The addition of a supplemental labeling for apples (Stinger) will allow growers to control several important problem weeds. Right now, prior to bud stage, in mid-May, is especially good timing for Canada thistle control with clopyralid.
Clopyralid is effective on weeds in the legume (white clover, vetch), composite (thistles, dandelion, horseweed, asters, goldenrod) and nightshade (black and hairy nightshade) families. Some weeds in these families are very attractive to honey bees and other pollinators. By eliminating these weeds from both orchard row middles, as well as in-row areas, we lessen the impact of insecticides on pollinators that may visit these weeds during bloom.
Some weeds such as goldenrod, Canada thistle, asters and horseweed can grow three to six feet tall within the tree row, interfering with harvest and other orchard operations, so there is more than one reason to eliminate these weeds.
Trees must be established one year before treatment. The treatment is not for first year plantings. The pre-harvest interval for clopyralid in apples is 30 days and it may not be used during apple bloom. At this point, a single application of the highest labeled rate, 2/3 pint per acre, should provide the best control of Canada thistle. Split applications of 1/3 pint per application are also labeled. Do not exceed 2/3 pint per acre, total per year.
If you miss the treatment window of opportunity that has opened in mid-May, consider clopyralid as a postharvest spray, targeting both row middles and in-row areas where target weeds exist. Rosettes of Canada thistle are good targets in early fall and often persist late in the growing season. Addition of a labeled 2,4-D material in the fall applications will further help to reduce weed populations.