By Megan Taylor and Nevin Lawrence et.al
As October approaches and harvest starts, don’t forget about thistles plaguing your pastures. Fall, specifically October and early November, is a key time to chemically control thistles in pastures. This is an ideal time to control biennial and perennial thistles. Proper identification of thistles is key to picking the proper chemical control.
Thistle Species in Nebraska
In Nebraska we have several biennial thistles, but we mostly deal with musk, plumeless, Scotch, and bull thistles in our pastures. Biennials require portions of two growing seasons to flower/reproduce. They develop from seed the first season as a flat rosette (Figure 1). The rosette overwinters and the cold causes the rosette to bolt the next season and produce seed. Once the plant puts on seed it will die and that seed spreads by wind. When trying to control biennial thistles, destruction of rosettes prior to flowering (bolting) is an effective means of preventing seed formation and subsequent spread. Fall is a key time to target these rosettes and to help mitigate seed production the following season.
Figure 1. Musk Thistle seedling having a flat growing rosette (photo courtesy of Pamela Borden Trewatha)
Another thistle to look out for is Canada thistle. Canada thistle is a creeping perennial that can be controlled with fall spraying, in conjunction with other management options in the spring. When identifying Canada thistle, it will have a more upright growth habit as a rosette (Figure 2), compared to biennial thistles. Previous research from Robert Wilson (UNL Emeritus Professor) indicated that control of Canada thistle went from 33%, when an herbicide was applied in the spring, to 90%, when fall applications were made. Dr. Wilson reported that fall herbicide applications caused changes in sugars stored in the roots of Canada thistle, and decreased the chances of the weed surviving the winter.
Figure 2. Canada thistle young upright shoots from creeping roots. (Photo courtesy of Pamela Borden Trewatha)
While in the rosette stage thistles are more effectively controlled using herbicides. It is important to note that fall spraying of thistles is not a silver bullet and effective control often needs repeated applications. It will take several years of timely control before the soil seed bank is reduced. Choosing the right products for your program is another key step to controlling your thistles.
Picking the Right Product
There are many herbicides labeled for thistle control. When choosing an herbicide for spraying thistles, the proximity of thistles to waterways and sensitive plants, grazing and haying restrictions, and the type of thistle are all important considerations. Several products are effective for all thistles, but some herbicides have higher efficacies depending on the thistle species. Note that some products traditionally recommended for spraying thistles have recently changed product names. Take care when purchasing products and always read/follow label directions before use.
GrazonNext® HL, Milestone®, Chaparral®, Graslan® L, Stinger®, Overdrive®, and Tordon 22K® are all products that are labelled for use on biennial thistles as well as Canada thistle. 2,4-D mixed with dicamba is also an effective option, but should be sprayed when temperatures are warmer for the highest efficacy. When using Tordon 22K® or Graslan® L, both products are redistricted use and contain picloram, use extreme caution around other vegetation, especially trees. Both products will kill woody plants. Most of the herbicides used for control of thistles also kill desirable forbs, so spot spraying individual plants or patches rather than broadcast spraying the entire pasture can spare valuable forbs.
Source : unl.edu