Answer: First, have your soil tested. Sometimes regular fertilization without testing can cause nutrient and pH imbalances, and can lead to thatch as well. The test results will guide you in taking the right approach.
Thatch is an accumulation of a thick layer of dead stems and roots. It can build up over a period of a few years and choke out a lawn. If soil is healthy and biologically active, dead grass parts will naturally decompose, and thatch won't develop. I don't recommend dethatching by hand, because it's very hard work. You can rent a power dethatcher at most equipment supply outlets.
The quickest and easiest way to thicken your lawn is to overseed with a good quality grass seed mixture. The seeds will sprout in 10-14 days so you'll see almost instant results. (If your lawn has excessive thatch, you should remove it before overseeding.) Continue to water and mow on a regular basis, and apply fertilizer as recommended by your soil test results, and your lawn should grow thick and lush this season.
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