Hot, Humid Weather Is Tough on Grass Seedlings
The hot and humid weather conditions often experienced across Canada during the summer months are likely to lead to problems in young grass seedlings. Mature turf is more tolerant. Extreme summer-like conditions are not a good time to plant grass seeds, such as bluegrass, ryegrass, and fescues, or to have young seedlings fighting to survive. The best time to plant grass seed is during early spring and early fall when soil temperatures are between 60-80 degrees, which occur in mid- to late-spring and again in early-fall.
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 may 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 the compacted soil layers to provide aeration and good drainage of water through and across the soil. Level the area, filling in any holes, so water doesn't continue to pool.
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 degrees combined with excess moisture and humidity are particularly stressful. Not only is soil oxygen depleted, but warm-season diseases such as damping off (Pythium) and brown patch (Rhizoctonia) become very active. They can attack and possibly kill young grass seedlings. For all these reasons listed above, its best to stick to the cooler fall months when planting grass seed.