The easiest and cheapest way to begin thickening up your lawn is to properly feed the existing grass. Even if it is thin and anemic, it is easier to thicken what you have than to start from scratch.
Keep in mind, lawns are like people, the more they are fed, the thicker they get! With that in mind, when attempting to make your lawn thicker, feed it regularly. I recommend a treatment every 4 to 5 weeks.
Secondly, you need to use the right fertilizer. Nitrogen is not really the best nutrient for making a lawn thick. While nitrogen will give your lawn a spurt of growth, it won't do much for the long term root structure. Potassium and Phosphorus, on the other hand, will help strengthen those grass roots, and in the long run, thicken the top as well. Look for fertilizer composition with rates like 10-10-10 or 20-20-10 which is sometimes referred to as "starter fertilizer."
Water Regularly and Deeply
Many homeowners allow their lawns to go dormant during the hot and dry summer. While this is a common and acceptable practice, it won't do much to help your thin lawn get thicker. In fact, if a summertime lawn is allowed to go for too long without proper moisture, you will get dead loss in the turf, thus thinning it out.
When you water a lawn during a thickening regimen, you need to be consistent and deliberate. I always recommend 4 irrigation cycles per week laying down a minimum quarter inch of water each time. This is equal out to one inch of water weekly in each area of the lawn. Invest in a rain gauge if you are unsure of how much water your particular sprinkler is putting down.
Mow High
This is a good recommendation for any lawn, but a must for a lawn that is being fed and watered on the schedule recommended above. By mowing tall, you don't run the risk of scalping the turf and drying it out. Taller grass also retains more leaf surface area that can photosynthesize and strengthen the root system of the turf.
Aerate in the Fall
Grass roots do most of their growing and spreading in the fall. Aeration helps loosen compacted, pressed lawn soil, thus allowing roots more room to grow. It is also a good idea to over seed your lawn with fresh cultivars directly after the aeration. New grass seed is always welcome, but more will germinate after aeration as the soil has been disturbed enough to offer a slight covering.
If you follow the simple steps above, your lawn will be noticeably thicker in less than one season.
