South Florida Fertilizer Calculator

Most South Florida counties permit lawn fertilization from October 1 through May 31. The summer blackout period (June 1 through September 30 in most counties, October 31 in Palm Beach) prohibits applying nitrogen or phosphorus fertilizers. The best times to fertilize are March-April for spring feeding and October-November for fall feeding. The application just before the blackout in late May is your most important one — use a quality slow-release product that feeds the lawn through summer legally.

Florida's Model Fertilizer Ordinance, adopted by most South Florida counties, prohibits applying phosphorus to established lawns without a soil or tissue test confirming deficiency. Phosphorus runoff is a major contributor to algae blooms in Biscayne Bay, the St. Johns River, and South Florida's coastal waterways. Most established South Florida lawns have adequate phosphorus in the soil — a no-phosphorus blend like 15-0-15 is compliant without any testing required.

UF/IFAS recommends a slow-release nitrogen fertilizer with no phosphorus for established St. Augustine lawns — a blend like 15-0-15 or 16-0-8 works well. At least 50% of the nitrogen should be in slow-release form as required by most Florida county ordinances. Apply no more than 1 lb of nitrogen per 1,000 sq ft per application. For St. Augustine in South Florida feed in March, again in late May before the blackout, and again in October after the blackout ends.

For most South Florida lawn grasses the target is 1 lb of actual nitrogen per 1,000 sq ft per application — that is the UF/IFAS maximum recommendation. The amount of fertilizer product you need depends on the nitrogen percentage on the bag. A 16% nitrogen fertilizer (like 16-4-8) requires about 6.25 lbs of product per 1,000 sq ft to deliver 1 lb of nitrogen. A 32% nitrogen fertilizer requires only about 3.1 lbs per 1,000 sq ft. Use this calculator above to get the exact amount for your specific bag and lawn size.

Slow-release nitrogen is coated or chemically treated so it releases gradually over weeks or months rather than all at once. Florida counties require at least 50% of nitrogen in fertilizers to be slow-release because it dramatically reduces runoff into waterways. Fast-release nitrogen not absorbed by the lawn washes into storm drains and canals during South Florida's heavy summer rains, contributing to algae blooms in Biscayne Bay and the Florida Keys National Marine Sanctuary. Slow-release products also feed the lawn more evenly and reduce the risk of burning.

Most indoor tropical plants in South Florida benefit from fertilization monthly during spring and summer when growth is most active, reducing to every 6-8 weeks in fall and winter. The key rule is less is more — indoor plants grow slowly and cannot use heavy fertilizer doses. Always use fertilizer at half to quarter the label rate for indoor applications. South Florida's year-round warmth means indoor plants rarely go fully dormant, so some light feeding year-round is appropriate. Indoor plants are exempt from Florida's summer fertilizer blackout ordinances which apply only to outdoor turf and landscape fertilization.

Yellowing fronds on indoor palms in South Florida are most commonly caused by magnesium or manganese deficiency — both are common in our hard water conditions. South Florida's municipal water is high in calcium and bicarbonates which compete with magnesium uptake in container-grown palms. The solution is a palm-specific fertilizer containing magnesium (look for Epsom salt or magnesium sulfate as an ingredient) and a manganese supplement. Also check that the palm is not sitting in standing water — root rot causes yellowing that mimics nutrient deficiency. A licensed interiorscaper can diagnose whether the issue is nutritional, cultural, or pest-related.