During the long hours of harvest, growers need to remember that a very narrow window of opportunity exists for applying fall pre-emerge herbicides.

These herbicides include products such as Dual, Outlook, Parallel and Verdict. The product labels on these four herbicides prohibit fall applications until soil temperatures at a 4‑in. depth are sustained at less than 55 degrees F. The labels also prohibit applications after the soil freezes.

In Minnesota, typical dates when soil temperature drops to this threshold ranged from mid-October to early November, according to Ron Struss, Minnesota Department of Agriculture pesticide management scientist. And the dates when the soil froze ranged from late November to mid-December.

 With the application window pinched between these two limits, Struss said the timeframe for proper application can be as narrow as 35 days.

“There are both economic and environmental reasons for farmers to keep these limitations in mind as they plan their fall field work,” Struss said. “Delaying fall herbicide applications until soil temperatures drop below 55 degrees reduces the amount of herbicide lost to degradation before spring, and ending fall application once soils freeze reduces the chance of product runoff during thaws.”

Label requirements vary from one product to another. In all cases, Struss said, applicators should take the time to carefully read and follow all label requirements for herbicides they plan to apply.

Soil temperature can be taken in the field using a soil thermometer. Soil temperature should be measured when it is likely at its maximum — from 3 p.m. to 4 p.m. on sites with good sun exposure. Soil thermometers should be left in the soil for five minutes before a reading is taken.

Two websites are now available to help applicators in Minnesota time their herbicide applications. The University of Minnesota and Minnesota Department of Natural Resources provide maximum and minimum soil temperature data at 4-in. depth for eight locations at http://climate.umn.edu/cawap/soilpan/soilpan.asp. The MDA provides soil temperature data at 6‑in. depth for nine locations around the state at http://gis.mda.state.mn.us/maps/csgsoil.htm.