One of the best techniques to use to thicken your lawn is to apply an appropriate grass seed and fertilizer to the existing turfgrass in the spring. Applying new grass seed in the spring will allow the seed to germinate, sprout and become well established before the weather becomes too hot in mid- to late-summer. Fertilizer will help the both new seed and the existing turfgrass to grow healthily. When the new seed is established the whole turf will be thick, green and welcoming.
Overseed in the spring when the existing turfgrass is still dormant. Mow the lawn so it is closely cropped to the soil. Remove existing thatch, if any, with a metal rake. Rake all of the thatch into a pile put it into plastic yard bags. Take the thatch to a green waste dump for recycling.
Spread new grass seed. Rotary spreaders or drop spreaders make spreading grass seed evenly and uniformly quite easy. Different spread rates will apply differing amounts of seed to the existing turf.
Apply a starter fertilizer according to package specifications.
Spread a thin (1/8 inch to 1/4 inch) layer of peat moss over the seeded and fertilized turfgrass. The peat moss will help protect the seeds and help the seeds sprout. Keep the turfgrass consistently moist. The moisture will help the seeds to sprout in a timely manner.
Continue to water the turfgrass regularly until the new grass becomes long enough to mow for the first time. Mow the new grass when the blades are long enough that the mower will remove the top third of the length of the blade.
Apply another course of fertilizer to the over-seeded lawn after six to eight weeks to continue to deliver the needed nutrients to the over-seeded turf. The fertilizer will also help to keep the lawn thick and green.