How to Control Crabgrass in St Augustine | Lawn-Safe Methods

Before buying any product, learn how to control crabgrass in St Augustine grass by understanding the one timing rule that makes or breaks the effort: pre-emergent herbicide applied at the right soil temperature stops the weed before it starts. Once crabgrass is visible and growing, no herbicide kills it without risking the lawn itself. This guide covers the prevention window, the one post-emergent option that is safe for St. Augustine, and what to do when crabgrass has already taken hold.

Controlling Crabgrass in St. Augustine Grass: Prevention Rules That Work

Pre-emergent herbicides form a chemical barrier that kills crabgrass seeds as they germinate. The critical number is soil temperature: apply when it reaches 55°F. In Northeast Florida that usually falls before March 1, and similar southern regions follow the same soil-temperature trigger. Apply pre-emergent too late—after the seeds have sprouted—and it does nothing.

Products labeled safe for St. Augustine include Prodiamine (Lesco Stonewall) and Atrazine (Hi-Yield). Apply to established, healthy turf only. Thin or bare areas will not fill in after a pre-emergent, because the barrier blocks all seed germination, not just crabgrass. Follow up with a second application six weeks later to extend coverage through the full germination window. According to the UF/IFAS Extension service, fertilizing before mid-April feeds the weeds you are trying to stop—wait until April 15 to apply nitrogen.

Can You Kill Active Crabgrass Without Damaging St. Augustine?

Only if you catch it immediately after sprouting. Celsius WG is the single post-emergent herbicide widely considered safe for St. Augustine, but it must be applied at the higher rate listed on the label during late spring or early summer when crabgrass is still young. It contains iodosulfuron, thiencarbazone, and dicamba. A 10-ounce bottle costs around $100, but the concentration is high enough that one bottle lasts several seasons for a typical home lawn.

For small patches, Blindside (sulfonylurea) works as a spot treatment when the weed is correctly identified. Atrazine offers some post-emergent crabgrass control and is safe for St. Augustine, but repeated applications can build up in the soil and damage the turf over time—limit use to what the label specifies.

For a full comparison of products that actually work on St. Augustine, see our tested roundup of the best crabgrass killers for St. Augustine grass.

Products That Will Kill St. Augustine (Avoid These)

Quinclorac—found in many grassy weed killers such as Quintessential—causes rapid and severe damage to St. Augustine. It works on Bermuda and zoysia but is a confirmed danger to this turf type. Glyphosate (Roundup) kills everything it touches, including St. Augustine. The only scenario where glyphosate makes sense is spot-treating a patch you are willing to replace with fresh sod.

Mechanical Removal: Hand-Pulling and Digging Methods

Pulling crabgrass works when you get the entire root system and the weed is actively growing. The risk is spreading seeds—bag everything immediately after pulling, and never mow over seed heads with the discharge chute open. For established clumps, dig the weed out with a shovel, till the soil to bury any dropped seeds deeper, and lay fresh sod over the bare spot. This method is slow but avoids chemical exposure entirely.

Common Mistakes That Wreck St. Augustine

The most expensive mistake is reaching for a quinclorac-based product—it kills the weed and the lawn together. Applying pre-emergent after crabgrass is already visible is the second most common error; the barrier only prevents new germination. Misidentification also trips up homeowners: dallisgrass and tropical signalgrass look similar to crabgrass but need different treatment timing and chemistry. And baking soda, sometimes suggested as a home remedy, burns the leaf surface but never reaches the roots—the crabgrass returns, and the St. Augustine takes unnecessary stress.

Here is the seasonal game plan that keeps St. Augustine ahead of crabgrass:

Checklist: Your St. Augustine Crabgrass Control Plan

1. Measure soil temperature in late winter—apply pre-emergent when it reaches 55°F.
2. Use Prodiamine or Atrazine labeled safe for St. Augustine.
3. Skip granular fertilizer until April 15.
4. Reapply pre-emergent after 6 weeks for continuous coverage.
5. For young crabgrass in late spring, apply Celsius WG at the higher label rate only.
6. Hand-pull and bag visible crabgrass before seed heads appear.
7. In fall, apply pre-emergent again when soil cools below 70°F.