7 Best Crabgrass Killer For St. Augustine Grass | Kills Crabgrass

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Quinclorac is the most reliable post-emergent active ingredient for killing crabgrass without harming healthy St. Augustine when applied correctly. This 64-ounce concentrate from Quali-Pro contains 17.79% quinclorac, which stops photosynthesis and causes cellular collapse in grassy weeds within 4 days of application. Users report visible wilting and a dried brown appearance within under a week, even on large, multi-tiller crabgrass clumps. The chemistry is selective enough to spare your St. Augustine runners when used as a spot spray with a surfactant.

This product works on over 15 weed species beyond crabgrass, including foxtail, torpedograss, and field bindweed. The coverage estimate of 1,500 square feet per bottle makes it an excellent choice for medium-sized yards with persistent crabgrass patches. For best results, mix one ounce per gallon of water and add a non-ionic surfactant or methylated seed oil to reduce surface tension and prevent the solution from beading off waxy crabgrass leaves.

Some users note that quinclorac is less effective on clover and may require a second application for heavily infested areas. While the upfront cost is higher than granular options, a single bottle often lasts multiple seasons for spot treatment, making it a mid-range investment that pays off through reliable results.

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Primesource’s Quinclorac 1.5L Select offers a slightly higher concentration of 18.92% quinclorac compared to the Quali-Pro version, meaning the same volume of concentrate covers more ground. Users consistently report near-total crabgrass death — estimates of 60 to 98 percent — after a single application when paired with MSO surfactant. The one-gallon jug provides enough material for homeowners with larger properties or multiple applications throughout the growing season.

This post-emergent formula excels on smooth and large crabgrass, foxtail, and torpedograss. The addition of a blue dye indicator helps you track where you’ve already sprayed, preventing overlap that could stress the St. Augustine. Many reviewers note that lawn grass surrounding the treated crabgrass remained completely untouched even at 4 ounces per 2 gallons of water, confirming the selectivity of quinclorac on warm-season turf.

Some users point out that efficacy drops below the 70 percent mark on very dense, multi-year infestations, and heat can make application uncomfortable during peak summer. The bottle’s narrow opening also makes pouring the concentrate a bit fiddly, so a measuring cup is recommended.

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Preen Lawn Crabgrass Control uses dithiopyr, an active ingredient that provides both pre-emergent and early post-emergent activity. This means it can stop crabgrass seeds from germinating and still kill young crabgrass that has already sprouted within the first few weeks after application. The 15-pound bag covers 5,000 square feet, making it a strong mid-range option for homeowners who want a single, easy-to-spread treatment each spring.

Listed as safe for St. Augustinegrass when applied at the directed rates, Preen can be used on both cool-season and warm-season lawns. Users who apply it before soil temperatures reach 55°F consistently report a nearly crabgrass-free season. The granular formulation reduces the risk of drift and accidental overspray that liquid concentrates carry, which is a major advantage for St. Augustine owners who worry about burning their turf.

A small number of users report that the product failed to stop crabgrass on their lawns, possibly due to late application timing or improper watering-in. Preen requires rainfall or irrigation within a few days of application to activate the dithiopyr barrier, and missing this window drastically reduces efficacy.

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Southern Ag Atrazine is one of the few herbicides specifically labeled for broadcast application on St. Augustine grass, making it a trusted fallback for homeowners who want a blanket spray without spot-treating each weed. The 4% atrazine formula targets broadleaf weeds like dollarweed, henbit, and clover while providing moderate suppression of crabgrass. It is also safe for centipede grass, another warm-season turf type with similar sensitivity.

This product works slowly over 2 to 4 weeks, which is normal for atrazine’s root-uptake mechanism. Users who pair it with a surfactant and a secondary herbicide like Weed Free Zone report excellent weed elimination, though they caution that the lawn may temporarily brown before bouncing back. The one-gallon size provides a generous supply for multiple applications through the spring and fall.

The most significant limitation is that atrazine only suppresses crabgrass rather than fully killing it. For an established crabgrass infestation, a quinclorac-based product is a better choice. Additionally, some customers received jugs without labels or safety instructions, which raises a serious safety concern and requires careful inspection upon delivery.

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Mesotrione is a unique active ingredient that works both as a pre-emergent (preventing crabgrass seed germination) and as a post-emergent (bleaching susceptible weeds white before killing them). Liquid Harvest’s 8-ounce concentrate is a direct comparison to the branded Tenacity formula, offering similar chemistry at a more budget-friendly price point. It is explicitly labeled for use on St. Augustine grass that was grown from sod, which is the standard for most Southern lawns.

Users report exceptional results on crabgrass, clover, and wild violet after a single application of 1 teaspoon per 2 gallons of water. The bleaching effect is dramatic — treated foliage turns white within 7 to 10 days before collapsing. For pre-emergent use, the product requires 0.2 inches of rainfall or irrigation within 24 hours of application to activate the soil barrier. It also works on 46 different broadleaf and grassy weed species, making it one of the most versatile spot-treatment options available.

The biggest caveat is that mesotrione can discolor or stunt healthy St. Augustine grass if applied too heavily or to stressed turf. The product is not recommended for use on Bermuda, Zoysia, or bentgrass, and users must be meticulous about measuring and avoiding overlap. A spray dye indicator is highly recommended to track coverage.

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Barricade 4FL from Syngenta is a professional-grade pre-emergent concentrate built around prodiamine, one of the longest-lasting soil barrier chemistries available. A single application can prevent crabgrass germination for an entire growing season when applied before soil temperatures reach 55°F. The 4-ounce bottle treats a substantial area, making it a mid-range option that outperforms many granular products in longevity and precision.

Prodiamine is safe on established St. Augustine grass when applied at label rates, and it targets up to 30 species of both summer and winter annual weeds. Users who apply this product early in the spring consistently report zero crabgrass breakthrough, even in high-pressure yards. The liquid formulation allows you to mix only what you need and adjust coverage for oddly shaped lawns or beds.

This is a strict pre-emergent — it will not kill crabgrass that has already germinated. Users who miss the early spring window will need to switch to a post-emergent product. The concentrate also requires careful mixing measurement to avoid over-application, and some homeowners prefer granular spreaders for simplicity.

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Hi-Yield’s granular formula uses Dimension (dithiopyr), the same active ingredient found in many premium crabgrass preventers, at a more budget-friendly price point. The 12-pound bag covers 5,000 square feet and can be applied up to 4 weeks later than many other pre-emergent products, giving you a wider application window if you miss the early spring timing. It also provides both pre and early post-emergent activity, catching young crabgrass that has already germinated.

St. Augustine owners who use this product consistently report significant reductions in crabgrass, goosegrass, and henbit after two applications — one in early spring and another in fall. The granular form eliminates the need for a sprayer and reduces the risk of drift damage. Users also praise the dimension chemistry for not damaging ornamentals and landscape beds when applied nearby.

A common complaint is that the bag weight can be inconsistent; some customers received a 9.6-pound bag instead of the advertised 12 pounds. The product also requires consistent use over multiple seasons for full results — a single season of application will not eliminate a long-established crabgrass seed bank. Timing remains critical: applying after the spring window opens reduces the first-year benefit considerably.