Question
When over sowing do you have to add topsoil or can you just spread seed over grass and water in? Also when and how can you harvest the seed from the actual grass? My granddad used to do it when I was little using the clippings and a strainer but I don't know when.. Thanks Matt

Answer
Grass goes to seed in late spring. You can see the seed heads in the lawn. Note: many modern cultivars are actually sterile.

if you don't have actual bare spots, but just a wish to thicken it up, it helps to core aerate, or slice the soil with a machine and then spread grass seeds and water. topsoil is optional.