Answer: To some extent the answer will depend on why the ground is uneven and why there are bare spots. If it is shady for example, and the lumps are caused by tree roots, then trying to grow a smooth lawn is probably not feasible because lawn grasses need full sun to grow well and filling over the roots can kill the tree. If there are old excavations there that are still settling, or other unique reasons for the lumps, that could be an ongoing issue.
I would suggest you consult with your county extension to run some basic soil tests and determine whether or not the existing soil can support grass growth. If not, based on the test results, they should be able to help you determine what would need to be done to prepare the soil for growing a lawn. It may involve removing the existing grass/weeds and starting over, or perhaps it can be overseeded and patched. Either way it may involve adding organic matter as well as other amendments such as lime or fertilizer based on the test results.
If you do need to remove the existing vegetation and till the soil and then level it, it is best if you can do that a few weeks ahead of planting so that the soil has time to settle a bit and can be re-leveled or fine graded again prior to planting. This also allows time for weed seeds at the surface to sprout and be controlled prior to planting.
Generally speaking, sod is the faster way of getting a good lawn but it is far more expensive than seeding. Both ways require proper soil preparation to provide a good environment for the roots. If the soil is not adequate, your seed or sod will simply not thrive there.
Your county extension should be able to tell you which particular grass varieties do best in your area and in your particular soil. They should also have available directions for lawn establishment and maintenance programs suited to your local area.
June is really too late in the season already to try to establish a new lawn, but end of August/beginning of September is actually the best time of year to do so. So you can work on the soil this summer and be ready to go then. Good luck with your project!
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