Our readers keep the lights on and my morning glass full of iced black tea. As an Amazon Associate, I earn from qualifying purchases.3 Best Crabgrass Killer For St Augustine Grass | Saves Your Lawn

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The potent concentrate that bleaches crabgrass white before wiping it out.

You reach for this when you have visible crabgrass that needs to go now. Its active ingredient mesotrione works by blocking photosynthesis in the weed, and buyers report it “bleached out crab grass white” within days of the first spray. That visual feedback is faster than the 15-pound Preen bag, which is a pre-emergent and shows no visible results for weeks. This bundle is labeled safe for St. Augustine grass (sod only), so you get aggressive weed control without worrying about your main turf. The bundle includes an 8-fluid-ounce mesotrione concentrate and an 8-ounce surfactant, which is a chemical that helps the spray stick to the leaves for better absorption.

You do need to plan around watering. The instructions say that if no rainfall occurs within 10 days of application, you must water in the product with about 0.15 inches of water to activate it. Some reviewers noted it takes two to three applications to fully kill stubborn weeds, and one reviewer noted that a second dose was needed to finish the job. At 8.32 ounces total weight, this is a spot-treatment tool for targeted areas, not a full-lawn broadcast solution — its coverage is listed at 2,000 square feet, versus the Preen bag’s 5,000 square feet, so it suits smaller infestations.

Unlike the Preen granules that work through the soil, this Liquid Harvest bundle acts as both a pre- and post-emergent, meaning you apply it to weeds that have already sprouted or as a pre-emergent. It targets 46 broadleaf species and grasses including crabgrass, clover, and dandelion, so it is versatile beyond just one weed type. One reviewer praised it as a “Poa killer!” and noted that surrounding fescue got white tips but recovered after a couple of weeks. If you are dealing with crabgrass that is already several inches tall, this is the faster, more targeted option than the Preen granules.

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The 15-pound pre-emergent bag that covers a whole season and a whole 5,000 sq. ft.

You want to stop crabgrass before it even shows its face, and this is the play. The active ingredient dithiopyr (a chemical that stops cell division in germinating seeds) prevents crabgrass seeds from germinating. Unlike many pre-emergents, Preen says you can apply it up to 4 weeks after the crabgrass has already emerged, giving you a bit of forgiveness if you are late on your spring treatment. It covers 5,000 sq. ft. per 15-pound bag, while the 2-pound Agralawn powder covers about 100-200 feet.

Owners mention mixed but generally positive results. One reviewer in New Jersey noted that after four years of using Preen products, their lawn was nearly weed-free. Another user mentioned, “I have seen darkening of crabgrass in my lawn,” after four weeks, though they felt a second application was still needed. The granular form is easy to apply with a standard broadcast spreader, and it is labeled for use on St. Augustinegrass as well as many other warm and cool-season grasses like bermudagrass, zoysiagrass, and centipedegrass.

Unlike the Liquid Harvest mesotrione which works on contact with leaves, this Preen product works through the soil to stop germination. That makes it the better choice for early spring prevention, but it is less useful if you already have a full-blown crabgrass invasion in midsummer. One thing to note: the coverage number is for prevention, not for killing mature crabgrass that is already several inches tall — in that scenario, you will want to pair it with a post-emergent like the Liquid Harvest bundle.

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The cinnamon-based powder that is so gentle you almost forget it is a herbicide.

This is the option for anyone who is nervous about using harsh chemicals on their St. Augustine grass. The active ingredient is cinnamon, a naturally derived substance that the manufacturer says is absorbed through the leaves to kill weeds and roots with “very little reaction” to St. Augustine, Bermuda, Bahia, and Centipede grasses. It comes as a powder in a 2-pound container (the lightest of the three picks) and covers about 100-200 feet, so it is designed for small spot treatments rather than whole-lawn broadcasting.

Customers note that it works, but with a catch. One buyer mentioned, “At first we were afraid it was killing the good grass as it yellowed,” though after regular watering the crabgrass died and the lawn recovered beautifully. Another reviewer gave it 1 star, claiming it killed their Bermuda grass and not the weeds, and noted the powder “blows back at you when apply.” The manual nature of the powder application means you need to be careful on windy days, and the 2-pound bag is dramatically smaller than the 15-pound Preen bag, so it suits small infestations only.

Compared to the Liquid Harvest mesotrione concentrate, which arrives as a liquid you mix with water, this Agralawn powder is applied dry directly to damp weeds. It is post-emergent only and works on visible crabgrass, similar to the Liquid Harvest pick, but the cinnamon ingredient is a much softer approach. One reviewer summed it up as “mixed feelings but it works,” noting that it kills crabgrass but requires full coverage — missed spots survive.