The calendar flips to late summer and a St. Augustine lawn starts telegraphing its needs—slowing its top growth, holding onto a faded green, and bracing for the chill. That transition window is the single most critical time to feed it if you want dense turf next spring and a carpet that shrugs off winter weeds. But dumping the wrong ratio of nutrients now can leave your St. Augustine soft, disease-prone, or pale heading into dormancy.
I’m Mo Maruf — the founder and writer behind The Tools Trunk. I analyze seasonal lawn chemistry and compare formulations across granular and liquid delivery systems to find what actually works for warm-season turf under real soil conditions.
This guide breaks down the specific NPK ratios, micronutrient profiles, and application strategies that suit St. Augustine grass during its autumn feeding window. Whether you need a slow-release granular or a fast-acting liquid boost, these picks help you dial in the right fall fertilizer for st augustine grass.
How To Choose The Best Fall Fertilizer For St Augustine Grass
St. Augustine grass has a lower tolerance for cold and a higher sensitivity to certain nutrient loads than Bermuda or Zoysia. Picking a fall fertilizer means looking beyond the NPK numbers toward release timing, iron content, and disease prevention.
Nitrogen Type and Release Speed
Quick-release nitrogen gives a fast green flush but fades fast and can push soft growth that invites large patch fungus in cooler weather. Slow-release or controlled-release nitrogen feeds steadily over weeks, supporting root development without forcing a vulnerable growth spurt right before frost. For fall feeding on St. Augustine, a blend of both sources works best—visible color now and sustained energy later.
Potassium for Winter Hardiness
Potassium is the nutrient that strengthens cell walls and improves a grass plant’s tolerance to cold, drought, and traffic. A fall fertilizer with a potassium number close to or higher than the nitrogen number (like a 15-0-15 or 8-0-8 ratio) helps St. Augustine store carbohydrates in roots and rhizomes, giving it better survival odds through winter stress and a faster green-up in spring.
Iron and Micronutrient Density
St. Augustine is notorious for showing iron deficiency as yellowing between leaf veins, especially in high-pH or compacted soils. A fall fertilizer that packs chelated iron or iron sulfate delivers that deep blue-green color without forcing excess nitrogen-driven growth. Humic acid and sea kelp also improve nutrient uptake and root mass, which matters when soil temperatures drop below 60°F.
Quick Comparison
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| Model | Category | Best For | Key Spec | Amazon |
|---|---|---|---|---|
| Espoma Organic Lawn Food Summer Revitalizer | Organic Granular | Gentle iron-rich feeding | 8-0-0 with Bio-tone | Amazon |
| PetraMax Neighbor’s Envy | Liquid Concentrate | Rapid deep-green color | 7-in-1 formula with 19% N | Amazon |
| Scotts Turf Builder Healthy Plus | Granular + Fungicide | Fall disease prevention | 19-0-10 with fungicide | Amazon |
| GS Plant Foods Green Envy | Organic Liquid | Root and soil health | Humic acid + kelp | Amazon |
| The Andersons Premium Weed and Feed | Granular Herbicide | Broadleaf weed control | 24-0-16 with herbicide | Amazon |
| Scotts Turf Builder Fall Mix | Seed + Fertilizer | Overseeding bare patches | Seed mix with root builder | Amazon |
In‑Depth Reviews
1. Espoma Organic Lawn Food Summer Revitalizer
Espoma’s 8-0-0 formulation drops the phosphorus entirely and leans on organic nitrogen sources combined with iron for a slow, steady color response without forcing a rapid top-growth flush that leaves St. Augustine vulnerable to large patch fungus. The Bio-tone microbial additive supports root soil biology, which matters when the grass is stockpiling energy for winter. Users consistently report that this fertilizer reverses yellowing without the chemical burn risk that comes with synthetic high-nitrogen products.
At 30 pounds covering 5,000 square feet, the granules are large enough to feed through a broadcast spreader without dust or clumping, and the organic smell is mild compared to manure-based blends. Several long-term users mention switching from synthetic brands and seeing better resistance to heat stress and summer drought after three seasons.
One weakness: the 8-0-0 ratio lacks potassium, so it works best as a mid-fall touch-up rather than a complete winterizing feed. Pair it with a potassium supplement if your soil test shows low K. The granular size also requires a spreader with larger openings—bag settings tend to be slightly light, so calibrate before covering the whole lawn.
