St. Augustine Care: Mow High, Watch for Chinch Bugs

St. Augustine is the grass people choose when they have shade and want a southern lawn. It has the broadest blade and the most coarse texture of the common warm-season grasses, it spreads with thick above-ground runners, and it is managed almost exactly opposite to Bermuda — mow it high, not low.

On the weed side this grass is the most herbicide-sensitive common lawn grass in the country, and the sensitivity differs by cultivar — see best weed killer for st augustine before buying anything labelled for southern lawns.

What Makes St. Augustine Different

Four traits drive every care decision, and three of them differ from Bermuda:

• Coarse, broad blades. Distinctive and easy to identify — nothing else in a southern lawn looks like it up close.
• Thick above-ground stolons. It spreads across the surface with visible runners rather than knitting tightly at soil level. Good for covering ground, and the reason low mowing exposes and damages it.
• The best shade tolerance among warm-season grasses. This is its main selling point over Bermuda and the usual reason people pick it.
• No seed. Established only from sod, plugs or sprigs. Repairs cost money rather than a handful of seed.

In Tennessee, St. Augustine is grown alongside Tall Fescue, which is the dominant grass here. Mixed yards are common, and the two need opposite mowing heights — a single deck setting cannot serve both. Your USDA range is 6a–8a, and local soil reads: Three different states in one: wind-blown loess deep enough to farm around Memphis, thin clay over limestone and cedar glade in Middle Tennessee, and stony acidic mountain soils east of the plateau.

Floratam and the other cultivars

Floratam is the most widely planted St. Augustine cultivar and the one most often named specifically on product labels. It is coarser and needs more sun than some alternatives, and it carries herbicide restrictions that a generic “St. Augustine” instruction may not cover. If you do not know your cultivar, treat label warnings about Floratam as applying to you until you find out.

NC State’s summary is worth having in one line before the maintenance detail: St. Augustinegrass “is a fast-growing turfgrass best adapted to the coastal plain” with “very coarsely textured leaves”, and it “spreads by stolons” — above-ground runners rather than underground ones. That last point explains most of what follows: everything it uses to spread is sitting on the surface where a low mower can reach it.

Mowing High Is Not Optional

The single most common failure is someone applying Bermuda habits to a St. Augustine lawn.

• Height: 3–4 inches. Floratam at the top of the range. Higher than most people expect.
• Never remove more than a third of the blade. At a 3.5 inch target, mow when it reaches about 5 inches.
• Sharp blade matters more here, not less — a coarse blade shreds visibly when the mower is dull, and the frayed tips brown across the whole lawn.
• Do not scalp. Scalping is a Bermuda practice. On St. Augustine it removes the leaf area the plant depends on and exposes stolons to sun scald.

Mowing high is also weed control: a tall, dense St. Augustine canopy blocks light from reaching the soil, and weed seeds cannot establish in the dark.

Feeding and Watering

St. Augustine is a moderate feeder — lighter than Bermuda — and the same warm-season timing rule applies: only while it is actively growing.

Your window in Tennessee is Fescue: September through November; Bermuda and zoysia: May through August. Soil pH here reads: Acidic (5.0–5.8) in the east, near-neutral over the Middle Tennessee limestone, so the lime a Knoxville lawn needs is money wasted in Murfreesboro, which generally keeps iron available, so pale growth here is more likely genuine hunger or a root problem.

On watering, the drainage of your soil matters more than a fixed schedule:

Tennessee drainage reads: Loess drains freely and erodes in the west; Middle Tennessee's shallow clay over bedrock ponds in spring and cracks open by August. Slow-draining soil means the real risk here is overwatering — St. Augustine sitting in wet soil is how brown patch and take-all root rot get started, so let it dry between waterings.

Chinch Bugs: The Signature Pest

If a St. Augustine lawn is browning in summer, chinch bugs are the first suspect. They are strongly associated with this grass.

How to recognise it:

• Damage starts in the hottest, sunniest strip — beside a driveway, a south-facing wall, a curb — and expands outward in heat waves.
• The turf yellows, then browns, and does not green up with watering.
• The pull test stays firm. Chinch bugs feed on stems and crowns, not roots, so the turf does not lift like a rug. That is what separates them from grubs.
• Part the grass at the margin of a damaged area and watch the soil line for small moving insects.

Drought damage looks similar and is the usual misdiagnosis. The difference is that drought yellowing follows your irrigation coverage, and it recovers when you water deeply. Chinch bug damage does not.

Our chinch bug guide has the float test that confirms it in five minutes, and the documented insecticide resistance that decides what is worth buying. The other Southern lawn pest worth knowing is not a grass feeder at all — fire ants build mounds instead, and the fix is bait rather than anything sprayed on the turf.

