Question
Why is dethatching a lawn good for it please?
Sacramento, California

Answer
Good morning Sue:
Thatch is a tight, brown, spongy, organic layer of both living and dead grass roots and stems that accumulates above the soil surface. The rate and extent of thatch accumulation is the result of environmental conditions, soil conditions and management practices (irrigation, mowing, fertilization). Thatch is more of a problem on Kentucky bluegrass, bentgrass and fine fescue lawns. It is seldom a problem with tall fescue or buffalograss. Contrary to some beliefs grass clippings do not contribute to thatch accumulation and should be returned to the lawn during mowing to recycle the nutrients that they contain.
You can measure the thatch buildup by taking a knife, trowel or shovel and remove a plug of turf . Take a deep enough plug so you can see the grass, thatch, roots and soil. Measure the thatch, and if it is greater than 1/2" inch, you should take measures now to reduce it.. As the thatch layer thickens, it becomes the main rooting medium for the grass. This will allow the turf to be more susceptible to drought stress or winter kill and increases the possibility for insect, disease and weed problems. Also, fertilizers and pesticides applied to a thatch filled lawn work less effectively because they cannot penetrate into the root zone.

The best way to remove thatch is to rent A Power Rake or Vertical Mower. Check with your local lawn and garden supply store or tool rental store for a power rake (or vertical mower). Most machines have changeable blades: one for vertical mowing and one for power raking. Do not attempt to dethatch a St. Augustine  grass or zoysia grass lawn as damage may result. You should only vertical mow these grasses.
You might consider hiring someone to do it for you.
For smaller lawns, you can remove thatch by simply using a thatch rake, which has sharp, rounded tines. Typically these are sold at lawn and garden supply stores.
Prepare the lawn for dethatching by mowing at the lowest recommended height for your grass type. Two to two and a half inches would be best for Kentucky bluegrass. Next, lightly water the lawn. The soil should be moist but not saturated otherwise you could tear up the turf.
Using the power rake with the proper reel and blade settings for your lawn, make several successive passes over the lawn just as if you were mowing the grass. Do the same thing again, but perpendicular to the first pass.
If you are using the thatch rake, rake the lawn in one direction, then again perpendicular to the original direction.
Use a lawn rake to clear the removed thatch from your lawn. Rake it into piles and collect it with trash bags or lawn and leaf bags. If you have a compost pile, you can dump the removed thatch in there. It will work well as mulch on your vegetable or flower garden.
If thatch is minimal, you can have your lawn core aerated. This process simply removes "cores" (or "plugs") from your lawn. Doing so will allow air, water and nutrients to easily reach the root zone where they will aid in the breakdown of organic matter, thus reducing thatch. If this is done regularly, thatch will rarely be a problem.
You can rent a core aerating machine, or hire a lawn company to do it for you.
Core aerating also helps with compaction. It is recommended that core aeration be done at least once a year for heavily used and maintained lawns in the spring for warm-season grasses and during the fall for cool-season grasses.
Thoroughly water the lawn to prevent drying out the turf. If you haven't fertilized recently, now is a good time to do so.
A good web site that you may be interested in is as follows:
Thatch Management from University of Missouri:
http://muextension.missouri.edu/explore/agguides/hort/g06708.htm
I hope I have answered your question.
Have a Good Lawn!
Floyd McMahon