What works
- Safe organic formula that won’t burn St. Augustine
- Iron corrects yellowing without forcing soft growth
- Bio-tone improves soil biology for root storage
What doesn’t
- Zero potassium means it doesn’t harden for winter
- Large granules need spreader calibration
- Bag expansion can cause slight weight variance
2. PetraMax Neighbor’s Envy 7-in-1 Liquid Fertilizer
PetraMax’s Neighbor’s Envy packs 19 percent nitrogen with 2.5 percent iron plus humic acid and sea kelp into a single 64-ounce concentrate that covers up to 8,000 square feet. The 7-in-1 approach eliminates the need to buy separate root stimulant, color enhancer, and soil conditioner—everything arrives in one bottle. Users report visible darkening within 7 to 10 days, even on Bermuda and St. Augustine lawns recovering from summer stress or freeze damage.
Liquid delivery means the nutrients hit the leaf surface and root zone almost immediately, which is useful for a quick color lift before cooler weather sets in. The humic acid content also chelates micronutrients in the soil, making them available to the grass even when soil temperatures start to drop. Several reviewers note that three applications turned a yellow, thin lawn into a dense green carpet.
The main catch is longevity: the fast-acting nitrogen produces a dramatic flush that can fade after about 10 days, which means some users find themselves reapplying sooner than expected. The liquid formulation also tends to settle, so frequent shaking is required to avoid clogging sprayer filters. For small lawns, the per-application cost runs higher than a granular alternative.
What works
- Very rapid greening with high nitrogen and iron
- All-in-one formula simplifies fall feeding schedule
- Covers large area with concentrated liquid
What doesn’t
- Color fades faster than slow-release granulars
- Frequent shaking needed to prevent clogging
- Premium cost per square foot for large lawns
3. Scotts Turf Builder Healthy Plus Lawn Food
Scotts Healthy Plus delivers a 19-0-10 ratio combined with a systemic fungicide that controls 27 different lawn diseases, including brown patch—the single most common fungus that attacks St. Augustine as nights cool down in early fall. The dual-action approach means you can feed the lawn and prevent disease in one pass, which simplifies the schedule during a season when large patch pressure peaks. The 27.4-pound bag covers 8,000 square feet, making it a good fit for medium to large St. Augustine lawns.
The nitrogen blend includes both quick-release for fast color and slow-release for extended feeding, so the grass gets an immediate green-up but doesn’t produce a soft, vulnerable growth flush that worsens fungal pressure. Potassium at 10 percent supports root hardiness through winter dormancy. Users with St. Augustine in humid southern climates consistently report that alternating Healthy Plus with a straight fungicide every three to four weeks prevents outbreaks entirely.
It is a synthetic product, so users looking for an organic approach should look elsewhere. The fungicide component is a broad-spectrum systemic, which means it kills beneficial soil fungi alongside the harmful ones. The price is also a step up from standard fertilizers, though the dual fungicide-fertilizer aspect replaces a separate fungicide application.
What works
- Fungicide prevents brown patch before it starts
- Balanced N and K supports root health for winter
- Covers 8,000 sq ft with dual-action formula
What doesn’t
- Synthetic chemistry may not suit organic lawns
- Fungicide is non-selective against soil microbes
- Higher price than standard fall fertilizers
4. GS Plant Foods Green Envy Liquid Lawn Food
GS Plant Foods Green Envy is a liquid organic concentrate designed to complement a granular fertilizer rather than replace it. The formula relies on humic acid to improve nutrient uptake and sea kelp to supply trace minerals and natural growth hormones that encourage root branching. For fall use on St. Augustine, the real value is that it helps the grass extract more from whatever fertilizer you apply, making your primary feeding more efficient.
Users report that spraying Green Envy after a dethatch or aeration helps the lawn green up faster and fill in thin spots. The organic nature means there is zero burn risk even at full concentration, and it mixes well with any other liquid or granular product. Several long-term users say they see a thicker lawn after two seasons of consistent use, especially on soils that tend to be compacted or high in clay.
The downside is viscosity—the liquid is noticeably thick, and users have reported clogs in hose-end sprayers that aren’t designed for humus-based concentrates. The smell is earthy and strong, though it fades quickly after drying. Visible color change is also slower than synthetic liquid feeds because the organic nutrients need soil microbial activity to break them down.
What works
- Improves fertilizer uptake with humic acid action
- Zero burn risk for sensitive St. Augustine
- Sea kelp supports root mass and stress tolerance
What doesn’t
- Thick liquid can clog hose-end sprayers
- Slower visible results than synthetic liquids
- Strong earthy smell during application
5. The Andersons Premium Weed and Feed 24-0-16
The Andersons delivers a high-nitrogen 24-0-16 formulation with a herbicide that targets over 250 broadleaf weeds, making it a strong choice for a fall cleanup pass before winter sets in. The 18-pound bag covers 5,000 square feet, and the dual-release nitrogen provides both immediate green-up and sustained feeding over several weeks. This is a granular product, so it works well with a standard broadcast spreader and doesn’t require mixing or sprayer equipment.
