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Alien Plants
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The Monday Garden
September 14, 2003, issue no. 77
Meadow Killer: Spotted Knapweed
by Sue Sweeney
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| 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. |
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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) |
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