Question
To get rid of thatch, my county agent has discouraging "dethatching" with a "dethatcher" and has encouraged aerating.

1) renting the aerator is pricey so I only do it once a year.  I share the rental with a neighbor, so finding a good time gets tricky.  Is August too early?  What is too late?  We live in the Champlain Valley of upper NY state.

2)  I aerated two years in a row (three times over the lawn), missed a year, aerated, and then missed last year.  I saw no impact on the lawn during this period.  Do you think aerating is an effective way to reduce thatch?

3) I have not top-dressed after aerating, but intend to do that this year.  I will topdress with a thin layer of composted cow manure after the top dressing.  Would there be any value in top dressing with cow manure again in the spring and mid summer? (but without aerating)

4) When I take up sod I try to salavage as much of the dirt from the root zone as possible.  I've saved some of this dirt/fine roots material.  Would it help to spread it over the thatchy areas, or should I put that same energy into another layer of true compost?

Rick  

Answer
1) you do not need to core aerate more than once per year. In your area, early september is the ideal time. I would core aerate once the summer highs are starting to go down. Water the lawn a few days before aerating (but not right before or you get a muddy mess and the plugs will not release the tines). This to avoid the lawn being draught stressed and to ensure you can penetrate deeply into the soil with the tines. For same reason, I recommend the types you rent at equipment rental stores. They are typically large gas powered machines which due to weight pushes into the soil.

2) moderate levels of thatch can be removed by aerating. Thatch can create a barrier where water, oxygen and nutrients can not reach the root zone. By aerating you open up the soil. Core aerators are preferred to spike aerators as the hollow tines from core aertors will pickup plugs and deposit them on the surface of the soil to decompose. Spike aerators do punches through the thatch, but can make the soil compacted around the spike holes.

3) Top dressing is a much debated topic. Most people agree that topdressing with compost or decomposed cow manure is a good idea in moderation. If you do so while core aerating it is very beneficial. If you do not core aerate and put a layer of compost over a thatchy lawn, you can actually make it worse.

4) same as above. if soil is put back on the lawn, it certainly does not hurt it. but if you put it back over a very thacthy lawn it can aid the problems. soil itself does not provide any nutrients or benefits to the lawn like compost would.