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The Monday Garden
Meadow Killer: Spotted Knapweed

Issue No. 77 - September 14, 2003
by Sue Sweeney

This is a really bad guy, sneaking under a fence along the sidewalk near the main hospital in Stamford, CT.   While millions of acres of pasture in Western USA and Canada have been over run by this evil doer, we can stop it on the East Coast where it has not yet gotten a foothold.


Spotted knapweed (Centaurea maculosa) is probably not even welcome where it came from (Central Europe and Russia to Siberia).  Indeed, this stuff's so bad that no one will own up to bringing it here.  Instead, its introduction in the late 1800's is blamed contaminated seed and discarded ship ballast.

The problem is that many grazing animals won't eat knapweed if they have any other choice, be they domestic cattle or free deer or elk.  To add insult to injury, knapweed also leads to greater water run off and thus soil erosion and sedimentation in the rivers and ponds than the plants that it replaces.  And now it's been learned that knapweed literally poisons the earth so that other plants can't grow.   I guess this is about as good an example as you can get of  "noxious weed".   Not even birds will eat it.

Knapweed is a member of the Sunflower-Aster (Composite) family.  It looks like a thornless thistle (same purple color and downy flower head).  The flower heads also resemble the closely related (but lovely blue and benign) bachelor's button, and the leaves have the same powdery look as bachelors' buttons.

Knapweed infests dry, sunny meadows and roadsides.  It survives harsh winters and droughts by possessing a long taproot (the underground bunker of the plant world).  It blooms midsummer through fall, so its easy to spot knapweed now that so many other plants have stopped flowering for the year.

Knapweed tends to get started in disturbed areas (e.g. newly plowed land) where it establishes a small and seemly harmless colony.  However, after several years, it spreads quickly.  According to the September 9, 2003 New York Times, biologists at Colorado State University have discovered that knapweed does it by releasing a toxin that kills the neighboring plants roots so quickly and effecting that the researchers took out a patent on the stuff as a herbicide.  Use of chemical warfare by a member of the sunflower clan comes as no surprise to any one whose yard has been contaminated by sunflower seed husks.

Knapweed lives several years, producing thousands of seeds each year.  The seeds mature and scatter by late summer and sprout in fall and spring. Since birds don't eat knapweed, the seeds are generally carried short distances by wind and foot traffic.  Longer distance movement is caused by transposition of soil, compost, wood chips, hay bales, and trash containing the seeds, running water, and vehicle undercarriages and wheels. The seeds can lie dormant for years, waiting for the right conditions to germinate.

Control: pull it up and get the WHOLE root; place in the sun in a black plastic bag for several months to sterilize the seeds.

Further reading:
http://www.mtweed.org/Identification/Knapweed/knapweed.html (Montana)
http://www.ext.nodak.edu/extpubs/plantsci/weeds/w842w.htm (North Dakota)
http://www.ext.colostate.edu/pubs/natres/03110.html (Colorado)
http://www.oneplan.org/Crop/noxWeeds/nxWeed30.htm (Idaho)
http://royal.okanagan.bc.ca/mpidwirn/plantsandanimals/knapweed.html (British Columbia)
http://www.dnr.state.wi.us/org/land/er/invasive/factsheets/knapweed.htm (Wisconsin)
http://www.invasivespecies.gov/profiles/spotknwd.shtml (USA Feds)
From The Monday Garden. Copyright © by Sue Sweeney. Reproduced with permission. 

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