First round with thistles is over The first round in the battle with thistles is about over, says Leon Wrage, Extension weed specialist at South Dakota State University.
Musk and plumeless biennial thistle have seeded and completed their life cycle. Most of the Canada thistle that wasn't controlled earlier also has seeded.
"The situation with Canada thistle looks critical," Wrage said. "Control in fields looks good, but fencelines, field corners, pastures and some roadsides were missed in the program. The tremendous seed load from some of these areas that can mean more problems next year."
For the second round, wait for fall rosettes of musk and plumeless thistles, said Wrage. Bull thistle was just beginning to bloom in mid-August; for them, mowing or chopping is the best bet, where practical. Canada thistle will produce new fall growth if moisture is favorable. Mowing or tilling will set up the plant for new growth. Herbicides on mature plants that are drying down will not be very effective.
Treating is still an option if plants were controlled early in the season and still are actively growing; this will reduce but not eliminate seeding if flowers have been open for more than 7 to 10 days.