Why can't I use 2,4-D on St. Augustine?

2,4-D — especially the ester formulations — can severely damage or kill St. Augustine, particularly during the heat of summer. The wide, thin blades absorb 2,4-D readily and the grass has no built-in tolerance. Use Celsius WG per label instead. Do not reach for an atrazine-based southern weed-and-feed either — Floratam is not tolerant of atrazine (Texas A&M EHT-144). NEVER use Trimec, Spectracide Weed Stop, or Scotts Turf Builder Weed and Feed (Northern) on St. Augustine.

What's the safest weed killer for St. Augustine grass?

Celsius WG, applied per label — it is labeled for St. Augustine including Floratam, has no upper temperature limit, and kills 200+ weeds. For nutsedge use SedgeHammer (halosulfuron) or Image (imazaquin). Avoid all 2,4-D products, quinclorac, and atrazine — Floratam is not tolerant of atrazine (Texas A&M EHT-144), and most homeowners cannot tell which cultivar they have.

How short can I cut St. Augustine grass?

Floratam needs 3.5–4 inches and Palmetto 3–4 inches (Palmetto is a semi-dwarf, per UF/IFAS ENH5). Dwarf varieties like Seville and Captiva tolerate 2–2.5 inches. Anything lower scalps the stolons and the lawn turns brown for weeks. Tall mowing is the #1 free defense against weeds and chinch bugs on St. Augustine.

How much nitrogen does St. Augustine grass need per year?

2–5 pounds of actual nitrogen per 1,000 square feet per year, regional: North FL 2–4, Central 2–5, South FL up to 6, split across 3–5 applications from spring green-up through August. Stay closer to the low end in shaded lawns and on poor soils, higher on well-irrigated full-sun lawns. Exceeding your regional max invites brown patch, gray leaf spot, and chinch bug pressure.

How do I know if I have chinch bugs?

Yellow-to-brown patches in hot, sunny edges of the lawn that don't respond to watering. To confirm, do a float test: cut both ends off a coffee can, push it into the soil at the damage edge, fill with water — chinch bugs float to the top within 5 minutes. Treat with bifenthrin or imidacloprid drench, and back off the nitrogen.

Can I seed St. Augustine grass?

No — St. Augustine has no commercial seed option. Bare spots and new lawns must be plugged or sodded. Plugs are cheaper and fill in within 2–3 months during peak growth (May through August). Sod is the fastest fix for large bare areas.

Is St. Augustine grass good for shade?

Yes — St. Augustine is the most shade-tolerant warm-season grass, thriving with as little as 6 hours of sun. Dwarf varieties like Seville and Captiva handle even more shade. It outperforms Bermuda, Zoysia, and Centipede in partially shaded lawns across the Gulf Coast and Florida.

When should I fertilize St. Augustine grass?

Fertilize only when the lawn is actively growing — typically April through August in zones 8–10. Apply 0.5 lb of nitrogen per 1,000 sqft per application, 3–5 times per season. Match your region for the yearly total — up to 4 lbs North FL, 5 Central, 6 South FL — and stop 60 days before the first expected frost.

Warm SeasonZones 8 – 10Sun + Part Shade

The Complete Guide to
St. Augustine Grass

Everything you need to know — tall mowing heights, no-2,4-D weed control, chinch bug treatment, gray leaf spot fixes, and a month-by-month plan built for the Gulf Coast.

Right now: slowing

October — Apply fall pre-emergent (split with August) for Poa annua control

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Quick Reference

Mow Height

3.5 – 4"

Floratam, BitterBlue & Classic: 3.5–4". Palmetto & Raleigh: 3–4" (semi-dwarf). CitraBlue: 2–3". Dwarfs (Seville, Captiva, Delmar): 2–2.5". Tall mowing shades out weeds.

Water / Week

0.75 – 1"

Deep and infrequent. 2x per week in summer, early AM only. Avoid evening watering.

N per Year

2 – 5 lbs

Per 1,000 sqft, regional: North FL 2–4, Central 2–5, South FL 4–6. Split across 3–5 apps; over-feeding beyond your regional max invites brown patch and chinch bugs.

Best Zones

8 – 10

Gulf Coast and Florida. Cold-sensitive — damaged below 25°F.

Soil pH

6.0 – 6.5

Ideal 6.0–6.5; tolerates up to 7.5. Iron chlorosis appears in alkaline soils above 7.5.

Sun Needs

Sun + Part Shade

The most shade-tolerant warm-season grass. Needs 6+ hours direct, more in heavy shade varieties.

