Fertilize when the roots are growing! When to apply lawn fertilizer is almost as important as the fertilizer you apply. Leading scientists recommend applying nitrogen fertilizers in the spring and fall (for cool season grasses) and throughout the summer (for warm season grasses), since this is when the grass plant is actively growing. They recommend a fertilizer application that delivers 2.5 to 3.5 pounds of nitrogen per 1,000 square foot per year. The recommended timing of fertilizer applications is as follows: Spring lawn care: April–May The best time to apply fertilizer in spring is when soil temperatures have warmed and the grass plant is actively growing. A general rule of thumb is after the first 3 mowings. Apply 0.5 to 1 pound of nitrogen per 1,000 square feet to the lawn to support growth and to build-up nutrient reserves, which help the lawn withstand the heat and stress of summer. If you are using a weed and feed fertilizer, timing is much more critical to get the best results from the weed control. Be sure to read and follow application directions carefully. We recommend applying it after the forsythia has bloomed but before the dandelions go to the "puff ball" stage. The combination of weed control ingredients kill over 200 broadleaf weeds and prevent crabgrass. Summer lawn care: June–August Cool season grasses should not be fertilized in summer. Agronomy experts do not recommend the application of nitrogen fertilizers to cool season grasses in the summer. Cool season grasses go dormant in the summer and fertilizing at this time can burn the grass and contribute to watershed pollution. Warm season grasses do need fertilizer in the summer. Warm season grasses grow most vigorously during the hot summer months. At this time the grass plant benefits from the nutrients in fertilizer to grow dense, lush turf. Fall lawn care: September–November The most important time to fertilize your lawn is in the fall to help you lawn develop a strong root system. Apply 1.5 to 2.5 pounds of nitrogen per 1,000 square feet. Use a lawn fertilizer with a higher amount of nitrogen, if your grass type is Kentucky bluegrass or perennial ryegrass, or soil is sandy, or grass clippings are not returned to the lawn. Source