If your St, Augustine lawn has been denied proper care for any length of time it will be under stress and in danger of dying. St. Augustine is a warm-season perennial grass that tolerates heat and cold well compared to other species, but it does require ample moisture, regular feeding, some sun and proper mowing to perform well as an ornamental lawn turf. When beginning a rescue effort, start with proper watering, as this is the lifeblood of any lawn grass.
Feel the grass, thatch and soil for moisture. If the thatch or soil feel anything less than very moist, water immediately, as the grass is likely under drought stress. Keep St. Augustine grass moist at all times under the thatch over the root zone to a depth of 6 inches. Apply a minimum of 1 inch of water per week in one or two deep-soaking waterings unless rains have been prevalent. The grass will under-perform and then die back in drought.
Hold off on mowing until some new green growth appears. Once you resume a mowing regimen, keep the grass blade height within the recommended range for the St. Augustine species. Mow the lawn to 1 to 3 inches in blade length and no shorter to prevent scalping and sun burn. The longer the blade length, the less stress on the plant roots, the greener the lawn and the less frequently mowing will be required.
Apply a lawn fertilizer once the lawn is well hydrated and new green growth is visible. Fertilizing a drought-stressed lawn can burn the foliage tips and roots exacerbating your problem. Use a complete lawn fertilizer formula that is rich in nitrogen. Apply according to the product label directions, not exceeding 1 lb. of actual nitrogen for every 1,000 square feet of lawn expanse. Apply over moist grass and water in well after feeding to drive the nutrients down into the root zone and prevent burning of tender plant tissues. Repeat once per month from spring through fall.
Correct yellowing, chlorotic St. Augustine lawns growing on alkaline soil or over iron deficient soil, with an iron sulfate or iron chelate foliar spray product. Apply according to the product label directions when yellowing is noticed and repeat as needed throughout the growing season to compensate for the deficiency and keep the lawn green.