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Walla County Noxious Weed Control Board
Weed
of the Month
Spikeweed (Centromadia pungens)
Spikeweed is a native to California and was introduced to the Columbia river basin of northeastern Oregon and southeastern Washington within the last century. Also known as common tarweed, it behaves as an invasive weed and agricultural pest. It occurs on roadsides, waste areas, and in low alkaline sites of grain fields and rangelands, sometimes forming dense stands. Spikeweed is a tough, spiny plant which is avoided by livestock.
Spikeweed is an annual with rigid, bristly, branching stems that grow 1 ½ to 3 feet tall. Basal leaves are pale, straw colored, stiff and several inches long with narrow lobes. Leaves along the stems are up to ½ inch long, sharp pointed, with dwarf stems or short flowering branches in their axils. The leaves and stems are rough to the touch with both long and short spreading hairs which may have tiny glands which exude a strong-scented resin.
Plants germinate in fall or late spring, growing into rosettes with leaves that are several inches long and sharply divided. Rosettes bolt in late spring and early summer. Yellow, 1/3 inch ray flowers bloom at the tips of short leafy branches from July to September. Large plants may have well over 100 flowerheads and are heavy seed producers. Seeds are about 1/16 inch long with wart-like projections and a short spine.
A broadleaf weed killer with a good surfactant is effective on small rosettes. Choices found in garden centers typically include 2,4-D (2,4-dichlorophenoxyacetic acid); mecoprop or MCPP (2-(2-methyl-4-chlorophenoxy) propionic acid); or dicamba (3,6-dichloro-o-anisic acid); with two and three-way combinations available. More mature plants can be controlled with clopyralid (Curtail, Redeem), available over the counter, or Chlorsulfuron (Telar) or metsulfuron methyl (Escort), restricted to licensed applicators.
Hand pulling can be effective in small areas. It may also be useful as a follow-up method in areas that were treated with herbicides earlier in the growing season. Plants should be pulled while they are still green and relatively 'soft', and even then gloves must be worn.
No approved biological control agent is available.
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ALL PESTICIDE LABEL INSTRUCTIONS AND PRECAUTIONS
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