If your lawn is a cool-season grass type (tall fescue, usually), it should be overseeded in early autumn, not spring. (Warm-season turf like zoysia is not overseeded.) If you have to repair specific areas of the lawn, you can sow seed soon (April is generally a decent time so the soil has warmed) as needed to discourage erosion or weeds from causing problems, or patch bare areas with sod (easier than seed). There are a number of reasons sowing fescue seed in spring is discouraged, but if you can't wait until September or so (the ideal seeding timeframe), then you can put down seed in a couple weeks. Spring-sown seedlings will face potential problems from weed competition (and few herbicides can be used in conjunction with seeding, especially given Montgomery County lawn pesticide restrictions), late frosts, and heavy rainstorms that could erode or dislodge seedlings. In early autumn, warmer soil temperatures can help roots establish faster, and while winter annual weeds may be germinating, they usually don't need to be treated as they'll merely die out on their own by late spring.

We recently overhauled all of our lawn care web content. You can find connections to all of the new pages on our Lawns for the Maryland Home Landscape page. The Maintaining an Established Lawn page includes recommendations and procedures for overseeding.

Any time you sow grass seed, it needs careful attention to watering, both to initiate germination and to avoid killing the seedlings if they dry out while growing roots. (They are not very tolerant of getting too dry during this critical stage.) We would not suggest overseeding during any period of time that you can't monitor the lawn for watering needs, since seed/seedlings may need a light watering several times per day depending on soil moisture-holding capacity and the weather.

Miri