Question
When is the best time to dethatch my warm weather grass lawn (St Augusting/centipede).

Also, how often should I dethatch?

thanks

Answer
Good Morning Jon:

Although I do not give advice on warm season grasses I will do the best I can to help.
A couple of good sites are as follows: The last one is a PDF file so it may take time to load it. Just copy and paste to your browser.

Centipede lawns at:
http://www.ces.uga.edu/pubcd/L313-w.html

Lawn care Georgia:
http://www.oznet.ksu.edu/library/hort2/MF2131.PDF

Thatch in lawns is often misunderstood; both its cause and control. Some lawns have serious thatch problems while others do not. Thatch is a layer of living and dead organic matter that occurs between the green matter and the soil surface. Excessive thatch (over 1/2 inch thick) creates a favorable environment for pests and disease, an unfavorable growing environment for grass roots, and can interfere with some lawn care practices. The primary component of thatch is turfgrass stems and roots, and accumulates as these plant parts buildup faster than they breakdown. Thatch problems are due to a combination of biological, cultural, and environmental factors.
Cultural practices can have a big impact on thatch. For example, heavy nitrogen fertilizer applications or overwatering frequently contribute to thatch by causing lawns to grow excessively fast. Avoid overfertilizing and overwatering. Despite popular belief, short clippings dropped on the lawn after mowing are not the cause of thatch buildup. Clippings are very high in water content and breakdown rapidly when returned to lawns after mowing, assuming lawns are mowed on a regular basis (not removing more than one-third of the leaf blade).
As thatch levels accumulate to greater than 1/2 inch, lawn problems may begin and thatch needs to be controlled. Thatch may be torn out with a dethatcher or vertical mower, but will most likely return unless the cause is corrected. Mechanical dethatching is also very destructive to the lawn because roots are in thatch instead of soil, so plants tear out easily. Overseeding is usually required afterwards.
Core aerification, followed by topdressing are two methods which will generally correct the reasons thatch is accumulating. Core aerifying machines will pull up small soil cores to the surface, which are left there to act like topdressing. The holes created help solve problems such as compaction or poor drainage. Topdressing is simply adding a thin layer(1/8 to 1/4 inch) of compatible soil over the thatch, which adds microorganisms to help in breakdown.
Aerifying equipment may be rented or services are available to do it for hire. Aerifying is an excellent lawn practice with many benefits, as it helps solve soil problems which in turn leads to better root systems and healthier lawns. Aerify in spring or fall, making sure adequate moisture exists in the soil. Make two trips over the lawn, the second perpendicular to the first. An average of 15 to 20 aeration holes per square foot is suggested.

Warm Season Grass Dethatching (Taken from the Arizona Master Gardener Manual)

Wait until the grass has greened up for 2-3 weeks and the night time air temperatures are above 60癋 if you want to dethatch in the late spring. Early and midsummer dethatching is best! Do not dethatch heavily in the fall, since the warm season grass will be damaged and be weak for the winter. Collect the thatch by raking or by mowing up debris. Follow with 1.0 lb. -Nitrogen- per thousand square feet of either a balanced (N-P-K) or a fertilizer such as ammonium sulphate, ammonium phosphate, or ammonium nitrate.
Thatch can be removed by hand raking or with a power rake. Hand raking is laborious and is only practical for small areas. Power rakes can be rented from rental firms, or the service hired from a professional lawn care company. Power raking devices use rigid wire tines or steel blades to lift thatch debris and a small amount of soil to the lawn surface. The soil should be moist, not dry, for best results. Power raking during excessive soil moisture conditions tear and pulls the turf from the soil instead of slicing and lifting the thatch debris as desired. Remove clippings and thatch debris immediately. Always dethatch in at least two directions.
After thatching in early summer, apply a preemergence herbicide to prevent the potential encroachment of crabgrass. This application can be done in combination with fertilization (1.0 -1.5 lb. N/1000 sq. ft.). Make sure that the preemergence is applied at least 75-90 days before fall overseeds take place.
Have a good lawn!
Floyd McMahon