Aerating and Overseeding – One process
Aerating your lawn should always be coupled with overseeding. After all, if you open up the surface, all the competing weeds and possibly fungus will get a head start and all that oxygen and nutrient and moisture will go to the wrong species if you don’t put down some grass seed. You may wish to cut corners with the cost, so just aerate every other year, although I would try to cut costs elsewhere if you can, aerating is very important, once done, together with overseeding, you will notice an acceleration and greening, with strong growth. Don’t cut the resulting outburst too short at first and water well.
Cold Climate Issues
Don’t try aerating too early in the spring, if you live in very cold climate, wait until the ground is thoroughly thawed. Don’t forget that the ground can fall well below freezing at night whilst the average temperature is above freezing. It is these cycles of frost and thaw that will destroy the new seeds. They can only resist for a short time, maybe a couple of frost thaw cycles,so beyond that, its just a waste of seed.
If you have a clay laden soil, then water drainage is going to be difficult, it will improve with successive aeration processes but it may take many seasons, in extreme cases, try aerating twice or more times. Brushing sand over after aerating a particularly stubborn clay soil lawn will ease the problems a bit faster. You really need that water to pass through the soil layers to the grass roots of the existing plants and to reach the new seeds put down when overseeding. It may be worth overseeding in a small patch, if it is particularly difficult in one area.
Corn Gluten Spray
Don’t apply corn gluten sprays after reseeding/ overseeding. If you are going to use these products pay great attention to the instructions, its basic mechanism to the stop weed seeds from absorbing moisture.
Minimum depth to Aerate
Really, to do a proper job, you need to be removing plugs from the lawn, not just sending a solid spike down, it will kind of work, but horizontally it is actually doing more compaction ! I would aerate if possible to 2.5 or 3.5 inches with a hollow tine aerator and leave the plugs on the surface. Micro organisms within the removed plugs will help to break down the thatch. Indeed by using hollow tines, some of that thatch material itself will be removed. Once watered over, and seeded over, new deeper roots will develop, making the grass plants stronger and more dominant over the weed species.