Water is a critical step anytime you plant new grass seed. Without the proper amount of moisture, the seeds will fail to germinate. Although a garden hose or sprinkler is usually used to water the area for planting, there is no reason why you can't wait until it rains. The key to planting grass seed in the rain is to ensure that it is a light rain rather than a downpour. If the rain is heavy, it will wash away the grass seeds.
Determine how much grass seed to use for your size lawn by reading the application rate on the grass seed package. The rate will be in lbs. per 1,000 square feet.
Pour five parts of moist construction sand into a bucket and add one part of grass seed to it. Mix the contents of the bucket thoroughly with a trowel or garden spade. The sand will weigh down the grass seed so that it does not wash away in the rain.
Pour the sand and seed mixture into a seed spreader and set the rate of the spreader dial to the application rate specified on the grass seed package.
Move the spreader to the edge of the lawn and push down on the dispenser handle to begin applying the seed. Walk along the entire length of the lawn and turn around when you reach the end. Continue walking until you have applied grass seed to the entire surface of the lawn.
Rake the entire surface of the grass lightly to cover the grass seeds with soil. The seeds only need to be approximately 1/8 inch below the surface, so you do not need to rake very deep.
Keep the ground constantly moist until the seeds germinate after planting. If rain does not continue, water the ground with sprinklers or a garden hose.