The best spring fertilizer for St. Augustine grass is a slow-release granular blend with an NPK ratio around 16-4-8 or 15-0-15, applied once the lawn is fully green and actively growing, typically mid-March through April in the Southeast.
Apply it too early—while the grass is still half-dormant—and you’ll waste product while encouraging tender growth that a late frost can kill. Apply the wrong ratio, and you fuel disease or violate local phosphorus restrictions. The trick is matching the timing, the nitrogen source, and the exact numbers on the bag to what St. Augustine actually needs that first round of the year.
Why 16-4-8 or 15-0-15 Works Best
St. Augustine in spring wants a high first number (nitrogen) to push green growth, a moderate third number (potassium) for root and stress tolerance, and a low or zero middle number (phosphorus). The nitrogen should be partly slow-release—30–50% of it—to feed steadily over weeks without burning the grass or leaching into groundwater on sandy soil. Phosphorus above 5% is rarely needed on established turf and is banned or restricted in Florida and other Gulf states because it runs off into waterways. A zero-phosphorus 15-0-15 blend is a safe default if you haven’t tested your soil.
Spring Feeding Schedule for St. Augustine
Three to four total applications between March and October deliver the 2–4 pounds of actual nitrogen per 1,000 square feet that St. Augustine needs annually. Spring round one lands in March or April, as soon as the lawn has been mowed at least twice and shows full green growth. Round two follows in late spring, around May or June.
| Round | Timing (Southeast & Texas) | Rate (lb N / 1,000 sq ft) |
|---|---|---|
| Spring opener | Mid-March – late April | 0.5–1.0 |
| Late spring | May – June | 0.5–1.0 |
| Summer (optional) | July – August | 0.5 (use lower rate to avoid disease) |
| Early fall | September – early October | 0.5–1.0 |
Regional differences: Central Florida waits until mid-to-late April. South Texas and South Georgia start in March–April. The unbreakable rule: do not feed before the grass is fully out of dormancy and actively growing. If you’re looking for tested product recommendations for spring St. Augustine feeding, our roundup covers the top blends that match these ratios.
How to Apply Spring Fertilizer Correctly
Getting the right numbers on the lawn matters more than the brand on the bag. Follow this order:
- Soil test first. Check pH (target is 6.0–6.5) and phosphorus levels. Skip this, and you might apply what’s banned or already abundant.
- Mow and water. Mow 1–2 days before, removing no more than 1/3 of the blade. If the lawn is dry, water it the day prior so the granules dissolve into damp soil.
- Apply to dry grass. Wait until morning dew dries. Use a calibrated spreader in overlapping passes. Set it to deliver 0.5–1.0 pounds of actual nitrogen per 1,000 square feet.
- Water it in. Run irrigation for 15–20 minutes within 24 hours to move nutrients to the root zone and wash granules off leaf blades.
The common mistakes that waste the feeding: applying before full green-up, using weed-and-feed combos whose herbicides damage St. Augustine, and piling on heavy nitrogen during summer heat when gray leaf spot is active.
The Early-Bird Trap
Fertilizing in late February or early March when the lawn still has brown patches encourages a flush of growth that can’t survive a late freeze. If that flush gets killed, the grass enters summer already stressed and thin. Let the lawn tell you it’s ready—consistent green across the whole yard and at least two mowings behind it.
Stick to the 2–4 pounds of nitrogen per year total (favor the higher end on sandy soils). And never fertilize between November and February—that window is for dormancy, not growth. A good spring start with a 16-4-8 or 15-0-15 slow-release granular, applied at the right time and rate, sets up St. Augustine to handle summer heat, foot traffic, and the occasional drought without a second thought.
FAQs
Can I use a weed-and-feed on my St. Augustine lawn in spring?
Most combination weed-and-feed products contain herbicides that damage or kill St. Augustine, especially during spring green-up. Apply fertilizer and weed control separately, using a herbicide labeled specifically for St. Augustine, or skip the weed killer entirely until you confirm the weed type.
Do I need phosphorus for my St. Augustine lawn?
Only if a soil test shows a phosphorus deficiency. Established lawns on healthy soil rarely need it, and many Gulf Coast states restrict phosphorus application by law. Using a 15-0-15 or similar zero-phosphorus blend avoids the issue entirely while still delivering the nitrogen and potassium St. Augustine needs.
How much actual nitrogen does St. Augustine need per year?
St. Augustine requires 2 to 4 pounds of actual nitrogen per 1,000 square feet annually. Sands and sunny lawns need the higher end; shaded lawns or clay soils stay at the lower end. Split that total across three or four applications between early spring and October, never exceeding 1 pound per single application.
References & Sources
- Clemson Cooperative Extension. “St. Augustinegrass Maintenance Calendar.” Details fertilization timing, rates, and soil test guidance for the Southeast.
- Texas A&M Agrilife Extension. “Maintaining St. Augustine Grass Lawns.” Regional guidelines for Gulf Coast and South Texas spring feeding.
- LSU AgCenter. “Fertilizing St. Augustine Grass in Spring.” Covers nitrogen rates, safe products, and the early-application risk.