Tennessee Lawn Threat Matrix

What to scout for on Tall Fescue lawns in USDA zones 6a–8a.

High-Risk Weeds

• Dallisgrass (a perennial the quinclorac that clears crabgrass will not touch)
• Crabgrass
• Wild garlic
• Annual bluegrass (Poa annua)
• Henbit
• Nutsedge

High-Risk Diseases

• Brown patch (tall fescue's summer nemesis statewide)
• Large patch (zoysia)
• Spring dead spot (Bermuda)
• Dollar spot
• Gray leaf spot

Common Pests

• White grubs
• Fall armyworms (August invasions)
• Billbugs (zoysia)
• Imported fire ants, first confirmed as an isolated Shelby County infestation in 1948 and now covered by a Tennessee Department of Agriculture quarantine whose county list is reissued as the range moves; UT Extension publishes the two-step bait method as SP419
• Sod webworms

The Tennessee St. Augustine Year

Generic calendars are less useful than your own. For Tennessee — Wet (~52 in/year), spring-heavy with humid summers; late-summer dry snaps stress fescue — extension guidance for this climate runs:

• Spring: Pre-emergent sweeps west to east — Memphis in early March, Knoxville three weeks later
• Spring: Lime only east of the plateau; Middle Tennessee limestone counties rarely need it
• Spring: Hold spring nitrogen on fescue to a minimum — the fall feeding is the one that counts
• Spring: Green-up feeding for Memphis zoysia and bermuda from late April
• Summer: Brown patch watch on fescue every humid week — preventive fungicide often pays
• Summer: Mow fescue at 4 inches in summer heat
• Summer: Feed warm-season lawns monthly through August
• Summer: Scout for armyworm moths and egg masses in late July
• Fall: Aerate and overseed fescue mid-September to mid-October
• Fall: Feed fescue September, October and November
• Fall: Fall pre-emergent for Poa annua on warm-season lawns

For patching, note that St. Augustine cannot be seeded — plan sod or plugs inside your local establishment window: Overseed fescue mid-September to mid-October; sod warm-season lawns in early summer. Local last frost: Mid-April (east) to late March (west).

The Herbicide Problem

This is where St. Augustine lawns die, and it deserves its own section.

Several common broadleaf herbicides that are routine on Bermuda will injure or kill St. Augustine, and Floratam is singled out by name on many labels. Atrazine-based products are widely used on St. Augustine precisely because the standard three-way mixes are risky on it.

Three rules, none of them optional:

1. The label must name St. Augustine. Not “southern lawns”, not “most grasses” — St. Augustine, and ideally your cultivar.
2. Respect the temperature ceiling. Most turf herbicides injure grass above roughly 85–90°F, and St. Augustine has less margin than Bermuda does.
3. Spot-treat rather than blanket-spray. Less product, less risk, same result on scattered weeds.

Our lawn weed identification guide covers matching the weed family to the right product family before you buy anything, and how to kill dandelions without killing grass walks the selective-herbicide basics.

The Care Plan

Products for a St. Augustine Lawn

One piece of good news on this grass: gray leaf spot names St. Augustine constantly, but NC State ranks it the MOST resistant host — rarely badly damaged when it is managed.

And the weed that most often follows over-watering on this grass has its own page — dollarweed, where the first fix is the irrigation and the herbicide table is sorted by grass.

Use that last one only once you have confirmed the insects, not as insurance. It treats the entire lawn, and the bifenthrin in it is toxic to bees and aquatic life — so it belongs on a lawn with a real problem, kept off blooming weeds and away from drains. If you only need to feed, feed.

Note what is not here: a general-purpose three-way broadleaf herbicide like Weed B Gon. On St. Augustine, applying common herbicides blind is how lawns get killed. The Texas A&M herbicide tolerance table lists Quinclorac as “NR” (Not Recommended) for St. Augustine, and common actives like 2,4-D and dicamba as “C” (Caution). The only fully “T” (Tolerant) consumer active is Atrazine. Identify the weed first, then find an Atrazine-based product or confirm the label specifically names your St. Augustine cultivar.

If you are considering a robotic mower, check its maximum cutting height first: UF/IFAS notes most were designed for grasses mowed below 2.5 inches, while St. Augustine wants 2.5–4. Robot mowers on St. Augustine grass covers UF’s trial at 2.4 inches and the delayed dormancy that came with it.

If a patch thins in the same place each spring and the runners lift off the soil like a rug, that is not drought — take-all root rot covers the field test that separates it from large patch and chinch bugs, and why fungicide sprayed on a visible patch does nothing.