Spotted and Diffuse Knapweed (Centaurea maculosa, C. diffusa) Aster Family / Thistle Subfamily By Thomas J. Elpel with additions by Pamela G. Sherman
About Spotted and Diffuse Knapweed: There are about 500 species of Centaurea around the world, including the harmless annual corn flower or bachelor's button (C. cyanus). Spotted knapweed (C. maculosa) first arrived in this country in about 1893 and quickly spread across both western states and eastern states. By 1920 the plant was well established in several infestations near Missoula, Montana, and now it has spread to 5 million acres just in this state. Spotted knapweed covers an additional 2.5 million acres in other states. Diffuse knapweed (C. diffusa) was first reported in Washington state in 1907. Today the plant has spread to more than 3.21 million acres across Washington, Idaho, Oregon, Montana, and British Columbia. Both Diffuse and Spottted knapweed are typically annual, sometimes biennial or short-lived perennial, thus they reproduce mostly by seed.
Spotted knapweed, though, can also resprout from roots.
Identification: To help identify Diffuse knapweed, look at the shape of the plant, the shape of the buds, the bracts beneath, and the color of the flowers. The buds are shaped like urns, the plant is shaped like a ball, and the flowers are typically white, sometimes pink, with rigid spines on bracts beneath. Diffuse knapweed flowers and sets seed in late summer or fall. It then breaks off at the base and becomes a tumbleweed, spreading seed far and wide.
Spotted knapweed, on the other hand, has pink to purplish flowers with prominent black spots on the bracts beneath. No sharp spines as on its closely related cousin, Diffuse. Spotted is more prevalent at higher elevations.
Spotted knapweed generates between 400 and 25,000 seeds per plant. It is not known how often stock and wildlife eat knapweed seed heads, but tests indicated that 11% of the seeds eaten by mule deer and 4% of the seeds eaten by sheep pass through their digestive systems, with a viability of up to 22%. Viability was near zero for seeds that took longer than two days to pass. The experiments were carried out by feeding mule deer and sheep capsules of spotted knapweed seeds.
Spotted knapweed has a reputation for exuding a poison that kills other plants nearby. Infested lands may consist of nothing but acres and acres of knapweed with seemingly lifeless soil beneath it. I've even seen infested plots where purple slime molds oozed out of the ground, further contributing to the eerie aura of a dying landscape.
Research at the University of Montana in Missoula indicates that spotted knapweed kills other plants not by poisoning them, but by draining them of nutrition. Knapweed forms a symbiotic association with a particular fungus in the soil. The fungus steals carbon away from nearby grasses, reducing their ability to compete. Grasses that evolved with knapweed were able to adapt and survive, but our native plants did not have the opportunity to develop the defensive mechanisms. Durar hard fescue is one such plant from Eurasia that can be seeded to out-compete knapweed, but it should only be introduced where native species have already been replaced with other exotics.
Medicinal: Knapweed is both bitter and astringent.
Prevention: Motorized vehicles are the greatest contributor to the spread of knapweed. Nearly 2,000 seeds may be caught underneath a vehicle after driving through one knapweed patch. Up to 10% of those seeds can still be stuck there ten miles down the road. Off-road vehicles are especially notorious for spreading knapweed up and down trail systems. If you drive through a patch of knapweed it is essential that you at least stop at a carwash and thoroughly spray clean the underside of your vehicle.
Knapweeds love disturbed soils, so they are often the first to move in when new subdivisions are cut out of native rangeland. Any kind of road construction also contributes to its spread, especially when road dirt is moved from one location to another. Legislation to require weed-free fill dirt along highways was killed in the 1999 session of the Montana legislature. The cost of acquiring weed-free fill dirt is unknown.
Mechanical Controls: Pulling knapweed by hand is effective on very small patches, but it requires persistence over a number of years to exhaust the seeds in the soil. The patch may even get bigger the second year, as over-looked seedlings from the first year mature into full-sized plants. Even one missed plant can produce a thousand or more seeds to regenerate the seedbank, so real diligence is required. I find it is helpful to pound in stakes around the site to help identify it in subsequent years.
A rumor has been circulating about a forest ranger who reportedly developed a form of cancer in his fingers after a long day of pulling knapweed by hand. According to the rumor, the plants contain properties that promote cell division. The plant juice reportedly entered cuts in his hands and led to the amputation of two fingers. Research suggests that there is little truth to the rumor, but you can still wear gloves, just to be on the safe side.
Repeated mowings only encourage knapweed to bloom low to the ground, but one well-timed mowing when the plants are in the bud to early flower stage does reduce seed production by 79-99%.
Fire: Burning knapweed does little to control it, and it opens up more bare ground for the plants to colonize. However, burning away old dead stands of knapweed can help reduce the amount of herbicide required to kill the living plants that sprout up afterwards.
Biological Controls: A dozen insect species have been introduced in the U.S. to control spotted and diffuse knapweeds, with varying results. The larvae of several insects feed on the roots or root crowns of the knapweeds, stealing energy from the plants and making them susceptible to fungal or bacterial infections. These root-boring insects include a weevil (Cyphocleonus achates), a yellow moth (Agapeta zoegana), a brown moth (Pterolonche inspersa) and the European buprestid beetle (Sphenoptera jugoslavica). The latter two only attack diffuse knapweed.
