I have had numerous folks call in about how to control thistles. There are some tough weeds and grasses out there that are hard to control and almost impossible to eradicate. If left uncontrolled, thick thistle stands can reduce grazing, result in less forage production and ultimately lower calf weaning weight. A single thistle plant can produce at least 4,000 seeds, which increases the chance for higher thistle populations in the pasture the following year. Consequently, management practices need to be conducted prior to flower formation for effective thistle control.

Even if thistles have not infested your pasture in the past, it is ideal that your pastures are scouted in late fall through mid-spring to ensure that thistles do not get out of control. New infestations are easier to manage than large-scale populations. Right now we are in the summer-blooming thistle flower stage which is the most recognizable growth form. Herbicides are not nearly as effective when they reach this stage. And contrary to what people think, the spring is not the best time to control them. The best time to control them effectively is in the fall when they are small in the flat rosette stage. This is usually October and early November which is one of the best times to control pasture thistles with herbicides. Delaying treatment until spring can limit your herbicide and control options.

The importance of application timing cannot be over stated. Thistles are normally not visually evident as a problem until flowers are produced. However, the plants are there in the rosette form long before flowers emerge, and early scouting should allow early detection and optimum control. Quickly scouting the pastures in the fall to early winter will reveal the presence of thistles (rosette stage) and allow for an inexpensive herbicide application. If you wait until thistles flower, mowing or herbicide options are limited, less effective, and more expensive. Take the time to scout early, because it is the key to better and more economical thistle control. Look close this fall and you’re likely to find many thistle seedlings in a small, flat, rosette growth form. At this stage they are very sensitive to certain herbicides.

Several herbicides are effective and recommended for thistle control like ForeFront, Milestone and Chaparral; all are very effective. Two other very effective herbicides are Tordon 22K and Grazon. Be careful with all these herbicides, but especially Tordon and Grazon, since they also can kill woody plants, including trees you might want to keep. 2,4-D also works well while it’s warm, but you will get better thistle control by using a little less 2,4-D and adding a small amount of Banvel or dicamba to the mix. Other herbicides, such as Redeem, Cimarron, and Curtail, also can control thistles in pastures. I think GrazonNext HL does a good job controlling flowering thistle, but if seeds are already produced and the plant is beginning to die, mowing may be the best (temporary) option.

No matter which weed killer you use, though, be sure to read and follow label instructions, and be sure to spray on time. Next year, avoid overgrazing your pastures so your grass stands get thicker and compete with any new thistle seedlings.

Good luck battling thistles and all the other unwanted weeds and give me a call if I can help you. Consider your management options and plan accordingly! After all, you’re the boss!

Marty Morgan is Cooke County Ag Agent for the Texas A&M AgriLife Extension Service. Reach him at 940-668-5412 or marty.morgan@ag.tamu.edu.