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Thistle and Other Hard to Control Weeds

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Posted: November 26, 2013

Stinger, a selective, postemergence herbicide for control of broadleaf weeds, is now labeled for use on apples. It is especially effective against thistles, dandelion, nightshades, and goldenrod, with some activity against clovers. Its mode of action is somewhat similar to 2,4-D, acting as a synthetic auxin, and it is in the same HRAC category as triclopyr (Garlon®).
Dr. Rob Crassweller, Department of Plant Science

Dow AgroSciences has recently announced the release of a supplemental label for the use of Stinger (clopyralid) in apples. This herbicide has long been labeled in stone fruits and is now labeled for use in apples (sorry not pears). Rates are 1/4 to 2/3 pint/treated acre. Preharvest interval is 30 days with a REI of 12 hours. The maximum total yearly rate is 2/3 pint/A. It can be applied to nonbearing and bearing trees established at least 1 year in the orchard. 

A couple of reminders about the use of Stinger on any crop are in order. Stinger can seep or leach through soil and under certain conditions can enter the groundwater table where it might be incorporated into drinking or irrigation water. Avoid using the material on sandy soils or where the water table is close to the surface. In Pennsylvania this would also include avoiding the use around areas with high limestone outcroppings or where sinkholes are prone to develop. I know that this may sound like all of Pennsylvania but you should be aware of this possibility.

If you are specifically going after thistles be aware that there are actually two windows for control. The first opportunity is a spray in early to mid-June from the rosette stage to when the flower buds are just forming but have not opened. This can help reduce the plant vigor. The second window of opportunity is in August to October when thistle regrowth is about 10 inches high.

Since thistles can exist in clumps or patches the best approach may be to use a hand sprayer for spot spraying. The table below gives the amount of Stinger per gallon of spray for specific rates based upon treating 1000 square feet of treated area.

 


1/4 pt/A 1/3 pt/A 1/2 pt/A 2/3pt/A
Amt/gal 2.7 ml 3.6 ml 5.4 ml 7.3 ml