I understand that a lawn may need to be dethatched periodically so the roots of the grass can reach into the soil (instead of into the thatch layer). How do I tell if my thatch layer is getting too thick? (Preferably before the lawn starts to look bad.)
According to your profile, your location is Bellefonte, PA, therefore unless you correct me, I am going to assume you have a cool-season lawn.
Thatch: Enemy of Lawns from University of Missouri Extension:
If you follow the above link you will see a photo that clearly shows what thatch build up in a lawn looks like.
Examine the lawn closely regardless of how healthy it appears. Cut several plugs 2 or 3 inches deep. Lift and examine the profile. If thatch is present, it will appear as a distinct horizontal layer of brown spongy or feltlike material.
When about 1/2 inch of thatch develops in bluegrass, remove it before the grass is damaged. Thatch is best removed by power raking, verticutting or coring with a machine that may be hired or rented. Coring causes less lawn stress than dethatching with a power rake or vertical mower and provides the additional benefit of reducing soil compaction.
Lawns should be dethatched only when conditions favor rapid turf recovery. For cool-season lawns, early fall dethatching is most desirable.
Cool-Season Grasses: Lawn Maintenance Calendar from University of Missouri Extension:
Remove excessive and heavily diseased clippings. Watch for leaf spot and mildew diseases.
Aerate if thatch is heavy or soil is compacted.
Aerate where needed.
Rake. Dethatch. Kill weed patches. Overseed thin spots. Resume top-dressing if needed.
Clippings need not always be removed. When they are short enough to filter down to the soil surface, they decay and recycle nutrients back to the soil. Remove clippings when they remain on the surface or when excessive thatch is already causing a problem.
Thatch is a layer of undecayed plant parts accumulating at the turf base. It forms a barrier to water and air movement in the same manner as compaction.
Thatch is primarily a problem of intensely fertilized and watered lawns. Even though clippings are removed regularly, thatch still can form because old plants and basal leaves are more resistant to decay than are the clippings.
Thatch removal should be initiated whenever accumulation exceeds 1/2 inch. Early fall is the preferred time for dethatching lawns.
For more information on thatch, see MU Extension publication G6708, Thatch - Enemy of Lawns.
You may also find the information found in the below links well worth you time:
Video: Identifying Thatch from University of Illinois Extension
Cool-Season Grasses: Lawn Establishment and Renovation from University of Missouri Extension.
Natural Lawn Care from University of Missouri Extension.
Lawn Dethatching tools here on SE.
If you decide you need to dethatch, I would recommend you aerate at the same time.
I dethatch my lawn when the layer is about 3/4" thick . A thick layer of thatch can also:
prevent proper aeration.
impede the flow of water to the roots.
prevent the normal thickening of the turf in autumn.
I would avoid dethatching (or'scarifying') in spring, when the grass has not yet produced side shoots - if the lawn is opened up, it will become more vulnerable to weeds. The best time to scarify is probably early autumn, when removal of the thatch will stimulate the grass to produce runners and side shoots.
If your lawn has a bouncy feel to it when you walk on it, thatch is probably building up. As a general rule, plan to dethatch your lawn when the thickness of the thatch is more than 1/2" deep. To determine the thickness, remove a small square of your lawn to a depth of about 3" and measure the brown layer between the grass blades and the soil surface. Note the spongy layer of material above the mineral soil. If this layer is more than 3/4" — 1" thick when you compress it, you should consider having your lawn dethatched or beginning a management program which will encourage thatch decomposition.
See also my answer to Lawn Dethatching Tools here.
I'd go by the way it looks - if it looks like there's quite a bit of yellowing, dry, and dead grass then I'd scarify.
Alternatively, if you don't want to wait until it begins to look bad then periodically just scarify a small patch to see how much thatch is lifted. If there's a lot, the lawn will probably benefit from a good scarification.
This video shows what I mean: Lawn Care & Landscaping : How to De-Thatch a Lawn
A useful side-effect of scarifying by hand is that's also very good exercise!