Working together to strengthen and support noxious weed management efforts in Montana.

Diffuse Knapweed

Many Kinds of Knapweed Found in Montana

Friday, September 2nd, 2011


The September MSU monthly weed post focuses on knapweeds.  Did you know that there are eight kinds of knapweed found in Montana?   Dr. Jane Mangold’s publication this month takes the flower of each of the knapweeds and teaches us how to tell the difference.

Download this two page publication today.   Learn how to tell spotted knapweed from brown knapweed.

MWCA has pages devoted to identification of the knapweeds  listed on the Montana Noxious Weed list:  Russian Knapweed, Spotted Knapweed, Diffuse Knapweed and Yellow Starthistle.

Ecology and Management of Diffuse Knapweed

Friday, August 5th, 2011


By Jim Jacobs, Invasive Species Specialist, NRCS, Bozeman, Montana and Sharlene Sing, Assistant Research Professor, Montana State University, Bozeman, Montana


A close relative of spotted knapweed in the Asteraceae taxonomic family, diffuse knapweed is typically biennial, reproducing exclusively by seed. It forms a rosette with a central crown and tap root in the juvenile stage and a single upright stem one to three feet (0.3 to 0.9 m) tall with numerous spreading branches at maturity. White (occasionally purple) flowers are borne in heads with spiny bracts (see Figure 1). It is generally found on more arid sites than spotted knapweed. Native to the Mediterranean region, diffuse knapweed was first recorded in Montana from Mineral County in 1951 and by 2008 had been reported from 39 of Montana’s 56 counties. Its dense, spiny overstory reduces the availability of desirable forage plants to livestock and wildlife, and grass production can be reduced by over 90% in heavy infestations.

Declines in diffuse knapweed populations in western North America have been attributed to the lesser knapweed flower weevil (Larinus minutus), one of 14 approved biological control insects it is host to. Herbicidal management can best be achieved by application of any of the following chemicals at the rosette to bolt stages: aminopyralid, clopyralid, dicamba, picloram, and 2,4-D. Grazing management using sheep or goats should be applied during the rosette and bolt stages before the formation of spiny flowerheads, and animals should be removed when 50% of grass forage has been utilized. Tillage can be used to remove plants and release the seed bank. Persistent hand pulling and grubbing may be practical on small scale infestations. Diffuse knapweed re-sprouts following fire, including intense wildfire. Cultivating and re-vegetating competitive plants will improve the longevity of control applications and reduce re-establishment from the seed bank.

Download and read the complete publication.

2010 Statewide Biological Control Monitoring Report

Thursday, October 21st, 2010


APHIS-PPQ completes a second season of the statewide biological control monitoring program in cooperation with Montana Dept. of Ag. and the BLM. Over the past two years, this project has been helping counties, state and federal agencies, and Indian Reservations with monitoring past biological control releases and assessing the potential for collectibility when they find established populations.  Download your copy of  the report for the work completed in 2010.

Impact of Knapweed on MT Economy

Tuesday, July 23rd, 1996


Steven A Hirsch and Jay A Leitch

The economic impact of thee invasive, exotic weeds–diffuse, spotted, and Russian knapweed (Centaurea diffusa, C. maculosa, and Acroptilon repens)–on Montana’s economy was estimated using a procedure developed for another invasive weed species. Published data and that from a survey of county weed boards were used to estimate direct negative impacts of over $14 million annually due to infestation of over 2 million acres of rangeland and wildland. This amounts to about $10.63 on each infested grazing land acrea nd $3.95 on each infested wildland acre. Direct plus secondary economic impacts, estimated using an input-output model, are about $42 million annually,, which could support over 500 jobs in the state’s economy. This first approximation suggests the knapweed infestation problem in Montana deserves attention, although more work could be done to refine these estimates and to allow estimation of he impacts at sub-state levels.

Link to Full Article