actively growing at this time.  ‘Skeletons’ of last year’s knapweed plants, the dried-out stem and flower seed heads, are a tell-tale sign of where knapweed grew last year.  Upon closer examination of the skeleton, the current year’s growth (rosette) can typically be identified.  Rosette leaves of spotted knapweed are indented or divided about halfway to the midrib.  Rosettes first initiate growth in mid-spring, plants then bolt (or produce their flowering stem) in early summer and by mid-summer the purple blooms are typically seen. 

Controlling knapweed while in the rosette stage is much
more effective than waiting until flowers appear.  Photo by Kari Lewis.

                Spotted knapweed is a Montana noxious weed that greatly decreases land productivity, forage production, and property values.  Now, while knapweed is in the rosette stage, is the time to begin spotted knapweed control.

                Regular and careful hand pulling can control spotted knapweed.  Knapweed regrowth occurs from the crowns, so the entire crown portion of the plant must be removed.  Plants can be pulled most effectively when the soil is moist.  Plants should be bagged and disposed of in a manner to prevent seed dispersal which would exacerbate the spread of knapweed.  Knapweed plants produce from 500 to 4,000 seeds per plant and seeds can be viable for up to eight years, so proper disposal of plants is critical. 

                Mowing is most effective in areas where there are healthy perennial grasses that will respond to mowing with renewed growth.  If the plant community is dominated by forbs or annual grasses, mowing may open up the plant community to further weed infestations and increase knapweed density.  Mowing early in the season can prevent seed production and lower the plant’s carbohydrate reserves.

'Skeleton's from last year's knpaweed
plants are a tell-tale sign of where to find
the current year's rosettes. 
Photo by Kari Lewis.

                Herbicides labeled for spotted or diffuse knapweed application during the rosette stage include those with the following active ingredients: Aminopyralid (Milestone), Clopyralid + 2,4D (Curtail), Picloram (Tordon 22K), or Triclopyr + clopyralid (Redeem R&P).  The herbicide 2,4-D can be applied at the early stage of bolting.  The MSU Extension publication, ‘Biology, Ecology, and Management of Montana Knapweeds’ includes herbicide application rates as well.

                Another tool in the toolbox for controlling knapweed includes biocontrols.  Biocontrols are insects that naturally feed on the knapweed plants, decreasing the plant density, plant height, and over time, plant population.  Biocontrols work best in large, dense infestations.  The Glacier County Extension Office will again be coordinating a bulk order of biocontrols.  Orders for the Larinus minutus/obtusus (Knapweed Seed head weevil) will be due June 7 ($50/carton) and orders for the Cyphocleonus Achates (Knapweed Root Weevil) will be due July 26 ($90/carton).  Shipping will be free to the Glacier County Extension Office and with the purchase of five cartons of a biocontrol species, there will be one free carton added.  Order forms are available at the Glacier County Extension webpage or by contacting the Glacier County Extension Office at (406) 873-2239.