Spotted Knapweed – June 2017 Weed of the Month

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Spotted knapweed in King County in late May.

What’s five feet tall, has pink or purple flowers with spotted bracts, and costs the state of Montana $42 million each year? If you answered spotted knapweed (Centaurea stoebe), you’re right! A native to grassland steppes between central Europe and western Siberia, this Class B noxious weed first hit North America via Victoria, British Columbia, in 1893. It arrived and spread largely as a contaminant in alfalfa (and possibly clover) seed as well as in discarded soil used as ship ballast. By 1928 it had reached King County, and by 1940 was a serious problem on gravelly soil in nearby Whatcom County. It now grows in the entire contiguous U.S. except for Texas, Oklahoma, and Georgia—and when it invades an area, it really takes over. In some places, spotted knapweed comprises more than 95% of the available plant community.

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Spotted knapweed is an especially big problem in rangelands and wildlife grazing areas—mostly those of deer and elk—throughout the intermountain west. For instance, it currently covers between two and five million acres in Montana. But it manages to thrive here in wetter western Washington, too. The plant prefers well-drained soils in full sun, but can tolerate some pretty harsh conditions. In King County, you often find it growing along roads and railroads, as well as on industrial lands, gravel pits, and similar sites with dry, disturbed soils.

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Surprisingly, given the association of knapweed with dry sites, spotted knapweed is also a common sight on the riverbanks and gravel bars of the White and Snoqualmie Rivers. In spite of being flooded in the winter, these areas are very dry in the summer, and knapweed seeds are annually washed down from above to re-infest the riverbanks.

sp knap - infest, riverside

In spite of our wet climate, spotted knapweed has been one of the top five regulated noxious weeds in King County every year since the Noxious Weed Control Program started. This is a testament to the plant’s highly adaptable nature and its tenacity as well as its invasiveness. To learn more about current and past spotted knapweed sites (as well as those of other noxious weeds), check out this interactive map.

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Spotted knapweed rosettes growing in gravel.

Spotted knapweed is a short-lived perennial with up to twenty upright branched stems and a stout taproot. It starts out as a basal rosette with deeply lobed, grayish-green leaves. It can be very hard to distinguish from diffuse knapweed at this stage, but it stands out in western Washington where we don’t have many native plants with this sage-like color.

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Close-up of young spotted knapweed leaves.

When spotted knapweed blooms, it can be distinguished from diffuse knapweed by it’s pink to light-purple flowers with bracts that have distinct black spots with vertical lines below (bracts are those scale-like structures growing around the base of the flower head). In contrast, diffuse knapweed has white flowers and little yellow spines on its bracts instead of dark tips.

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Spotted knapweed flower with bracts showing distinct black spots above vertical lines.

Spotted knapweed plants in King County are just starting to bolt in early June but generally haven’t started flowering yet. This is an excellent time to control spotted knapweed since it is easier to see but hasn’t started making seeds yet. Make sure you control yours before they go to seed!

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Spotted knapweed rosettes starting to bolt in gravel.

Individual plants aren’t  that hard to remove by pulling or digging, but each one produces many seeds that can remain viable for up to eight years. It reproduces entirely by seed and it is highly successful at doing so. You’ll more often find spotted knapweed growing in dense infestations, and disturbing the soil tends to promote germination of their many dormant seeds. For this reason, completely eradicating an infestation can be a long and difficult process. A variety of broadleaf herbicides are effective on spotted knapweed, especially in the spring and fall months. However, simply mowing it won’t work because the plant will re-flower shorter and shorter each year until it escapes the mower’s blades and produce seeds in spite of the mowing.

Matt Below_uprooted spotted knapweed

Weed Specialist Matt Below displays an uprooted spotted knapweed plant. Repeated mowing often just increases the root mass and number of stems of this tough plant.

Fourteen biocontrol agents (thirteen insect species and one fungus) are currently fighting the good fight against spotted knapweed with varying degrees of success. While these species are less useful in King County, where we tend to have smaller, pioneering infestations, they deserve a nod. (For more information on biocontrol agents, visit Washington State University’s Integrated Weed Control Project. Also, see our recent blog post on the subject.)

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The knapweed root weevil (Cyphocleonus achates) on a spotted knapweed flower.  Photo courtesy of Jennifer Andreas, Washington State University Extension.

Fortunately, spotted knapweed has a harder time dominating other plants in intact native plant ecosystems or well-established grasslands in our area. In the long term, good land management can go a long way toward keeping spotted knapweed off of your property.

If you see spotted knapweed anywhere in King County, please let us know! As many sites as we’ve controlled over the years, there are still more to discover. Our program is always eager to work with landowners in keeping spotted knapweed and our county’s other noxious weeds under control. You can also easily report any noxious weed on your phone or tablet by using the Washington Invasive Species Council’s updated invasive species reporting app.

For more information on control methods and other facts about spotted knapweed, follow these links:

King County Noxious Weeds: Spotted Knapweed (Centaurea stoebe (syn. C. biebersteinii, C. maculosa)

King County Weed Alert: Spotted Knapweed (Centaurea stoebe)

King County Best Management Practices: Spotted Knapweed (Centaurea stoebe ssp. micranthos) (synonyms C. biebersteinii and C. maculosa)

Washington State Noxious Weed Control Board: Spotted Knapweed (Centaurea stoebe)

Good luck out there!

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A lone spotted knapweed.

P.S. At different times in the past, spotted knapweed went by the scientific names Centaurea biebersteinii and Centaurea maculosa. Centaurea stoebe is its current scientific name, though the other two names continue to appear. Its common name has always been spotted knapweed.



Categories: Weed Identification, Weed of the Month

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