Canada Thistle Rebounds?

By Mark Loux

It can be nice to see old friends.  Except when they cause crop and yield loss, refuse to leave after a few days, and don’t respond to chemicals.  A while back we wrote about what appeared to be an increase in populations of dandelions and other winter weeds and made some guesses about why this was happening.  Canada thistle has once again become a problem in some fields in a big way, probably for some of the same reasons that dandelion has.  Our history with thistle during the past 30+ years is that it was a major problem before the widespread adoption of RoundupReady soybeans in the late 1990s.  Back then we had to take advantage of specific windows in the cropping cycle to try to get control with glyphosate, and the rest of the time we just tried to keep it from getting worse.  The “all glyphosate, all the time” strategy during the first decade of RoundupReady soybeans handled thistle and other perennials such as milkweed and hemp dogbane well, and we didn’t hear much about them.  Now we are though, and increases in thistle could be due to the following:

• lack of herbicide treatments in fall, when thistle is most effectively controlled
• lack of wheat in the rotation.  Including wheat allows a period after harvest for thistle to regrow to a large enough size in fall (compared with cutting it off during corn and soybean harvest)
• nonGMO soybeans, where options for control are ineffective/expensive and thistle gets a better foothold
• switch to the use of Liberty Link soybeans and the use of glufosinate in POST treatments.  Glufosinate is a contact herbicide that can burn down the top-growth but will not reduce populations
• applying POST glyphosate treatments too early, before thistle is large enough to respond well
• failure to use effective POST treatments in corn

The initial slow increases in the population of any weed are often ignored since populations are too low to cause a loss in stand or yield or interfere with harvest.  At some point though, the current year’s infestation will be substantial enough to provide the source for a much denser infestation the following year.  In this steeper part of a population’s growth curve, things can get out of control fast.  Canada thistle reproduces via windblown seed, and also spreads via a deep, dense network of creeping roots.  Left uncontrolled for a while, the patch of thistle that results from this can be thick enough to reduce crop stands and vigor considerably, literally sucking the life out of corn and soybeans.  Some suggestions for controlling thistle for those experiencing a rebound:

• Apply herbicides in the fall when the thistle plants are at least 10 to 12 inches tall.  This can occur into November as long as plants are still green, before freezes.  Our experience is that it’s probably not worth treating in fall when plants do not regrow to at least this size unless there are other weeds requiring fall application anyway.
• Use effective POST herbicides in corn, especially where wheat is not in the rotation and glyphosate is not being used POST in soybeans.
• if using glufosinate POST in soybeans that are resistant to glyphosate also (LLGT27, Enlist), add glyphosate to the mixture. 
• use effective burndown and residual herbicides in soybeans, to try to delay the POST glyphosate application until later in June when thistle are larger (ideally in the bud to flower stage). 
• Herbicides applied to small thistle plants in late spring only reduce the top-growth, without herbicide getting to the roots.  At small sizes, thistle plants are primed to just regrow again.  We have heard from several clients who observed ineffective activity on thistle from burndown applications and then resprayed as soon as the thistle plants had a few inches of regrowth.  While the basis for this approach in dense stands is to try to prevent suppression of the crop, it largely won’t work.  Steps need to be taken the previous years to prevent the development of dense thistle stands.
• In soybeans resistant to glyphosate, scout later in the season following the initial POST application for thistle regrowth and treat again as necessary.
• POST options in nonGMO soybeans are generally not good or are just expensive.  From the Weed Control Guide:  “Postemergence applications of Basagran (2 pt/A) will control above-ground parts of the plant or suppress the growth of Canada thistle. Regrowth usually occurs, but this treatment will reduce competition from Canada thistle in soybeans and help prevent the production of more rootstock. Apply when thistle plants are from 8 inches tall to the bud stage. COC should be included in the spray mixture. A second application at the same rate may be made 7 to 10 days later, if necessary.  Other products and mixtures with activity on thistle include fomesafen and mixtures of Basagran with fomesafen, Ultra Blazer, or Cobra. Postemergence application of Pursuit (1.44 oz/A), Classic (0.66 to 0.75 oz/A), FirstRate (0.3 oz/A), and Synchrony XP (0.75 oz/A) may also suppress thistle growth, but results have been variable.”

A reminder that the first page of the “Control of Problem Weeds” section of the “Weed Control Guide for Ohio, Indiana, and Illinois” contains a list of strategies for managing perennials.  This was first written before the availability of RR soybeans, so it can have some useful information for this type of situation where the use of glyphosate is being deemphasized.