| Walla
Walla County Noxious Weed Control Board
Weed of the Month
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. |
