If you've gone through the effort of making rich compost of your own, you can use it to top-dress your lawn for thicker, healthier grass. You can use compost to top-dress both new and existing lawns.
On a seeded lawn: After sowing lawn grass seeds, apply a thin layer — about 1/4-inch — of compost as top-dressing to help maintain consistent soil moisture while seeds germinate and tender grass seedlings get established. Top-dressing is especially helpful in arid climates or during dry or breezy spells, where the soil and seeds easily dry out within hours. (If a germinated seed dries out, it's a goner.)
On an existing lawn: Top-dressing with compost may also rejuvenate existing lawns. Lawns often become compacted over time from foot traffic, play, and mowing, which prevents air, water, and nutrients from circulating freely through the turf's root zone.
Top-dressing is more effective if the turf is core aerated before spreading the compost. To core aerate a small patch of turf, use a specialized foot press that you can find at your local home and garden store. For large lawns, rent a machine from an equipment supply company or hire a lawn maintenance firm.
When top-dressing with compost, you should only use screened compost or compost with particle sizes of 3/8-inch or less. Small compost particles infiltrate between blades of grass more easily than large particles, which may smother the grass. Also, take care to top-dress with compost that's guaranteed free of weed seeds, or you may be sowing a future weeding nightmare into your lawn!
No matter where you live, the best time to aerate and top-dress your lawn is when it is most actively growing. This allows the grass to vigorously rebound after having holes punched in it.
If you live in either a cool or "transition" climate and grow one permanent turfgrass (such as bluegrass or fescue), the best time to aerate your lawn is spring to mid-summer. Avoid aerating these grasses during summer's intense heat, which may stress roots. Although some growth occurs in early fall, these types of grasses go semi- or fully dormant as weather cools, making recovery after a late aeration more stressful. Also, early aeration promotes better penetration of summer and fall rains through the soil when it's most beneficial for growth. Improved soil penetration with rainfall creates a healthier, stronger lawn that has a better chance of making it through harsh winters unscathed.
If you live in a warm climate that allows year-round lawns, you have different options. The best time to aerate and top-dress is early to mid-summer when your warm-season lawn (such as Bermuda grass) is actively growing. You should also apply compost top-dressing (without aeration) after overseeding your summer lawn with a cool-season grass (such as ryegrass) in the fall. If you don't overseed, there's no need to top-dress in fall.
Irrigate immediately after top-dressing (unless rain does the job for you). Water disperses the compost evenly among the grass blades.