When I get a call about how to get rid of crabgrass in the lawn, I get a little tense. My first question is always, “What type of grass do you have?” When I hear the reply St. Augustine, I know the client isn’t going to like my answer.
There is no post-emergent chemical to control crabgrass in St Augustine lawns. So, what can we do? Hand dig it or wait until the frost kills it. Then you have to apply pre-emergence herbicides in the spring before the weeds emerge, so the crabgrass can’t take ahold the following summer.
We essentially have two seasons of annual weeds, warm season and cool season. Crabgrass is our peskiest summer annual weed, while poa annua and henbit are winter’s greatest annual invaders. The absolute best way to minimize issues with these weeds is to treat with a pre-emergence in a timely manner.
When soil temperatures fall below 70 degrees, our cool season weeds like poa annua and henbit begin to germinate. For warm season weeds like crabgrass, germination begins around 55 degrees and continues until 73 degrees. So applying pre-emergence herbicides prior to weed seed germination is essential. When seeds begin to germinate, these type herbicides will kill the tender seed by stunting the root development. Once you see the weed, you are too late.
Sept. 15 is our standard date in our area to get out the pre-emergence herbicides for the cool season, while the spring treatment is March 15. These dates are just recommendations. Check out the UGA weather site georgiaweather.net to view current soil temperatures. For clay soils like in Columbia County, use the weather station on the website for Clarks Hill, S.C. For the sandy Augusta area, the weather station in Dearing, Ga., is going to have a more similar reading. When visiting georgiaweather.net, look for the soil temperatures reading for “2-inch soil.” Currently, Clarks Hill is 78.3 degrees and Dearing is 84.4.
Allowing these pre-emergence products to disturb the natural germination process is important. Some of the common turfgrass pre-emergences have the active ingredient pendimethalin (Lesco Crabgrass Preventer) and prodiamine (Barricade). The easiest way to broadcast these products is with a rotary spreader, so get a granulated formula. I also like to get one with the 0-0-7 fertilizer to add a little potassium to lawns. We tend to lack potassium in our lawns, which is important for root growth and disease resistance. Different pre-emergence herbicides have varying lengths of effectiveness. Some will last for three months, while others may last half of a year. I use 90 days as a rule of thumb.
Adding a secondary treatment of pre-emergence to the lawn on June 1 and Dec. 1 can also be helpful. Weed seeds tend to accumulate along the edges of concrete walkways and roadsides, and then get blown into your yard by a car or a leaf blower. This second treatment will treat some of these seeds that find their way into the lawn. It is also good to get this second treatment out for seeds that may germinate later in the season.
Some things to be conscious of concerning pre-emergence herbicides are:
Apply only according to the written label on the packaging.
Do not apply to an area that is to be a newly sodded lawn. They will adversely affect the root growth and can kill the sod.
Do not apply where seeding may take place. This includes over-seeding with rye grass seed, new lawn bermuda and centipede seeds, wildflower seed or vegetable garden areas.
Make sure that this herbicide gets watered in within a reasonable time frame with rain or irrigation. That usually means about a ½ inch within one to three days.
There is evidence multiple uses of pre-emergence on centipede grass will cause an eventual decline. For the best long-term care, use only one pre-emergence application in the spring, and then use products like atrazine and imazaquin for post-emergence weed control.
Reach Campbell Vaughn, the UGA Agriculture and Natural Resource agent for Richmond County, by e-mailing firstname.lastname@example.org.