Preemergence Crabgrass Herbicide Timings

By:  Leon Warren and Dr. Travis Gannon

At this writing (Feb 28), North Carolina has been in the midst of a very mild winter with no cold weather predicted in the foreseeable future! What does this mean for turfgrass managers who plan to apply PRE crabgrass herbicides? Based on 30 year soil temperature averages, it means that there is very little room for procrastination as March approaches.

Much has been researched and written about the importance of applying PRE herbicides in a timely manner, and more critically, activating these herbicides with rainfall or irrigation before crabgrass germination. Yes, DNA herbicides (pendimethalin, prodiamine, oryzalin, trifluralin, etc.) are slightly forgiving as they can control crabgrass in the 1 to 2 leaf stage if activated. Dithiopyr can control crabgrass at a very slightly larger growth stage (<1 tiller), but none of these herbicides should be used as POST products. It is comforting to know that control can be achieved if applied and activated a little late for whatever reason. For oxadiazon (Ronstar, Quali-Pro Oxadiazon) users, remember that this particular herbicide has no POST activity at all and must be activated before crabgrass germination or control will be 0%.

January and February 24-hour mean soil temperatures averaged 47 and 48 F, respectively in 2012. The average for these two months is 42 and 44 F, respectively. Based on 30 year soil temperature data from Raleigh, the only January warmer than this year was in 1991 when soil temperature averaged 50 F. February of 1989 and 1991 averaged 51 and 50 F, respectively, to claim the warmest temperatures for that month. February 2012 comes in third place out of 30 years.

The concern with these warm winter soil temperatures from a historical perspective is that the warming trend continues throughout March. 1989 and 1991 data show that along with the warmest January and February soil temperatures, March temperatures for these years were also the warmest in the last 30 years. 51 F is the average March soil temperature but in 1989 and 1991, 57 F was averaged. Therefore, if this trend continues in 2012, expect March soil temperatures to continue warming at an above-normal rate.

Remember that crabgrass germinates when the 24-hour mean soil temperature approaches 55 F with goosegrass about 2 weeks behind, at 62 F. With the mild winter we’ve experienced so far and the showers we are finally recently receiving, soil moisture and temperature should be conducive for earlier than expected crabgrass germination. Be sure to have your PRE crabgrass herbicides down and don’t get caught with them sitting in your shed! In other words, apply ASAP!!! To date, no crabgrass seedlings have emerged, even in natural areas so all PRE herbicide options are still viable.



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