Penn State University weed scientists have been asked several times in the last couple of weeks whether we can still kill perennial weeds with herbicides this late in the fall as well as some cool season forages like alfalfa. Weeds like Johnsongrass, pokeweed, and other more tender herbaceous perennials have already entered dormancy; however, there may still be opportunities for some of the cool season species.
In the central part of the state, night-time low temperatures have routinely been right around 30° F and perhaps as low as 26 or 27° F. Usually, fall can be an excellent time to target herbaceous plants for control, but we have moved beyond the optimum time for effective control. Research conducted in Nebraska back in the late 1990’s showed that applications following the first frost of the season (temperature drops below 32 F, but leaf tissue is not damaged) actually provided a significant increase in control with several perennials. This research reported a 17 percent to 39 percent and 45 percent to 58 percent increase in control of Canada thistle and dandelion respectively if dicamba is applied after the temperature dropped below 29 F compared to 5 days before the first frost (Wilson and Michiels 2003). This would probably be similar for other effective herbicides as well, but this elevated control is dramatically reduced after a hard freeze that kills foliage of Canada thistle. With most plants it is possible to determine whether the foliage has been severely affected by frosts, thus scouting the field prior to application is important to ensure that active green foliage is still present. In general, if temperatures drop below 28 degrees at night for more than 4 hours then these plants may die and an herbicide application may not be effective. The study weed scientists have often quoted in an article like this is some work conducted by Nathan Hartwig back in 1970’s looking at the effect of glyphosate timing on quackgrass control. Dr. Hartwig applied a 1 lb/acre rate (1 qt of a 3 lb ae/gal formulation) in late September, early October, and November and rated the plots the next spring. The late September treatment provided greater than 90 percent control, the early October 80 percent to 90 percent, and the November treatment 50 percent to 60 percent control. Of course, more mild weather or an Indian summer would likely improve the November results.
In general, fall foliar applied herbicide applications should be made when daytime air temperatures are at least in the 50’s and preferably higher. If warm weather (greater than 65 degrees) returns for several days, the time is now. Applications when plants aren’t actively growing will limit herbicide uptake or movement, resulting in poor control the next year. Also, when applying systemic herbicides this late in the year, make sure to include adjuvant such as AMS and/or crop oil concentrate to insure adequate uptake of the herbicide.