Remove undesirable weeds from the lawn. If more than 50% of the lawn is infested with grassy weeds, you'll need to completely renovate and spray the entire lawn with a non-selective herbicide such as Roundup or Finale. In most situations only partial renovation is necessary. Use a selective broadleaf herbicide to control broadleaf weeds and non-selective herbicides to spot-spray patches of perennial grassy weeds. Be sure to read the herbicide label for guidelines on how soon you can seed or sod following the application.
The remaining desirable turf will compete with the new grass seedlings for water, sunlight and nutrients. To lessen this competition, scalp the existing turf with a lawn mower. Mowing at half the normally recommended mowing height will open the turf canopy and stress the existing grass plants slightly. This reduces their competitiveness and creates a more favorable establishment environment.
Follow the close mowing with cultivation. The key concept in renovating is seed-to-soil contact, which helps ensure young seedlings will successfully become established. The less desirable the existing turf is, the more you need the newly planted seed to develop into a solid stand, so the more extensively you need to cultivate (to create more seed-to-soil contact). In other words, match the type and extent of cultivation with the severity of the problem.
Slicing and slit seeding are less disruptive than aeration and dethatching, but create a poorer seedbed. If the need is great, aerate in several directions. This will help provide more sites for seed to establish. If dethatching before slit-seeding, several passes may be necessary. However, excessive dethatching can be harmful. It is important to leave a small amount of stubble or to prevent erosion of the seedbed and to shade new seedlings during establishment.
Obviously, you should avoid overseeding with new varieties or species that will clash with existing turfgrasses. Thus, warm- and cool-season turfgrasses should never be mixed, and it may be best to avoid certain cool-season mixes.
Be sure you choose suitable varieties. For example, look for varieties that are resistant to common diseases in the area. If shade contributed to the original lawns decline, look for shade-tolerant varieties.
Once the seedbed is ready, calculate the size of the area you'll seed. Knowing the square footage you will overseed you can then determine the amount of seed you'll need. Adjust the seeding rate according for lawns that may need more seed.