Answer: There are two basic ways to eradicate thistle. Cutting repeatedly and applying a chemical herbicide. Cut it all down, allow it to grow again; cut it all down again, let it grow again, etc. Two, three, four times?the more the better to deny the roots their solar energy collectors. In fact, if you just do this continual cutting for several years, the plants and roots will eventually die.
2,4-D is widely used for thistle control, but one application is seldom enough to kill the plants. Overapplication may reduce 2,4-D's effectiveness by killing the tops before much of the active ingredient moves to the roots.
A University of Wisconsin study showed that 2,4-D works best when thistles are in the rosette stage, usually in early May or again in mid-September. A rate of 1 pound of active ingredient per acre was sufficient to kill virtually all of the plumeless thistles at that stage. But once they started to bolt, a mix of .75 pounds of 2,4-D and .25 pounds of Banvel worked better.
Banvel at rates of 2 to 4 quarts per acre will produce a near-total kill on actively growing thistles, but these high rates may also stunt grasses temporarily. Roundup doesn't work very well on deep-rooted crops and will also kill surrounding forages, notes Cadwallader. But both Roundup and Banvel can be wick-applied when thistles are a foot or so taller than surrounding forages. In another Wisconsin study, two passes of Roundup with a rope wick only partially killed Bull thistles, but stopped their flowering totally.
Be sure to allow at least two or three hours between application and rain to avoid having the chemicals wash off the plants before they have a chance to take them up. Apply all herbicides following label recommendations.
If you still have problems after using these control tactics you might want to contact your local cooperative extension office or noxious weed control office for more assistance.
Best wishes with your project!
Q&A Library Searching Tips