North Dakota State University -- NDSU Agriculture Communication
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agcomm@ndsuext.nodak.edu

October 4, 2001

Canada Thistle Control May Still be Possible

Canada thistle, a growing problem of North Dakota farmers, can only be controlled by persistent efforts, notes a North Dakota State University weed researcher. Fall, a time when the plant is moving nutrients to the root system, is a good time to attack.

"Herbicide treatments following a light frost can still be effective," says Brian Jenks, a researcher at NDSU’s North Central Research Extension Center in Minot. "Wait a day or two for plants to recover. If they remain green and have a healthy appearance following a light frost, the herbicide treatments can still be effective."

Jenks says herbicides will be more effective if daytime temperatures are at least in the 60s. "We don’t recommend spraying if the daytime highs are in the 40s or if a hard frost is expected within the next few days."

These post-harvest treatments for Canada thistle are only one part of a year-long campaign to keep the weed in check, Jenks says. "The best tool for dealing with Canada thistle is to not let it persist and spread for several years without keeping it in check."

Canada thistle infestations in North Dakota grew significantly from 1993 to 2000, primarily because of increased rainfall. Manitoba research indicates that reduced tillage may also play a role in the weed’s proliferation.

Infestations in some areas of the state were reduced this year with dry conditions, Jenks notes. Other weeds such as kochia and Russian thistle, which grow better in dry conditions, were larger concerns for producers. The dryer weather may have given producers a chance to improve control of Canada thistle, but he cautioned that wetter conditions next year could bring increased problems with the weed.

Jenks outlined several strategies for controlling the weed.
  • Summer fallow followed by a fall treatment with Roundup. Repeated tillage will keep Canada thistle plants from growing and developing the underground root system. Once days get shorter (less than 14 to 16 hours of daylight) in the fall, Canada thistle plants will begin storing food reserves and will be less likely to elongate and flower. A final tillage pass in early August will provide enough time for thistle rosettes to develop for a mid- to late-September herbicide application. Research indicates that spraying as late in the season as possible before a killing frost is most effective.

  • Pre-harvest treatments. Treatments with Roundup have been approved for wheat and flax and may provide more consistent control than post-harvest treatments, because plants don’t always have enough time for regrowth after harvest. Roundup applied at 1 quart or more per acre has generally provided the best control. Treatments with 2,4-D and Clarity have been less effective than Roundup in long-term research trials.

  • Post-harvest treatments. For post-harvest treatments to be effective, crops must be off early enough to allow Canada thistle plants to develop new growth. Rainfall in August and early September is also important for weed growth. Crops harvested by mid August will provide a better chance for effective treatments than crops harvested later. In general, at least three or four weeks of warm, moist conditions following harvest are necessary to prompt sufficient weed growth for effective treatments.

  • Curtail treatment in wheat. In long-term research trials at the North Central Research Center, Curtail applied to wheat has maintained or decreased Canada thistle densities the following year after application. Curtail generally provides good Canada thistle control and virtually stops the weed from competing with the crop. If Curtail is used, there generally is not sufficient Canada thistle regrowth for a successful pre-harvest treatment. In some years a post-harvest treatment may be warranted.

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Source: Brian Jenks, (701) 857-7677, bjenks@ndsuext.nodak.edu
Editor: Tom Jirik, (701) 231-9629, tjirik@ndsuext.nodak.edu

 

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