For St. Augustine, the potassium at 16 percent is a highlight—that level helps the grass build cold tolerance and carbohydrate storage for winter dormancy better than most weed-and-feed products on the market. The herbicide is effective against dandelions, clover, chickweed, and other cool-season broadleaf weeds that try to germinate when St. Augustine growth slows. Several users report visible weed die-off within two weeks.
There are restrictions: the product is not for sale in New York, Florida, California, or Indiana, so check local availability. The herbicide component can also damage St. Augustine if applied during a stress period or if the lawn is wet, so follow the instructions precisely. A few users found the herbicide ineffective against established nutsedge, which requires a separate treatment.
What works
- High potassium for winter hardiness in St. Augustine
- Controls broadleaf weeds that invade in fall
- Dual-release nitrogen gives both quick and extended feeding
What doesn’t
- Restricted in NY, FL, CA, and IN
- Herbicide can stress St. Augustine if misapplied
- Ineffective against nutsedge and some perennial weeds
6. Scotts Turf Builder Grass Seed Fall Mix
Scotts Turf Builder Fall Mix is primarily a grass seed product with a built-in fertilizer and soil improver, rather than a pure fertilizer. The seed blend contains Kentucky bluegrass, perennial ryegrass, and creeping red fescue—cool-season grasses that establish quickly in fall. If you are patching bare spots in a St. Augustine lawn that has thinned from summer stress, this product helps fill those gaps with fast-germinating grass that will hold the soil in place until spring regrowth.
The 16-pound bag covers up to 12,120 square feet for overseeding or 4,000 square feet for a new lawn. The included Root-Building Nutrition helps the seedlings establish deep roots before the first frost. Users report visible sprouting within 10 to 14 days, even in southern states where St. Augustine is the dominant turf. The seed blend has moderate shade tolerance and medium durability.
The main limitation is that the seed mix is not St. Augustine—it’s a cool-season blend that will die off in summer heat if it outcompetes the warm-season grass. This product is best used only for temporary winter coverage on bare spots, not as a replacement for a St. Augustine fertilization strategy. The fertilizer component is lightweight, so you still need a primary fall feeding product for the established St. Augustine turf.
What works
- Quick germination to patch bare spots before winter
- Includes fertilizer for root establishment
- Large coverage for overseeding thin areas
What doesn’t
- Seed is cool-season blend, not St. Augustine
- Cool-season grass dies in summer heat
- Light fertilizer component, needs primary feeding
Hardware & Specs Guide
NPK Ratio for Fall
St. Augustine grass benefits from a fall fertilizer ratio that is high in nitrogen and potassium but low or zero in phosphorus. Phosphorus is rarely needed unless a soil test reveals a deficiency, and excess phosphorus can contribute to algae growth and runoff issues. Aim for ratios like 8-0-8, 15-0-15, or 19-0-10 to support color and root storage without stimulating excessive top growth.
Slow-Release vs. Quick-Release Nitrogen
Slow-release nitrogen sources such as urea formaldehyde, polymer-coated urea, or methylene urea provide a steady nutrient supply over 6 to 8 weeks. Quick-release sources like ammonium nitrate produce a fast green flush that lasts 2 to 3 weeks. For fall feeding, a blend of both allows immediate visual results while the slow-release portion continues feeding the roots as temperatures drop.
Iron Content and Source
Iron is particularly important for St. Augustine because it drives chlorophyll production and deep-green color. Chelated iron (such as Fe-EDDHA or Fe-DTPA) stays available in high-pH soils and alkaline water, while iron sulfate works faster but can stain concrete. Products with 2 to 5 percent iron provide visible greening without pushing excess nitrogen-driven growth.
Potassium for Winter Hardiness
Potassium strengthens cell walls and helps St. Augustine grass regulate water and nutrient movement as soil temperatures drop. A fall fertilizer should contain at least 8 to 16 percent potassium. This nutrient is especially important for St. Augustine in regions that experience frost or extended cold periods, as it improves the grass’s ability to survive dormancy and green up faster in spring.
FAQ
When should I apply fall fertilizer to St. Augustine grass?
Can I use a weed and feed on St. Augustine in the fall?
Should I use a liquid or granular fertilizer for St. Augustine in the fall?
Final Thoughts: The Verdict
For most users, the fall fertilizer for st augustine grass winner is the Espoma Organic Lawn Food Summer Revitalizer because it delivers safe, slow-release nitrogen and iron without the burn risk, and its organic formula supports soil biology during the critical fall transition. If you need a fast visible green-up and want to simplify with a single liquid product, grab the PetraMax Neighbor’s Envy. For lawns where brown patch or large patch fungus is a recurring problem, nothing beats the Scotts Turf Builder Healthy Plus with its built-in systemic fungicide.