· by Michael Blair, Nashville's Weed and Feed
Compaction is the normal condition of a Middle Tennessee clay lawn after a few years of use, and it limits root depth, water infiltration and air exchange.
What does core aeration do for a clay lawn?
Pulling plugs of soil out and leaving them on the surface. That relieves compaction, opens channels for water and air, and gives autumn overseeding an ideal seedbed.
Spike aeration, which pushes holes rather than removing soil, compacts the walls of the hole and achieves considerably less.
Timing
Autumn for fescue, immediately before overseeding so seed falls into the holes. Late spring to early summer for bermuda and zoysia, during active growth.
Never immediately after a pre-emergent, since it breaks the chemical barrier.
- Core aeration, not spikes
- Autumn for fescue, immediately before overseeding
- Leave the plugs to break down
- Not after a spring pre-emergent application
- Thatch over half an inch needs addressing
What is lawn thatch?
A layer of undecomposed stems and roots between the soil and the green growth. It is not grass clippings, which break down quickly and do not contribute.
A thin layer is beneficial. Over about half an inch it holds water at the surface, harbors disease and stops water reaching the roots.
Where thatch comes from
Over-fertilizing, shallow frequent watering and compacted soil that suppresses the microbial activity that would break it down. Bermuda and zoysia produce more than fescue.
Core aeration helps by bringing soil into the thatch layer and reintroducing the organisms that decompose it.
FAQ
Aeration, Thatch and Compacted Clay — questions
When should I aerate a lawn in Nashville?
Autumn for fescue, immediately before overseeding so the seed falls into the holes. For bermuda and zoysia, late spring into early summer during active growth. Avoid aerating shortly after a spring pre-emergent application, since it breaks the barrier you paid for.
Do grass clippings cause thatch?
No — that is a persistent myth. Clippings are mostly water and break down quickly. Thatch is undecomposed stems and roots, and it builds from over-fertilizing, shallow frequent watering and compacted soil that suppresses the organisms that would decompose it.
Is spike aeration as good as core aeration?
No. Spikes push soil aside and compact the walls of the hole, which on clay can make matters slightly worse. Core aeration removes plugs entirely, relieving compaction and creating open channels. Leave the plugs on the surface to break down rather than raking them up.
Should I rake up the plugs after aeration?
No, leave them where they fall to break down. The plugs are soil pulled from the lawn, and as they crumble they work into the thatch layer and bring in the organisms that decompose it. Raking them away throws out that benefit along with the soil you just pulled.