Grass seed needs warm soil, mild temperatures and regular irrigation to germinate and grow. That means the best times to plant grass in Seattle are spring and fall, when rainfall will make your job easier.
April and May are the best months for sowing grass seed in Seattle, according to Washington State University Extension. However, you can start a new lawn or overseed an existing lawn as early as mid-March if the soil isn't too wet, says Seattle gardening expert Ciscoe Morris. If the soil is wetter than a squeezed-out sponge, wait until conditions are drier to plant seed.
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Another good time to plant grass in Seattle is when the fall rains start in September. If Seattle gets an unusually hot and dry September, wait to sow grass seed until conditions improve–as late as Oct. 15.
Grass may be planted in Seattle during the summer, but it requires more work to coax the seed to germinate and keep the seedlings alive. During warm, dry periods, you must keep the planting area moist–not wet–with several light sprinklings daily.