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 New Mexico, St. Augustine is grown alongside Bermuda, 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 4b–9a, and local soil reads: Calcareous desert soils across the basins and mesas, with caliche — a cemented lime hardpan — at shallow depth in much of the state.

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 New Mexico is New establishment gets 0.5 to 0.75 lb of nitrogen per 1,000 sq ft every other week for six to eight weeks. After that the calendar follows the grass, not the state: cool-season lawns in the north feed spring and autumn, warm-season lawns in the south feed through the warm months and stop well before dormancy. Soil pH here reads: Alkaline, and specifically lime-buffered: NMSU's position is that soil pH here is controlled by the presence of calcium carbonate, which is why a percent-lime figure is a useful thing to test for and why pushing pH down is largely futile. Above pH 7.6 the iron chelate you buy matters — NMSU names EDDHA or DTPA over EDTA, and warns that DTPA should not be used above 7.6 at all, which locks iron away chemically — pale St. Augustine in this state is very often iron chlorosis rather than hunger, and chelated iron fixes it without forcing growth.

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

New Mexico drainage reads: Fast through the sandy desert soils until it reaches caliche, at which point it stops — a shallow lime pan perches water, blocks roots and turns a deep watering into a shallow one, so the depth to caliche matters more here than the surface texture does. Fast-draining soil means more frequent but still deep watering, and split fertilizer doses since nitrogen leaches quickly.

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.

New Mexico Lawn Threat Matrix

What to scout for on Bermuda lawns in USDA zones 4b–9a.

High-Risk Weeds

• Crabgrass in irrigated turf
• Puncturevine — goathead, the signature southwestern hot-dry-ground annual and a tyre and bare-foot problem as much as a turf one
• Field bindweed
• Kochia and Russian thistle at the dry margins
• Khakiweed and spurges in thin, hot, compacted ground
• Bermudagrass encroaching into a northern bluegrass lawn, which is the transition zone working against you
• Annual bluegrass (Poa annua) in over-irrigated cool-season turf

High-Risk Diseases

• Iron chlorosis — not a pathogen, but the most common reason a New Mexico lawn is yellow, and the one where buying the wrong chelate wastes the money entirely
• Salinity injury from irrigation water and from salts accumulating above caliche, which reads as drought stress that watering does not fix
• Spring dead spot on bermudagrass, appearing as the lawn greens up
• Summer patch and other root-zone diseases on cool-season lawns in the north
• Dollar spot
• Large patch on zoysia as soil passes through the 70s

Common Pests

• White grubs from May and June beetles and masked chafer
• Chinch bugs in hot dry sandy strips, which describes most of the state
• Bermudagrass mites in the south
• Sod webworm
• Cutworms and armyworms in monsoon years
• Bluegrass billbug in the northern cool-season lawns
• Red imported fire ants in isolated instances only, with Dona Ana County under quarantine: nursery stock, sod, soil, baled hay and straw, and soil-moving equipment leaving the county must be certified free of the pest

The New Mexico St. Augustine Year

Generic calendars are less useful than your own. For New Mexico — Roughly 8 to 15 inches across most of the populated areas, with a large share of it arriving in the North American monsoon between about June 15 and September 30 as heavy convective storms — which run off caliche and compacted ground rather than soaking in — extension guidance for this climate runs:

• Spring: Pre-emergent on soil temperature — February in Las Cruces, May in Taos, and treating those as the same date is the commonest mistake here
• Spring: Check the depth to caliche with a probe or a shovel before planning any irrigation change; it sets how deep a soaking can physically go
• Spring: Do not lime, ever, unless a test asks: pH here is held up by calcium carbonate already in the soil
• Spring: Audit the irrigation system before the pre-monsoon heat, not after it
• Summer: NMSU's water figures are the ones to plan against: cool-season turf uses about 0.26 to 0.50 inch a day in summer, roughly 1.8 to 3.5 inches a week, and about 38 inches over a year, against about 24 inches for warm-season turf
• Summer: Schedule irrigation for the driest quarter of the lawn rather than the average — residential systems here average a lower-quarter distribution uniformity around 0.50, so half the lawn gets far less than the timer suggests
• Summer: During the monsoon, turn the controller down: heavy convective rain on caliche runs off, but it still resets the soil moisture the system is watering for
• Summer: If the lawn is yellow, treat it as iron availability before nitrogen, and buy EDDHA-chelated iron rather than EDTA
• Fall: Cool-season lawns in the north: late August into September is both the seeding window and the most valuable feeding
• Fall: Warm-season lawns in the south: last feeding well before dormancy, and expect colour loss from the first frost regardless
• Fall: Take a soil test and specify the New Mexico set — Olsen phosphorus, saturated paste EC, sodium adsorption ratio and percent lime

For patching, note that St. Augustine cannot be seeded — plan sod or plugs inside your local establishment window: Warm-season lawns go in from late spring through midsummer once soil is thoroughly warm; cool-season lawns in the north go in late August into September. Establishment irrigation is the whole battle in either case. Local last frost: Late May to June at elevation; mid-March in the far south.

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.