Hot, Humid Weather Is Tough on Grass Seedlings
The hot, wet and humid weather conditions experienced across the northern US are likely to lead to disease and death problems in young seedlings. Mature turf is more tolerant. Extreme summer-like conditions are not a good time to plant northern grass seeds, such as bluegrass, ryegrass, and fescues, or to have young seedlings fighting to survive. Northern grasses are best planted during early spring and early fall when soil temperatures are between 50F and 75F. That's when those grasses will have time to mature. Unless you have to seed to stop erosion, hold off on seeding until nighttime air temperatures are consistently less than 65 and daytime temperatures are consistently less than 80 between the middle of August and Late September.
Good Drainage Is Critical
Seedlings do not survive where water accumulates and pools for long time periods during summer. After all, roots need oxygen as much as nutrients and moisture to survive. In areas next to driveways where auto tires have rutted the soil, all the tiny air pockets in the soil have been compressed, and water will accumulate in those low spots. Although seedlings may grow there for a little while, they probably won't survive for long. When preparing an area for planting grass seed, loosen compacted surface soil and sub-surface soil layers to provide aeration and good drainage of water through and across the soil. Fill the holes with soil similar to the native soil and then seed.
Hot Days and Saturated Soil Spell Big Trouble for Grass
Warm temperatures, combined with water-logged soils, create conditions where there is no soil oxygen for roots to breathe. If the roots can't breathe, they cannot absorb nutrients and water and they may die. As an example, you can see the stress of water-logged or saturated soils in many farm fields of young soybeans and corn that appear yellow and stunted. The same heat, humidity and soil aeration stresses that stunt those robust farm crops can kill tiny grass seedlings. So hold off on planting grass seed during times of severe heat, humidity, and excess water stress.
Hot, Sticky Nights Can Lead to Grass Diseases
High night time temperatures above 65 combined with excess moisture and humidity are particularly stressful. Not only is soil oxygen depleted, but warm-season diseases such as Pythium damping off and brownpatch (Rhizoctonia) become very active. They'll attack and possibly kill young grasses. Since there are no fungicides for these diseases on the market, you're better off waiting until the weather cools off.
Hot Weather and Scotts® EZ Seed®
Excessive nitrogen fertilization can increase heat stress and disease severity. If you have recently purchased EZ Seed, wait for cooler weather before applying it at the recommended rate. Keep nitrogen feeding to the minimum necessary to maintain healthy growth. If you have just planted and have seedlings coming up, try to maintain good drainage and prevent overwatering. If standing water is present, it could be damaging new seedlings. If you must start over, then loosen soil, fill holes, and grade the soil so that water doesn't pool for long.