In Houston, the vast majority of lawns are of the St. Augustine variety, although there are also some Bermuda-based and Zoysia lawns, too.
Most St. Augustine yards have bits and pieces of Bermuda mixed in, but some are overwhelmed by it. In fact, following the January freeze, some people have found they have more Bermuda grass than ever before.
One way of removing Bermuda is to kill it, till it, and replace it with new sod. But that's usually just for extreme cases ... where the Bermuda has grown to be absolutely solid. Still, if you want to start over, you should follow the rules in our Kill-Till-Fill-Sod Tip Sheet.
In most cases, though ... where there is a mix of Bermuda and St. Augustine ... cultural practices will help St. Augustine win the battle.
Simply mowing properly is the answer 90 percent of the time. If you mow as tall as your mower will allow, the St. Augustine will almost always crowd out the Bermuda. Conversely, if you want to accelerate the Bermuda, mow shorter and more often — then the Bermuda will win.
The reason for mowing taller? Bermuda can't grow in shade, and taller St. Augustine will shade the slower-growing Bermuda. Yes, you still have to mow once a week, and if your neighbors don't come up to your level, it is going to look like you don't mow at all. However, I promise your lawn will be a richer green with a thicker texture than your shorter-cutting neighbors. I call this the "Bring Your Neighbor Up to Your Level" challenge.
There's also an added benefit to growing taller-than-normal St. Augustine: you won't need to irrigate as frequently because tall grass supports deeper roots which have more water available to them deep in the soil.
A second benefit is more biotechnical. Grass blades have microscopic pores (stomata) to transpire carbon dioxide (CO2), oxygen and water vapor. The stomata open in the morning, and close when they get their fill of CO2. Since taller grass has more stomata exposed, it more readily collects CO2 released by soil microbes. And, as CO2 is heavier than air and tends to remain close to the ground, it's harder for wind to blow it away in taller grass. Therefore, tall grass gets more CO2 because it has more stomata and because the amount of CO2 is denser near the stomata. And, as a consequence, the stomata close off earlier in the day shutting off the transpiration of water vapor from the plant and allowing the plant (and soil) to retain water better.
I know ... that was way more technical than you probably needed. But I like researching this kind of stuff.
There is also the "chemical" approach. The liquid version of the herbicide Atrazine, if sprayed on Bermuda in the month of May, will diminish the Bermuda's health and enable the St. Augustine to take over. I know ... you die-hard Lemmonheads are calling me a "hypocrite."Liquid atrazine, if used per the label instructions, is not near as toxic to the environment as weed-and-feed formulas with granular Atrazine. I would prefer that you not use it at all, but in cases where there is a thick mix of St. Augustine and Bermuda, this aide comes in handy. Plus, if you follow the label instructions properly, all you'll do is spray the grass blades. You won't overuse or abuse the herbicide, so it won't poison the environment.
Finally, there are two absolutes you should keep in mind when trying to weed Bermuda from your St. Augustine. First, Bermuda does not suffer from drought damage like a St. Augustine yard ignored during a summer vacation. And second, it won't suffer chinch bug damage. So, if you do a poor job of maintaining your St. Augustine, Bermuda is going to take over.
And one more post script - if you want the Bermuda to take over, then get a reel lawn mower. These are the kind that cut from over the top, unlike a rotary lawn mower needed for St. Augustine.