Have you investigated the many factors involved in what is called Centipedegrass Decline? It would be worth your time to investigate. Improper fertilization, watering problems, soil pH, herbicides, and other abiotic factors can all contribute to Centipedegrass Decline. The first thing that comes to my mind, however, when reading your post is nematodes. When I lived in Beaufort County, the issues with several species of nematodes were becoming very serious. We tried to convince people to stay away from Centipedegrass because of it. Homeowners don't have the arsenal of nematicides available to them that golfcourses do. If you haven't already done so, check with your local Clemson Extension office for advice about how and when to collect soil for proper nematode assay. I know that I haven't addressed your herbicide question, but unless the cause of the problem with your grass is resolved, you'll always have a terrible stand of grass. Clemson can also be very helpful with recommednations for weed control, too. Ask to speak to an agent who specializes in turf, rather than a master gardener volunteer.

I believe Corn Gluten is considered an organic pre-emergence. Emergence meaning when the plant from seed can be seen above ground. Specifically what Corn Gluten does is attack the plant's rectal when the seed coat softens, opens and the stored energy within the seed begins growth by bringing the rectal into its escape route into your soil. Corn Gluten carries with it a low level organic nitrogen percentage that nurtures the existing plants while serving as a pre-emergence product. The cost of this and several chemical ways to do pre-emergence has always been debated. My opinion is that all ways of using pre-emergence tactics is only a relatively short time period of less that excellent control anyway. The chemical companies have always paid the universities to come up with warped facts to satisfy the use of their poisons. For instance as stated above: half life is a relatively short period of time but soil life comes out to be about a year. Few stop to read, think and question. First these chemicals are non-degradable. Half life I guess means only half of it has been flushed by rain or watering from your use point and is on its way to your rivers, bays and oceans. Full life must mean your soil is again void of the chemical. That is a direct statement that everything you put on is now en-route to the oceans of the world. The enormous poison pits and zones in the oceans are known and growing. There is no other place for them to go except to enter the evaporation facts and become even stronger where they are. Severe weather may lift them and redeposit them to complete the journey once again back to the ocean pits. If you use and love sea salt now how do you feel it will be enjoyed in another million years? Back to our soils now stated to be full life with no measurable poison in the soil. What they do not test and will not tell you is how long your natural biological recovery will take. The realistic recovery may be five years or longer. Is there a point of no return? I do not think anyone knows. No part of the answer to that question is very nice. Would growing the grass at four inches and mulching the cuttings make better sense? Would spot treatment in the early recovery make more sense than wholesale broadcast of some poison? Do we really need golf course picture perfect homestead grasses? Could we as homeowners recycle those leaves and plant tops by mulching open soil? Could legumes become cover crops that rejuvenate? I'm sure most could do some of these things to eliminate the use of some if not all man made chemicals. This reasoning and these questions apply to all man made non-biodegradable poison chemicals. All! There are no other statements possible that are absolutely true. How does all this relate to pre-emergence chemical concerns? Dumb question. 90% or more of those chemicals are non biodegradable.

A pre emergent is a product meant to stop a plant from continuing growth after it germinates so it needs to be applied early enough to prevent that from happening but not so early that the weather will dilute that material and render it ineffective. Just when you apply depends on what you are trying to suppress, but generally at least 7 days before your target "weed" will germinate.

Before you do anything read the label, at least pages 4-10 which talk about application in turf. The application rates for the transition zone are: 4 month rate - 0.36 oz/thousand sq ft 5 month rate - 0.4 oz/thousand sq ft 6 month rate - 0.46 oz/thousand sq ft Notice there's not a lot of wiggle room, so you need a good scale to measure. I recommend the 5 month rate for safety. The 6 month rate will extend into August and that's a little too close to overseeding time for me. Chances are it would be ok, up to you. If you don't plan on overseeding in the fall, definitely go for the 6 month rate. Page 4 has mixing and application instructions. Make sure to calibrate your sprayer before starting because again, applying too much will have consequences.