| Walla
Walla County Noxious Weed Control Board
Weed
of the Month
Scotch Thistle
Scotch Thistle (Onopordum
acanthium L.) is a native of Europe and eastern Asia and is
thought to have been first introduced into the United States as
an ornamental in the late 1800’s. This biennial, also known
as cotton thistle, may grow up to 8 feet tall. Its multiple stems
have broad, spiny wings with coarse, lobed, spiny leaves. Upper
leaves are alternate and basal leaves may be up to 2 feet long and
1 foot wide. Foliage is gray-green due to a thick mat of cottony
hairs covering upper and lower leaf surfaces.
Plants flower in mid-summer.
Numerous globe-shaped flower heads are up to two inches in diameter,
purple or purple-red with spine-tipped bracts, and occur in clusters
of two or three on branch tips. Seeds are plumed and usually germinate
in late fall, but germination can occur at other times, as well.
Plants grow in cultivated
fields, waste areas, roadsides, and often on the edges of forests.
Scotch thistle can choke out cultivated crops in fields and native
species in other areas. Its drought-resistant characteristic makes
it difficult to eradicate.
Small numbers of plants
can be removed by digging if they are cut off below the soil. Mowing
has limited effectiveness but prevents seed production if done either
immediately prior to flowering or when plants are just starting
to flower.
Herbicides are most effective
when applied to rosettes in fall or early spring: dicamba (Banvel
or Clarity); 2,4-D; dicamba + 2,4,-D; chlorsulfuron (Telar); metsulfuron
(Escort or Cimarron); picloram (Tordon); clopyralid (Stinger or
Transline) or clopyralid + 2,4-D amine (Curtail); glyphosate + 2,4-D
(Campaign).
BE SURE TO FOLLOW
ALL PESTICIDE LABEL INSTRUCTIONS AND PRECAUTIONS.
No biological controls
are currently available in the United States.
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