What makes St. Augustine different

St. Augustine is the dominant lawn grass of the Gulf Coast and Florida. Wide, coarse blades, aggressive stolons (above-ground runners only — no rhizomes), and the best shade tolerance of any warm-season grass. The catch: it has no seed option (must plug or sod), it's chinch bug candy, and 2,4-D will damage or kill it. The herbicide playbook is completely different from Bermuda or Zoysia.

Highest shade tolerance in warm seasonCoarse textureStolons only (no rhizomes)Plug or sod — no seedChinch bug magnetSensitive to 2,4-D

Month-by-Month Calendar

Tap a month

October

slowing

Mow

3.5 – 4" every 10 days

Water

Rainfall only unless drought

Fertilize

Light potassium only. No more nitrogen.

This month's tasks

Apply fall pre-emergent (split with August) for Poa annua control

Apply preventive brown patch fungicide as nights cool to the 60s

Spot treat winter broadleaf weeds with Celsius WG per label

See the St. Augustine month-by-month playbook

The full St. Augustine calendar — every task, every month, with priority and timing for every season.

Open the St. Augustine calendar

Pre-Emergent Timing

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Pre-emergent is the single most important application of the year. It prevents crabgrass, goosegrass and other weeds before they germinate. Once you see weeds, it's too late.

Spring (Crabgrass, goosegrass Prevention)

Critical

Apply when your 5-day average soil temperature reaches 55°F. Crabgrass seeds germinate when soil temperatures reach 55°F at the 4-inch depth for several consecutive days. Pre-emergent must already be down BEFORE that — once you see the first crabgrass blade, the chemical barrier is too late.

Typically late Feb – early Mar (zone 8 – 10)Varies by zone

Fall (Poa annua (annual bluegrass) Prevention)

Recommended

Apply when your 5-day average soil temperature reaches 70°F. Poa annua germinates as soil temps drop back through 70°F in fall. Apply your fall pre-emergent BEFORE that threshold to prevent the worst winter weed in southern lawns.

Typically Sep – OctVaries by zone

Timing matters more than product. Too early and it breaks down before weeds germinate. Too late and crabgrass is already growing — pre-emergent can't stop what's already up. Track your local soil temperature, not the calendar date.

What to use

Prodiamine 65 WDG (Barricade)

Specialty

Rate: 0.18 oz per 1,000 sqft (split spring + fall)

The gold-standard pre-emergent. Safe across the board on St. Augustine. Mix in a sprayer with marker dye and a quality surfactant.

Scotts Halts Crabgrass Preventer

Home Depot

Rate: 2.7 lbs per 1,000 sqft

Big-box pendimethalin granular — safe for St. Augustine. Stains concrete yellow, so sweep hardscape immediately after application.

Dithiopyr (Dimension 270G)

Lowe's

Rate: 2.3 lbs per 1,000 sqft

Pre-emergent with a few weeks of post-emergent reach-back on baby crabgrass. Safe on St. Augustine.

How to check your soil temperature

Use a soil thermometer at 2–4 inch depth, or check your local extension office's soil temp map. Look for the 5-day average, not a single reading — one warm day on a st. augustine lawn doesn't mean it's time.

What's Safe to Spray

St. Augustine handles most selective herbicides well. Here's what's safe and what to avoid — organized by where you'd buy it.

Safe for St. Augustine

✓

Scotts Halts Crabgrass Preventer

Pendimethalin

Big-box pre-emergent. Apply BEFORE soil hits 55°F at 4-inch depth. One of the very few big-box products that's safe on St. Augustine.

Safe

✓

Image Kills Nutsedge

Imazaquin

The big-box nutsedge option for St. Augustine. Slow acting (2–3 weeks) but effective on yellow nutsedge, dollarweed, and wild garlic.

Safe

Avoid — Will Damage Your St. Augustine

Scotts Turf Builder Triple Action

Benefin + 2,4-D + Dicamba (with fertilizer)

NOT safe for St. Augustine. Contains 2,4-D and dicamba — both will damage or kill St. Augustine. Northern lawns only.

Unsafe

Scotts Turf Builder Weed and Feed

2,4-D + Mecoprop + Dicamba (with fertilizer)

NOT safe for St. Augustine. Northern formula. Fertilize and spot-treat with Celsius WG per label as two separate steps instead — the southern weed-and-feeds substitute atrazine, which Floratam is not tolerant of.

Unsafe

Spectracide Weed Stop

2,4-D + MCPP + Dicamba

NOT safe for St. Augustine. Standard 3-way will damage or kill St. Augustine. Read the label before you spray.

