Late Summer is the best time of year for overseeding lawns in the Washington DC area. After August 15th, night time temperatures begin to cool off. This is an ideal time of year to start the grass planting process. You should kill any weeds first. Then aerate the lawn, preferably after a nice, ground softening, rainy day.
If your lawn receives full sun for most of the day the seed you should use is improved turf type tall fescue. Select a mixture with at least three different varieties. The more varieties in a seed mixture the better. Use only certified seed that has been recommended for use in our area.
Seeding rates should be anywhere from 2 to 5 pounds per 1000 square feet. The difference in rate depends on the current condition of the turf. Use the heavier rate for thin lawns and the lighter one well maintained, thick turf. Use care when overseeding near mulched beds and tree rings. There's nothing more depressing than having to weed a nice crop of fresh grass out of a mulch bed.
The seed can then be top dressed if the soil is compacted or lacks organic matter.
Depending on the variety, seed may take well over a week to germinate. This "clock" does not start until seed has been inbibed (watered). If rain is not expected within 24 hours, water in with a very light sprinkle. For best results it is wise to lightly water seed and young grass plants every other day.
Seed should come from varieties that are drought tolerant and have disease resistance. Varieties should have thin blades and dark color.
When growing grass in the shade, select a shade seed mix that contains the fine creeping fescues.