Most promising so far are the seedhead insects. The larvae of these insects eat the developing knapweed seedheads, usually destroying all of the seeds in the seedhead. The plants often produce swollen galls around the damaged area, further depleting its own energy reserves. Knapweed seed production has been cut in half where two species of gall flies (Urophora affinis and U. quadrifasciata) have been released together. These are the most widely distributed biocontrol agents for knapweed. Seed production has been reduced even more where the gall flies were released together with the seedhead moth (Metzneria paucipunctella).
The larvae of the seedhead flies (Chaetorellia acrolophi and Terellia virens) and the seedhead weevils (Bangasternus fausti, Larinus obtusus, and L. minutus) also destroy knapweed seeds. Adult Larinus minutus feed on leaves and their larvae feed on seeds. Be sure to check the flowerheads of the knapweed in your area for the presence of seedhead larvae; some of these insects may be at work there already.
The weevil seed-head feeder, Larinus minutus, and the weevil root-feeder mentioned above, Cyphocleonus achates appear to rapidly reduce Centaurea diffusa populations.
Different stakeholders have different economic interests and environmental concerns. Ranchers and farmers want knapweeds eradicated. But the success of these weevils has alarmed beekeepers throughout N. America, for whom knapweeds are crucial plants for the honey bee industry. In parts of N. America, therefore, there is ongoing discussion about knapweed management.
Releasing one or two biocontrol insects may only slow down the spread of knapweed, rather than reduce its present dominance of the landscape. A combination of all the above insects may be required to damage the knapweed population enough for our native plant s to better compete with it. In the long run, however, our native plants will need to develop defenses against the carbon-stealing ability of the knapweeds. Availability of knapweed biocontrols may vary, but start by asking your county weed coordinator or your local extension service.
Grazing: Knapweeds are remarkably high in protein, and there is some speculation that the plants were originally introduced here for forage, but they become tough and unpalatable late in the season. Knapweeds are also excellent plants for bees. Since knapweed is often sprayed while in bloom, one might wonder what effect the herbicides have on the bees and their honey.
Controlled grazing studies have been conducted with sheep on Idaho fescue rangeland infested with spotted knapweed. Sheep were put on the test plots for a few days in June, July, and September when cool season grasses were in summer dormancy. The intent was to repeatedly graze the knapweed with minimal impact to native grasses.
The sheep had little impact on the older knapweed, but significantly reduced the number of new, younger plants compared to the control plots. In essence, the older plants were not being replaced by younger ones. The effect on Idaho fescue was mixed with 40% greater plant density, but with 38% shorter leaves and 17% shorter flower stems. Kentucky bluegrass, an exotic, increased with grazing. The study concluded that repeated timed grazings could negatively impact spotted knapweed with minimal impact to native plant communities.
Similar tests were conducted with goats at the Lee Metcalf Wildlife Refuge. Controlled grazing reduced the number of mature plants, while eliminating new seedlings.
Cows often graze around knapweed, allowing it to bloom and flourish, however, they can be trained to prefer knapweed, as shown in the accompanying videos. In addition, well-timed, high-density grazing can hit knapweed hard, vastly reducing knapweed seed production while increasing vigor of other plant competitors. Portable electric fences are utilized to corral cows into ultra small paddocks thick with knapweed. The animals severely graze weeds and grass in one day, being rotated to the next paddock the following day, allowing the pasture to recover quickly. Paddocks that were grazed early in the bloom may be hit again later to graze knapweed that recovered enough to produce more blossoms. Read more about High Density Grazing for Spotted Knapweed Suppression.
Trampling: The soil surface in established knapweed patches often looks sick and poisoned. Sometimes the only thing growing between the knapweed plants is club moss. The sickly soils can be revived with a thorough hoof-pounding, so that more desirable species will compete better with the knapweed. More scientific studies need to be done in this area.
Chemical Controls: Knapweed is relatively easy to kill with herbicides, but it takes many repeat treatments to get any plants that were missed the first time through, plus all the new plants that germinate from seeds left in the soil. By itself, 2,4-D is effective only if applied in the early spring as the flower stem begins to elongate. Dicamba (Banvel®) combined with 2,4-D is also effective, if applied to the rosettes prior to bolting. Clopyralid by itself (Stinger®) or combined with 2,4-D (Curtail®) is also effective. Picloram (Tordon® 22K-applied to rosettes or early bolt stage) is probably the most favored selective herbicide to control knapweed because it is persistant in the soil, so it costs less over the long run.
Among non-selective herbicides, glufonsinate-ammonium (Finale®) is more effective than glyphosate (Round-up®). However, the knapweeds are spindly plants, and it is difficult to spray them without hitting surrounding vegetation. Caution is advised, since non-selective herbicides will kill everything and open up more habitat for the weeds to spread. But it is possible to use non-selective herbicides effectively during dry years when the knapweeds are still green, but the grasses are completely dormant and yellow.
