A fertile soil that provides the three main nutrients of nitrogen, phosphorus and potassium encourages the best green growth of lawns across the summer months. Cool-season grasses tend to brown and go dormant if soil is too dry and temps rise above 85 degrees Fahrenheit. Conversely, warm-season grasses become green and grow when temps consistently remain above 70 degrees. Fertilizing properly in spring and fall diminishes the need to apply fertilizers during the hottest part of summer, avoiding any problems.
Turfgrass fertilizers typically are rich in nitrogen, which promotes green leaves and production of plant cells tissues. Ample nitrogen guarantees a green color as well as vigorous growth of grass blades. Applications of fertilizer therefore increases the need and timing increments for mowing.
The University of Minnesota, Ohio State and Colorado State Universities generally comment that cool-season grasses are best only fertilized in spring or fall when temperatures are cooler. Fescue, bluegrass and ryegrass are cool-season lawns may be fertilized in summer, but in the waning days when day length is shortening and sun rays are not as intense: late summer and early fall. Warm-season lawns like St. Augustine, Bermuda grass and zoysia grass green up in spring and can be fertilized across the warm months when they most vigorously grown. However, being overzealous with nitrogen in warm season lawn feedings results in tremendously fast growth, leading to multiple mowing requirements weekly for best appearance.
When cool-season lawns start to turn brown in summer, many assume the soil is infertile and think a fertilization is warranted. These grasses naturally brown, look weak and spindly when soil moisture is low and air temperatures high. Increased watering is necessary, according to the Ohio State University Extension. Likewise, warm-season lawns naturally brown in late autumn when air and soil temps drop. Excessive drought in summer also leads to warm-season lawns to brown and go dormant, sprouting again when rains return and temps remain warm.
If fertilization is absolutely required in midsummer for cool-season lawns, make sure the soil is moist and product label directions are closely followed. Fast-acting water soluble fertilizers have potential to chemically burn leaf blades and roots in the soil. The University of Minnesota urges withholding quick-release fertilizer applications to cool-season grasses when the daily high temps are consistently above 85 F. Warm-season lawns also can be harmed by burning if fertilizers are applied at wrong times of day, to dry soils, or scattered too densely.
If properly maintained with consistent mowing and timely seasonal fertilizing in spring and fall, all lawns gain about 25 percent of their annual needs for nitrogen by decomposing mowing clippings. According to the Ohio State University, lawn clippings left to degrade on the lawn do not contribute to thatch build-up. With the organic nutrients on the lawn across summer, focus on proper watering to keep both cool- and warm-season lawns looking their best. To increase the green color of warm-season lawns across summer without the worry of increased mowing, consider applying iron rather than nitrogen.
When unsure, consult your local Cooperative Extension Office for recommendations for applications of nutrients to your lawn when summer's temperatures are warmest and sunlight most intense, especially if the weather is dry. Sandy soils leach nutrients more quickly compared to loam or clay soils, so regional differences may exist for proper lawn maintenance strategies. Moreover, improperly fertilized lawns in summer can lead to increased pest and disease problems.