The very best time to start a new lawn or repair an old one in the Washington area is the last week in August and the first week in September. The leaves haven't yet started to fall, the sun isn't so intense, and, except for annual bluegrass, most weeds are at the end of their growing cycle.

Young grass plants can establish themselves before cold weather sets in, and come spring they have a head start over weeds. Thick, healthy grass helps keep crabgrass and other weed seeds from getting the sunlight and space they need to germinate and take hold.

But don't start to work on your lawn if the soil is bone-dry. Wait until there is a good, soaking rain.

Specialists recommend using a mixture of seeds including such bluegrass varieties as Merion, Penn star and Fylking, and Pennfine perennial rye. Then, if a disease hits the lawn, it probably will affect only one or two varieties and the others will continue to provide a green lawn. The reason for the perennial rye is that it germinates in a few days compared to up to three weeks for bluegrass. It's as attractive and enduring, but it doesn't spread like bluegrass.

Buy quality seed, the kind that is up to 99.9 percent weed-free. Some of the better brands have a satisfaction guarantee on the package. Don't throw the seeds down on a bare, hard ground. Loosen or scratch up the soil so the seeds can take hold and grow. They should not be covered completely with soil because they need light to germinate. If necessary, work some organic matter such as compost into the soil.

Most soils in the Washington area are too acid for the best growth of lawn grasses. If you haven't had your soil tested, University of Maryland specialists recommend applying 80 pounds of ground limestone per 1,000 square feet of area and mixing it thoroughly into the top four to six inches of soil before seeding, also 30 to 40 pounds of 5-10-5 or 5-10-10 fertilizer mixed thoroughly with the top three to four inches of soil. The 5-10-10 is recommended for lighter sandy soils which are often low in potash.

On most soils, grass can be maintained satisfactorily by applying 50 pounds of ground limestone per 1,000 square feet every three to five years.

After planting the seeds, keep the bed moist until the seeds germinate. Sprinkle just enough to moisten the surface. After germination, when roots start to penetrate the soil, water less often but longer, to get the water down where the roots are.

If you already have a lawn and it needs renovation, specialists recommend that if 50 percent of the existing lawn is O.K., repair instead of starting all over. It's suggested also that you attempt to determine what caused part of the lawn to go bad and see if you can remove the primary cause of deterioration. Excessive shade, tree-root competition, poor drainage, soil too acid, inadequate fertility, soil compaction, improper mowing or improper watering could be among the causes.

For a shady area, try a mixture of fine fescues such as Banner, Pennlawn, Highlight and Ruby, and try Kenturck 31 fescue for heavy shade. But the odds are you are going to find, if you haven't already, trying to grow grass in heavy shade is a frustrating experience. Perhaps a ground cover would be best.