Herbicides are most effective as an aid to prevention. Use herbicides to prevent small outbreaks of knapweed from spreading into the broader area. If knapweed becomes integrated into the local ecology then herbicide may cause more harm than good. Spraying knapweed gives the temporary illusion of success as the knapweed dies out and is replaced by grasses, but at the cost of losing other native forbs and simplifying the ecosystem. Meanwhile, isolated knapweed plants survive and continue to spread, expanding the actual size of the outbreak. The next round of herbicides hits even more native plants, and the cycle continues as herbicides win each battle yet progressively lose the war.
Seeding: Heavily infested knapweed plots can be cultivated, bombed with herbicides, and reseeded with desirable plant species to restore productivity. Field tests have shown this treatment is most effective in the fall. The field is disced, then the weed seeds are allowed to germinate before being sprayed. Picloram (Tordon® 22K) proved to be more effective than glyphosate (Roundup®), probably since the picloram is persistant in the soil to kill weed seedlings in successive years. For maximum resistance to reinfestation it is advised that you plant a diverse seed mix.
Important: Most "weed problems" are really "people problems" from poor land management and a lack of ecological insight. It is easy to reach for a tool like fire, mowing, or herbicides to attack an out-of-control weed, but often those tools do little to get to the root cause of the weed infestation, and sometimes make the problems worse. Please read more about range ecology, desertification, and invasive weeds on this website before applying any tool of weed control Go to: Desertification and Invasive Weeds.
Notes
Thomas J. Elpel. Botany in a Day. HOPS Press: Pony, MT. January 2000.
C.A. Lacey, J. R. Lacey, P. K. Fay, J. M. Story and D.L. Zamora. "Controlling Knapweed on Montana Rangeland." MSU Extension Service. Circular #311. 1992.
Jim Story. Biological Control of Knapweed and Spurge: Principles and Status. Weeds as Teachers: Proceedings of AERO's 1995 Weed Management Alternatives Conference. AERO: Helena, Montana. 1997.
Roseann T. Wallander, Bret E. Olson, and John R. Lacey. "Spotted knapweed seed viability after passing through sheep and deer." Journal of Range Management. Volume 48, #2. March 1995. Pages145-149.
Caroline Lupfer Kurtz. "Plant Interactions: Knapweed Gets a Boost From Fungus." Research View (A Publication of the University of Montana-Missoula) Vol. 1, No. 2. June/July 1998. Pages 1-2.
Bret E. Olson, Roseann T. Wallander, and John R. Lacey. "Effects of sheep grazing on a spotted knapweed-infested Idaho fescue community." Journal of Range Management. Volume 50, # 4. July 1997. Pages. 386-390.
Tom D. Witson, Editor. Weeds of the West. The Western Society of Weed Science: Newark, CA. 5th Edition, 1996. Pg. 109.
Roger L. Sheley, James S. Jacobs, and Daniel E. Lucas. "Revegetating spotted knapweed infested rangeland in a single entry". Document.
Timothy R. Seastedt. Biological control of invasive plant species: a reassessment for the Anthropocene in New Phytologist Oct. 9 2014 https://onlinelibrary.wiley.com/doi/10.1111/nph.13065/full.
Identification and Management of Knapweeds in Colorado Produced by Larimer County Weed District.
Wildflowers-and-Weeds.com 
Spotted knapweed generates between 400 and 25,000 seeds per plant. It is not known how often stock and wildlife eat knapweed seed heads, but tests indicated that 11% of the seeds eaten by mule deer and 4% of the seeds eaten by sheep pass through their digestive systems, with a viability of up to 22%. Viability was near zero for seeds that took longer than two days to pass. The experiments were carried out by feeding mule deer and sheep capsules of spotted knapweed seeds.
Research at the University of Montana in Missoula indicates that spotted knapweed kills other plants not by poisoning them, but by draining them of nutrition. Knapweed forms a symbiotic association with a particular fungus in the soil. The fungus steals carbon away from nearby grasses, reducing their ability to compete. Grasses that evolved with knapweed were able to adapt and survive, but our native plants did not have the opportunity to develop the defensive mechanisms. Durar hard fescue is one such plant from Eurasia that can be seeded to out-compete knapweed, but it should only be introduced where native species have already been replaced with other exotics. 
Knapweeds love disturbed soils, so they are often the first to move in when new subdivisions are cut out of native rangeland. Any kind of road construction also contributes to its spread, especially when road dirt is moved from one location to another. Legislation to require weed-free fill dirt along highways was killed in the 1999 session of the Montana legislature. The cost of acquiring weed-free fill dirt is unknown.
Biological Controls: A dozen insect species have been introduced in the U.S. to control spotted and diffuse knapweeds, with varying results. The larvae of several insects feed on the roots or root crowns of the knapweeds, stealing energy from the plants and making them susceptible to fungal or bacterial infections. These root-boring insects include a weevil (Cyphocleonus achates), a yellow moth (Agapeta zoegana), a brown moth (Pterolonche inspersa) and the European buprestid beetle (Sphenoptera jugoslavica). The latter two only attack diffuse knapweed. 
