Question
My lawn has a moderate thatch build up. Is it better to have it dethatched, aerated, or both?

Answer
With moderate thatch building (less than 1") I prefer to core aerate. Core aerators will physically punch through the thatch and in the process pull plugs of soil from the lawn. The core holes will elivate compaction which will allow grass roots to expand. it will allow oxygen, water and fertilizer to reach the root zone and permit carbondioxide to escape (gas exchange). It is one of the most beneficial horticultural practices you can perform on your lawn.

De-thatching is mainly beneficial if the thatch layer is very severe. Core aerators may not punch deep enough and where the core machines do not remove a plug, no help will come. Therefore on severely thachy lawns, I would still de-thatch.

De-thatching is generally a lot more damaging to the lawn and you risk ripping out most of the grass on a severely thatchy lawn (but grass would never have grown properly so a mute point). Core aerating, while still damaging, usually allow the grass to recover quickly especially since it immediately improves conditions for grass growth.

You can do both, but with moderate thatch, only, I would probably core aerate 1-2 times annually to keep it at bay.

I would recommend that you de-thatch/core aerate in late spring or early fall (early fall preferred) for cool season grass, and early summer for warm season grasses.

Good luck.

Note: for most warm season grass (st. augustine, bermuda, zoysia, etc) some thatch is unevitable due to the growth pattern, but for cool season grasses (Kentucky Bluegrass, Perennial Ryegrass, Fescues) there should not normally be a thatch build up. If you have cool season grass, and find abnormally high thatch build up, review your fertilizing, mowing and watering routines may be required.