Quick definition before you start:
Preemergent needs to be applied before the weed begins to grow. Apply while the weed is still in the seed form.
Post-emergent kills the weed after the weed has leaves. Post-emergents work best on young plants. Older weeds will need several applications over several weeks or months to totally kill.
The key is to apply preemergents before the weeds emerge. However, the goal should be to control weeds through routine practices of: mowing, watering, and fertilization.
Spring application: Early March and a second application early to mid June
The optimum time for the application of preemergent herbicides for the control of warm season weeds is early March but you can apply the pre-emergent at any time and receive some benefit. A second application in June
Fall application: Early September
The optimum time for the application of pre-emergent herbicides for the control of cool season weeds is September, but you can apply the preemergent at any time in the fall and receive some benefit.
What to buy: Look for products with 0-0-0 or 0-0-7 composition on the bag. There are several on the market. Brand names like Halt, Dimension, Lesco's 0-0-7, Gallery (use primarily in fall). Corn Gluten has proven to be only 6% effective in Parker County.
Any time you intend to apply preemergent weedkillers and fertilizers at roughly the same time, apply the fertilizer first. Water it into the soil deeply. Wait a couple of days, then apply the pre-emergent granules and water lightly.
The link below gives you the Texas A&M recommendation on managing bermuda grass lawns.
Check out this link it is worth the read: