Question
Another "thatch" theory. A few years back my thatchy areas seemed to have some sort of fungus problem.  One county agent called it a fungus, another said there was some fungus but the real problem was drought.  The grass blades were nice and green at growth end, but turned brown from bottom up.  When I mowed at 3 inches the lawn first looked like I had scalped it.  I had this theory that this dying grass (which seemd so straw like) was causing thatch, but I was told that since the areas were already dying they would decompose and not add to the thatch.  What do you think?

Answer
Grass plants are monocots and they grow from "the bottom up".

Each grass leaf has a blade and a sheath. The sheath wraps itself around the blade to protect it, while the blade continues to grow upwards. Where the blade and sheath meet is known as the collar.

The "new growth" is closest to the base of the plant (inside the sheath) whereas the "old growth" is closest to the edge of the grass.

This is why you can mow a lawn. You cut off the old material and the plant is replacing with young material by pushing up the the grass blade (like a tape meassure)

Most dicots, such as flowers and trees, grow differently. They will put out new growth "on top" and the parts closest to the ground is the old growth. This is why flowers and trees can develop a harden stem (old material) and the fresh green material is at the top.

If you have grass which has died, mowing it will remove the top (old) layers first and then the new layers secondly. It is not uncommon to find dead grass with a green halo of old grass above.

Grass clippings contain material which decompose fast and leave very little left. Even if your entire lawn died, the material which is left after a short time is next to nothing. This is why grass clippings seldom produce thatch.

Some other plant materials, such as oak leaves or pine needles, for example, contain a large amount of ligning and suberin. Suberin and lignin are two waterproof, phenolic polymers which are difficult for microbes to decompose. It takes microbes a long time to dispose of these plant materials and left on the ground, or mixed/mulched into the lawn, they can contribute to thatch.

This is also why I previously have claimed that the benefits of topdressing with compost is more theoretical than real. In principle, the mircobe herd, which grew in size to eat the compost, would attack the thatch layer, also. In many instances, however, the microbes may not be able to decompose the thatch layer as it contain plant parts which are not easily decomposed.

Leaf sheaths, roots and crowns of grass plants do contain fibers, lignin and suberin. Even so, they contain much less than other plants, but more then the grass blades themselves.

Therefore, as long as you cut the grass blade off (and not cut significantly into the lower portions of the grass plants) clippings will not contribute to thatch.

Grass plants which produce stolons or rhizomes contribute more heavily to thatch, such as bermuda, st. augustine. On a much lesser scale, kentucky bluegrass also spreads by creeping stems underground known as rhizomes. As the rhizome continues to grow it will create additional plants that are capable of sustaining themselves by producing their own roots and shoots. These rhizomes are the main contributer to thatch in a home lawn in addition to other lingin rich yard debris which may be incorporated on the lawn over the period of the growing sesaon.

Generally, that is not a big concern unless it becomes too thick. A small amount of thatch is needed to help protect the grass plants.

To reduce chances of thatch:
- mow frequently and at the right height. Try not to remove more than 1/3 of the length of the grass in any mowing
- fertilize lightly in spring, and regularily in fall
- water infrequently and deeply