Crabgrass is a common summer annual weed that quickly invades warm-season lawns. Controlling it in St. Augustine turf is challenging because this grass variety is highly sensitive to many powerful herbicides. Applying the wrong chemical can severely damage or destroy the lawn. Effective control requires a careful, two-pronged approach of prevention and selective treatment.
Identifying Crabgrass and Optimal Treatment Timing
Crabgrass is a summer annual grass, meaning it sprouts in spring, grows throughout the summer, produces seeds, and dies with the first frost. It is identified by its coarse, light-green blades and a distinctive growth pattern that radiates outward from a central point. This growth pattern resembles a crab’s legs. It often grows low to the ground, allowing it to evade mower blades and shade the surrounding turf.
The timing of control efforts is directly tied to the weed’s life cycle. Prevention is more effective than treatment after the weed has emerged. Crabgrass seeds begin to germinate when the soil temperature at a depth of one to two inches reaches a consistent 55°F to 60°F for four to five consecutive days. This temperature threshold is the precise window to target for preventative measures.
Pre-Emergent Strategies for Prevention
Preventing crabgrass seeds from sprouting is the most effective control method for St. Augustine grass. Pre-emergent herbicides work by forming a chemical barrier in the top layer of the soil. This barrier stops the root development of germinating seeds. The barrier must be established before the critical soil temperature is reached in early spring.
When selecting a product, choose an active ingredient labeled safe for St. Augustine turf. Effective options include Prodiamine, Pendimethalin, and Dithiopyr. Some products containing Atrazine are also safe for use as a pre-emergent on St. Augustine. Atrazine offers the added benefit of broadleaf weed control.
For the chemical barrier to work, the product must be applied evenly and watered in immediately with about half an inch of irrigation. The water moves the chemical into the soil surface, activating the barrier where the seeds are located. Dithiopyr offers an advantage because it provides both pre-emergent control and limited early post-emergent control for new seedlings.
Post-Emergent Solutions for Active Infestations
Once crabgrass has fully emerged and is actively growing, chemical control becomes more difficult and hazardous for St. Augustine turf. Most post-emergent crabgrass killers, such as those containing Quinclorac or 2,4-D, will severely injure or kill St. Augustine grass. This sensitivity limits chemical options and makes non-chemical methods the safest approach.
The safest method for small or scattered infestations is manual removal. First, water the area to soften the soil around the weed’s root system. Next, use a small hand trowel or weeding tool to loosen the soil around the base of the plant. Finally, grasp the plant firmly and pull slowly to ensure the entire root system is removed. Remaining root fragments can allow the plant to regrow.
For larger areas, a few highly selective herbicides may be used, but only with extreme caution and strict adherence to the product label. Atrazine is a selective post-emergent option for St. Augustine, effective on young crabgrass. It carries a risk of turf discoloration if applied incorrectly or during high temperatures. Another selective herbicide, Celsius WG (active ingredient: thiencarbazone-methyl, iodosulfuron-methyl-sodium, and dicamba), is safe for St. Augustine and controls both crabgrass and other weeds. Solarization—covering the area with clear plastic sheeting during intense heat—can kill large, dense patches by superheating the soil. However, this process also kills the St. Augustine underneath, requiring replanting.
Long-Term Lawn Health Through Cultural Practices
A dense, healthy St. Augustine lawn is the best long-term defense against crabgrass. Crabgrass seeds require sunlight to germinate, so cultural practices that promote a thick canopy will suppress the weed.
Maintaining a higher mowing height is fundamental to this defense strategy. St. Augustine grass should be mowed at a height between three and four inches. This taller turf shades the soil surface, blocking the sunlight needed for crabgrass seeds to sprout. This effectively creates a natural pre-emergent barrier.
Proper watering techniques reduce weed pressure by encouraging deep, resilient root growth. Instead of shallow, frequent watering, provide deep, infrequent irrigation that delivers about one inch of water per week. Deep watering promotes a stronger root system that helps St. Augustine outcompete weeds for resources. Shallow watering favors weed growth.
A balanced fertilization schedule ensures St. Augustine maintains the density necessary to choke out invading weeds. Apply a nitrogen-rich fertilizer during the active growing season in spring and summer. This promotes lateral growth and fills in thin spots. A thick, vigorously growing lawn leaves little open soil for crabgrass to colonize.