Sep 28, 2012 – Crabgrass "Control"?


I was wondering if there is anything new or different advice for problems with the dreaded Crabgrass. I haven't tried any Pre-M because of the timing factor for one (not down at the right time and you have wasted your money) and I have seen stunted roots on others that have had Pre-M used. The Crabgrass is in St. Augustine in central Florida.


There probably is not anything strikingly new for crabgrass, and as one university fact sheet on this weed states it, "you cannot eradicate crabgrass and to expect a crabgrass-free lawn is unrealistic". These weeds produce copious numbers of seeds that can lay dormant in the soil for years, finally germinating when the conditions are appropriate. Typically this is when the soil temperature in the top 1/4 inch reaches about 60 degrees and stays there for several days. The seeds also could blow in from neighboring infested sites, so it will be a continual battle. For a homeowner it also pays to spend some time on a nice afternoon hand pulling the weeds, as they generally will not grow back from roots. Also coach the homeowner on proper watering and mowing, which, if possible, would be a deep watering every few days rather than a light daily watering, which encourages crabgrass and weakens the turf. 

Pre-emergent herbicides still may be the best choice, as they can kill the plant as it emerges from the seed and before it becomes visible or certainly well before it can produce more seeds. Some pre-emergents are root-absorbed and some shoot-absorbed, so perhaps changing to a different pre-emergent from the pendimethalin would be appropriate. Making sure the turf is healthy prior to the use of the herbicide will also help to prevent damage to the turf. A turf with healthy, deep roots is going to be less affected by herbicides that remain near the soil surface, and watering prior to the herbicide application is recommended. 

One herbicide on the list of possibilities is Dimension. According to the manufacturer it can be applied as much as 8 weeks prior to germination of the crabgrass seeds, so your window of opportunity is much wider. The active ingredient, dithiopyr, is also either pre- or post-emergent in activity, and it can kill crabgrass weeds that have already sprouted and are visible, but have no more than 5 leaves on the new plant. The a.i. is absorbed both by roots and foliage but is most active in the meristem area of the plant. It is labeled for use on St. Augustine turf, so this may be an alternative you could try. 

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