Gardening Tips: Pre Emergent Control of Crabgrass and Other Summer Weeds

— Written By N.C. Cooperative Extension

As you know, this winter has been unusually warm with very few days that have actually felt like January or February.  Now, as we get closer to spring, there simply isn’t much time left for us to experience that cold weather winter normally brings.  As such, plants are reacting a bit differently then they normally do at this time of year. 

            For example, today as I was driving around the county I noticed forsythia just starting to bloom.  As I write this article, it is still February and forsythia doesn’t usually bloom in our area until mid or late March.  This got me thinking.  Forsythia in bloom is often a good reminder that it is time to put down pre-emergent herbicides for control of crabgrass and other summer weeds.  Applying pre-emergent while forsythia is in bloom typically assures that the herbicide is in place when weed germination begins, without being so early as to allow the herbicide to degrade before germination.  Could it really be time for that already? 

            As it turns out, it certainly is time.  Crabgrass begins to germinate when the average daily soil temperature approaches 55 degrees.  The following excerpt, from a report by specialists in the NCSU crop science department posted to www.turffiles.ncsu.edu shows that 55 degree soil temperatures are just around the corner:

“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.”

There are many options for pre-emergent control of crabgrass and other summer weeds.  These herbicides include pendimethalin (Pendulum, Scott’s Halts Pro), prodiamine (Barricade), dithiopyr (Dimension), bensulmide (Bensumec) and others.  Make sure to follow the label applications of the product you use for safety and best results. 

Matt Stevens is the horticulture extension agent for the North Carolina Cooperative Extension Service of Halifax County.  If you have any questions about this article or any other aspect of horticulture or gardening, you may contact Matt at 583-5161 or matt_stevens@ncsu.edu.

Posted on Mar 1, 2012
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