Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
A few weeks ago, I wrote about lack of rain to activate herbicides, now I am being asked about how much herbicide is left after the big rains. Many areas of the state received at least 3 inches of rain two weeks ago and so what does that mean for fields that had the preemergence herbicide applied earlier. The easy answer is there is less in the top inch of soil where herbicides need to be for weed control. However, how much is still there, is hard to say. Fields sprayed before last weekend will need to be looked at regularly for “weed breaks” (weed emergence), which is likely to occur sooner than normal. Fields that need an early postemergence spray probably will need to include a residual herbicide to carry the field to canopy closure.
There were a number of fields with standing water, and we are likely to see the crop drown out in those spots. Continue to assess those drowned out areas, and even though there is no crop, they need to be treated. Do not give up on those areas; they need to be treated otherwise they will have weeds grow and produce many seeds (due to lack of a crop) which will be an issue for years to come.