While Canada thistles have become less prevalent in South Dakota over the past decade, this year’s wet summer may have stimulated thistle populations and prevented management, warns Mike Moechnig, South Dakota State University (SDSU) Extension weed specialist.

He encourages landowners to consider controlling the weed 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," says Moechnig.

The best time to apply herbicides in fall is generally mid-September, but they may be effective later if the thistles are still mostly green, he says.

"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. 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."

The primary limiting factor for herbicide efficacy is fall thistle growth, he says.

"The more fall growth, the better the herbicide's efficacy. 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."

Moechnig adds that spring is also a good time for thistle 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."

June is 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, the weeds 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,” says Moechnig “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 to neighboring property. With a little effort, we can continue to diminish Canada thistle populations in South Dakota."

More information on controlling this or other weeds may be found at www.sdstate.edu/ps/weed-mgmt. In addition, smart-phone applications for South Dakota noxious weeds are available on iTunes and the Android Market. Information may also be available from your local Extension office or weed and pest supervisor.