St. Augustine grass is beautiful, shade-tolerant, and tough enough to hold a lawn together through a North Texas summer — but it has a serious Achilles heel: it is one of the most herbicide-sensitive warm-season turfgrasses grown in the region. Apply the wrong product, or apply the right product at the wrong rate or temperature, and you can wipe out large sections of a St. Augustine lawn in days. We’ve been rehabbing damaged St. Augustine lawns across Arlington since 2006, and a distressing number of those calls come from homeowners who grabbed a weed killer off a store shelf without checking the label. This post covers what you absolutely cannot use on St. Augustine — and why the restrictions matter so much.
Why St. Augustine Is So Herbicide Sensitive
St. Augustine spreads through stolons — above-ground runners that carry nutrients and growth hormones to the expanding edges of the lawn. Unlike Bermuda, it has no underground rhizomes to serve as a backup recovery system. When stolon tissue is damaged or killed by herbicide exposure, that portion of the lawn simply stops spreading. It doesn’t regenerate from below. Coupled with the fact that St. Augustine has a naturally thicker leaf blade and more leaf tissue than Bermuda, it absorbs more herbicide per application and is less able to metabolize certain active ingredients quickly.
Add in the DFW summer heat — which amplifies chemical uptake and speeds phytotoxic reactions — and it becomes clear why product selection matters enormously on St. Augustine turf.
Products You Must Never Use on St. Augustine
Atrazine — Handle With Extreme Care
Atrazine is one of the few herbicides that is actually labeled for use on St. Augustine, and many products marketed specifically for St. Augustine contain it. But it has to be applied within a very narrow temperature and rate window. Applied when air temperatures exceed 90°F, atrazine causes severe bleaching and burn on St. Augustine. Applied at too high a rate, it persists in the soil and prevents spring green-up. Many homeowners assume “it’s on the St. Augustine label so I can pour it on” — that assumption causes expensive damage every year in North Texas. Atrazine is a late fall and winter tool, not a summer treatment.
MSMA and Organic Arsenicals
MSMA (monosodium methanearsonate) was once widely used for grassy weed control and is still available in some markets. It will injure or kill St. Augustine at rates required to control target weeds. Unlike Bermuda, which can handle MSMA at careful rates, St. Augustine lacks the tolerance needed to survive even moderate exposure. Do not use MSMA-containing products on a St. Augustine lawn under any circumstances.
Products Containing Bispyribac-Sodium
Bispyribac-sodium is a selective herbicide used to control grassy weeds including dallisgrass and crabgrass. It is highly effective on those targets — and highly injurious to St. Augustine. Even at low application rates, bispyribac-sodium causes yellowing, stunting, and long-term thinning of St. Augustine turf. It is not labeled for St. Augustine and should never be used on it.
Metsulfuron-Methyl at High Rates
Metsulfuron is used for broadleaf weed control and is present in some products labeled for warm-season turf. While it has a degree of tolerance window on St. Augustine, the margin between effective weed control and turf damage is narrow. Products formulated specifically for Bermuda often contain metsulfuron at rates that exceed what St. Augustine can tolerate. Always verify the label specifies St. Augustine compatibility before using any metsulfuron-containing product.
Quinclorac Without Caution
Quinclorac (sold under the brand name Drive) is commonly used to control crabgrass and is labeled for use on some St. Augustine varieties — but it can cause injury on Floratam, which is by far the most common St. Augustine cultivar in Texas. If your lawn is Floratam St. Augustine (likely if you’re in DFW), quinclorac applications require careful rate management and should be avoided under heat stress conditions.
Temperature Restrictions Apply to Everything
Even herbicides that are labeled safe for St. Augustine carry temperature restrictions that most homeowners ignore. As a general rule:
- Do not apply broadleaf herbicides when air temperatures exceed 85–90°F — heat dramatically increases uptake and injury risk
- Avoid application during drought stress — a stressed lawn is far more susceptible to herbicide injury than a well-watered one
- Do not apply immediately after fertilization — rapidly growing tissue absorbs herbicide faster and injury risk rises
- Early morning applications on cooler days reduce volatilization and minimize off-target movement
In DFW, this means the effective window for broadleaf herbicide applications on St. Augustine is primarily spring (April–May) and fall (September–October). Summer applications are risky and should only be done by professionals who understand the temperature-tolerance relationship.
What IS Safe to Use on St. Augustine
The approved chemistry list for St. Augustine is shorter than most homeowners expect:
- Prodiamine, pendimethalin, and dithiopyr for pre-emergent weed control — these are safe on established St. Augustine and are the backbone of spring and fall pre-emergent programs
- 2,4-D at labeled rates and temperatures for broadleaf weed control — applied correctly in spring or fall, 2,4-D effectively targets broadleaf weeds without turf injury
- Triclopyr-containing products at appropriate rates for tough broadleaf weeds — our post on triclopyr for tough broadleaf weeds in Arlington St. Augustine lawns covers this chemistry in detail
- Atrazine in winter only at labeled rates when turf is dormant and temperatures are below 70°F
- Image (imazaquin) for nutsedge and certain broadleaf weeds — labeled for St. Augustine with appropriate precautions
When in Doubt, Call a Professional
The cost of a professional weed control program is a fraction of what it costs to resod a St. Augustine lawn damaged by the wrong herbicide. We’ve quoted resodding jobs that ran into thousands of dollars for damage that started with a $25 bag of big-box weed killer applied at the wrong time. Our weed control and fertilizer services page explains how we build St. Augustine-specific programs that target weeds without putting your turf at risk — backed by 20 years of working with North Texas grass types.
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