(News article for January 30, 2021)
With the warm air temperatures of these past few days, soil temperatures have risen as well. In the Florida Parishes, soil temperatures have entered the upper 50s and low 60s, temperatures at which crabgrass begins to germinate, so it’s time to apply a pre-emergence herbicide for crabgrass, if you plan to use one.
Crabgrass is a warm-season annual weed that comes back each year from seed. Pre-emergence herbicides, as the name suggests, prevent weed seed germination. They do not kill weeds once they have germinated, so the timing of application is important.
One reason we talk more about using pre-emergence herbicides for crabgrass than we do for annual broadleaf weeds is that there are fewer post-emergence herbicide options for selectively killing weedy grasses in turfgrass areas than there are for selectively killing broadleaf weeds. The mentioned pre-emergence herbicides help prevent germination of some broadleaf weeds, as well.
Active ingredients to look for in crabgrass pre-emergence herbicides for lawns include dithiopyr, pendimethalin, prodiamine, indaziflam, and combinations of either benefin and trifluralin or dimethenamid-P and pendimethalin. Dithiopyr has the advantage of having some post-emergence activity on very young crabgrass, meaning that it can kill crabgrass within the first couple of weeks after it has germinated.
As when using any herbicide, be sure to read and follow label directions.
Note that this is not the time to use weed and feed products on lawns. It is too early to fertilize the warm season turfgrasses that we grow, including centipedegrass, St. Augustinegrass, zoysiagrass, and bermudagrass. The first application of nitrogen-containing fertilizer is not recommended until April.
On the other hand, now is a good time to soil test lawns in preparation for springtime fertilizer application. Good cultural practices (including maintaining an appropriate mowing height, soil pH, and fertilizer rate/timing for your turfgrass) are the foundation of good weed management.
Let me know if you have questions.
Contact Mary Helen Ferguson.
A pre-emergence herbicide must be applied before seeds germinate to be effective. It will not be effective on crabgrass plants like this. (Photo by Ron Strahan. Cropped)
Newly emerged crabgrass seedling. (Photo by Ron Strahan. Cropped.)
The LSU AgCenter and the LSU College of Agriculture