Crabgrass in a St. Augustine lawn is a common challenge for homeowners in warm-season climates. St. Augustine grass is highly susceptible to damage from many herbicides used for weed control. Eliminating crabgrass requires a strategic approach that prioritizes the safety of the desirable turf. Effective control involves understanding the weed’s life cycle, employing specific preventative chemicals, using carefully selected post-emergent products, and maintaining cultural practices that strengthen the lawn.
Understanding the Crabgrass Life Cycle
Crabgrass is an annual weed, meaning its entire life cycle occurs within a single year. Seeds begin to germinate when the soil temperature consistently reaches 55 to 60 degrees Fahrenheit at a one-inch depth for several consecutive days. This temperature threshold usually occurs in early spring, often coinciding with the blooming of local flowering trees, like forsythia.
The weed continues to grow throughout the summer, thriving in high temperatures. In the late summer and fall, mature crabgrass plants produce vast quantities of seeds before they are killed by the first hard frost. Understanding this annual cycle dictates the timing of all control methods, especially chemical applications.
Preventative Chemical Control
The most effective method for managing crabgrass is using pre-emergent herbicides. These chemicals create a barrier in the top layer of the soil that kills the seedling shortly after germination, before it sprouts above ground. Precise timing is required, meaning application must occur before the soil temperature reaches the 55-60°F germination window.
For St. Augustine grass, active ingredients like prodiamine, dithiopyr, or pendimethalin are considered safe and effective. These options generally do not harm established turf when applied according to label instructions. After application, water the herbicide into the soil immediately to activate the chemical barrier and intercept the germinating seeds.
Post-Emergent Solutions Safe for St. Augustine
When crabgrass has already sprouted and is actively growing, post-emergent herbicides are necessary to eliminate the established plants. St. Augustine grass is sensitive to many common post-emergent chemicals like 2,4-D and quinclorac, which can cause discoloration or severe injury. Therefore, careful selection of products is essential.
One reliable option for post-emergent crabgrass control in St. Augustine is atrazine, which is often used in warm-season turfgrass applications. Atrazine provides both pre-emergent and post-emergent suppression of crabgrass. Another highly effective and selective product is Celsius WG, which contains the active ingredients thiencarbazone-methyl, iodosulfuron-methyl-sodium, and dicamba.
Celsius WG controls a wide spectrum of weeds, including crabgrass, with a high degree of safety on St. Augustine grass, even when temperatures are high. Post-emergent treatments are most successful when the crabgrass is young, ideally with fewer than four tillers. Application should occur when temperatures are moderate, avoiding times of extreme heat, which increases the risk of turf injury and reduces the weed’s ability to absorb the chemical effectively.
Cultural Practices for Turf Resilience
Non-chemical cultural practices are a long-term strategy for minimizing crabgrass invasion by promoting a dense, healthy lawn. Crabgrass thrives in bare patches or thin turf where sunlight reaches the soil, encouraging seed germination. Maintaining a tall mowing height for St. Augustine grass, typically between three and four inches, creates a natural shade canopy.
This shade cools the soil surface, helping keep temperatures below the 55°F threshold necessary for crabgrass seeds to sprout. Proper watering also helps suppress crabgrass; deep, infrequent irrigation encourages the St. Augustine grass to develop robust roots. Conversely, frequent, shallow watering creates the moist, warm soil conditions near the surface that crabgrass prefers. A consistent fertilization schedule provides the necessary nutrients for the turf to maintain its density and successfully compete against weed encroachment.