Q: When the weather is unseasonably warm (60′s and 70′s) in January, should I put out a pre-emergent to prevent spring and summer weeds?
A: Good question. Here’s what my UGA weed control specialists say:
From Dr. Tim Murphy:
“Yes it has been warm, but for homeowners there is no need to rush the PRE application. I think over the next 3 nights we are forecast to have below freezing night temperatures. So, I am not overly concerned about any super early germination of crabgrass or other summer annual weeds.
“PRE herbicides are routinely applied in mid-Jan by many lawn care companies with a high number of accounts. These mid-January applications are always followed by an additional application approximately 60 to 90 days later. Early applications are primarily done so the lawn care company can get all lawns treated in case there is a early spring with significant warm-up in early March, or it rained the whole month of Feb.
“This program works because PRE herbicides undergo very little degradation, or breakdown, at cool soil temps (< 50-55 F). Crabgrass can initiate germination at around 53 to 58 F soil temps (average across several days). But in many instances the very early flush of crabgrass is suicidal, in that a hard frost will kill seedlings.
“Bottom line is: Yes, you can apply early in mid-Jan. if needed. But if one is self-treating home lawn I would wait about another month and apply around Feb. 15. Also, if a fertilizer/PRE herbicide product is selected make sure it is time to fertilize the turfgrass. IF not, then select a PRE herbicide that is not formulated on dry fertilizer carrier, or if it is, select a combo product that has a lot of slow release N.”
From Dr. Mark Czarnota:
“Dr. Murphy has done a good job of explaining the pre- herbicide situation this time of year. The same holds true for ornamental situations.
“I’m seeing a lot of winter weeds now, and they would have been controlled with a late fall pre- application. If there is a lot of winter weed pressure, I recommend an application of pre- in Oct / Nov and that should take folks to Feb / March.
“Two to 4 applications of a pre- herbicide (per year) should be sufficient for homeowners (but less or no applications if they have well-established ground covers and/or a good layer of mulch).”