Unsafe

Ortho Weed B Gon (Northern formula)

2,4-D + MCPP + Dicamba

NOT safe for St. Augustine. Has 2,4-D — the southern formula is different (no 2,4-D variants exist for St. Aug).

Unsafe

BioAdvanced All-in-One

2,4-D + MCPP + Dicamba + Quinclorac

NOT safe for St. Augustine. Both the 2,4-D and the quinclorac will damage St. Augustine. Use Celsius WG per label instead.

Unsafe

A note on Weed & Feed products

Combo products like Weed & Feed are convenient — one bag, one application. But you'll get noticeably better results using fertilizer and weed control separately. Here's why:

Timing: Fertilizer and herbicide have different ideal application windows. A combo forces both at the same time, so one is always off-schedule.

Application: Herbicides work best as a spray that sticks to weed leaves. Granular weed & feed relies on granules physically touching each weed — inconsistent coverage.

Precision: Most lawns don't have weeds everywhere. Spraying weeds where they are costs less and puts fewer chemicals on your lawn than treating the whole yard.

Common Weed & Feed products you'll see at the store

Image All-in-One Southern

Granular combo (fertilizer + sulfentrazone + penoxsulam)

Southern formula combo for warm-season lawns. Watch the rate on St. Augustine — start at the lower rate to avoid temporary thinning.

Scotts Turf Builder Weed and Feed (Northern formula)

Granular combo (fertilizer + 2,4-D + Dicamba)

WARNING: Northern weed-and-feed contains 2,4-D + dicamba. Do NOT use on St. Augustine — this is the most common cause of St. Augustine damage from a homeowner. Listed here so you can recognize and avoid the wrong bag at the store.

What to grab instead

Three separate products. Apply each on its own schedule. Better results, less waste, healthier lawn.

Fertilizer

A slow-release 3-1-2 ratio (e.g., 15-5-10 or 16-4-8) at 0.5 lb N per 1,000 sqft, applied when St. Augustine is actively growing. Total nitrogen is regional — 2–4 lbs North FL, 2–5 Central, 4–6 South FL per 1,000 sqft per year; over-feeding beyond your regional max invites brown patch and chinch bugs.

Pre-emergent

Prodiamine 65 WDG at 0.18 oz per 1,000 sqft. Apply when soil temps approach 55°F — before crabgrass germinates.

Post-emergent weed killer

Celsius WG per label for broadleaf weeds, SedgeHammer for nutsedge. NEVER 2,4-D, NEVER Trimec, NEVER quinclorac, and NEVER atrazine on St. Augustine — Floratam is not tolerant of it (Texas A&M EHT-144).

Always read the product label before applying. Labels and formulations change — verify the active ingredient and grass safety on the bag before you spray.

Need to check a specific product?Herbicide Safety Checker →

Common Problems

Tap any issue to see symptoms, causes, and exactly how to fix it.

See something weird on your lawn?Ask Larry — send a photo →

Mowing Guide

Mowing is the single most impactful thing you can do for your St. Augustine lawn. Here's how to get it right.

Height: 3.5 – 4 inches (Floratam)

Floratam — the most common St. Augustine variety — wants 3.5–4 inches, and Palmetto is a semi-dwarf a little lower at 3–4 inches (UF/IFAS ENH5). Dwarf varieties like Seville and Captiva go lower at 2–2.5 inches. Tall mowing shades out weeds, keeps roots cool, and reduces chinch bug damage.

Frequency: Every 5–7 days in peak season

Never remove more than 1/3 of the blade in one cut. At a 4-inch height, that means you mow when the lawn hits 6 inches. Skipping a week means scalping it back into shape — and scalped St. Augustine is brown for weeks.

Never scalp St. Augustine

Unlike Bermuda, St. Augustine has no rhizomes — only above-ground stolons. Scalping exposes the stolons to sun, kills them, and leaves bare patches that take months to fill. There is no spring scalp on St. Augustine, ever.

Clippings: Mulch, don't bag

St. Augustine clippings break down fast and return nitrogen to the soil — about 25% of your annual N if you mow consistently. Only bag if you're treating gray leaf spot or brown patch and want to remove infected tissue.

Sharpen blades every 20–25 hours

St. Augustine's wide blades show dull-blade damage faster than any other turf — torn tips turn brown within a day and invite gray leaf spot. A sharp blade is non-negotiable on St. Augustine.

Mow when the lawn is dry

Wet St. Augustine clumps badly under the mower deck and the wheels leave tracks in soft soil. Wet mowing also spreads brown patch and gray leaf spot from infected blades to healthy ones.

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