Wet summer may have set back South Dakota's control efforts.
Based on some estimates, South Dakotans have reduced densities of Canada thistle by 200,000-300,000 acres, says Mike Moechnig, SDSU Extension Weeds Specialist.
"This is approximately enough thistle to cover the entire surface of Davison county," he says. "Unfortunately, we still have about 1.5 million acres to manage."
Wet conditions this summer may have stimulated thistle populations and prevented them from being sprayed.
Moechnig encourages landowners to consider management this fall.
"Fall can be a great time to manage thistle populations as accidental drift may be less injurious to maturing crops, birds have completed their nesting, and time may be available prior to the crop harvest rush," Moechnig says.
Moechnig says the best time to apply herbicides in the fall is generally mid-September. As long as the thistle plants are still mostly green, he says herbicides may be effective later in the fall.
"SDSU research over the past couple years has indicated herbicide efficacy declines slightly in the fall as the thistle plants become more damaged by frost," he said. "Therefore, efficacy will decline slightly in October and even more in November as frost injury increases. Many people suggest that it is best to wait until the first light frost before applying herbicides, but I have found that this may not be true. In some cases, the first frost can be a heavy frost causing most of the shoots to turn brown and make herbicides less effective. Therefore, I don't recommend waiting for frost."
Moechnig says the primary limiting factor for herbicide efficacy is fall thistle growth.
"The more fall growth, the better the herbicide's efficacy," he said. "Canada thistle fall growth is best in years when moisture is adequate and the plant canopy is not dense. Consequently, it may be worthwhile to walk through areas to be sure new Canada thistle shoots are present prior to making fall herbicide applications."
If landowners are unable to manage Canada thistle this fall, Moechnig says next spring is also a good time for control.
"SDSU research has indicated that spring (June) herbicide applications may be slightly more consistent than fall applications because soil moisture is often more plentiful and thistles often grow taller than the actively growing grass canopy which makes the thistle more exposed to herbicides."
He adds that June is the time when most Canada thistle shoots have emerged and root carbohydrate reserves are lowest. If landowners miss the June timing, herbicides may be equally effective if applied in July or August. However, Canada thistle will have seeded by that time which could increase the potential for new patches in the future.
"There are several opportunities to control those expanding Canada thistle patches. The most important thing is to choose the best time based on your schedule to help ensure action is taken. Canada thistle is a statewide noxious weed so control is required to prevent its spread on to neighboring property," Moechnig SAYS. "With a little effort, we can continue to diminish Canada thistle populations in South Dakota."
More information on controlling Canada thistle or other weeds may be found at http://www.sdstate.edu/ps/weed-mgmt or do a Web search for "SDSU Extension weeds" and look for the publications tab on the left side of the page. In addition, smart phone applications for SD noxious weeds are available on iTunes and the Android Market. Information may also be available at your local Extension office or Weed and Pest Supervisor.