Ask Mr. Smarty Plants

Some of your problems Mr. Smarty Plants is not equipped to deal with, but we can help you out with the grasses. First, please step away from the spray can. Image is a post-emergent killer of broad-leaf weeds, which means it not only won't kill the St. Augustine, but can't get at the underground stolons of both grasses. Meanwhile, applications of too much is only polluting your soil and will eventually be washed away to drain into the water supply

Now, the Lady Bird Johnson Wildflower Center is committed to the growth, protection and propagation of plants native not only to North America but to the area in which those plants grow natively. Both grasses you are dealing with are non-native, and we would not recommend either of them. On the other hand, you seem determined to have a conventional lawn, for which there may not be many good native choices.

To answer your main question, will the St. Augustine crowd out the Bermudagrass? Not likely. We don't know what the sun and shade situations in the adjoining yards are, but St. Augustine is often used because it can tolerate shade. It also requires more water than most lawn grasses. Bermudagrass, on the other hand, can survive without so much water and does not thrive in shade. So, if your lawn is in the sun and you don't over-water it, the St. Augustine is not going to be able to compete with the Bermudagrass. And, even if it does, they are both grasses, they are both green. Just mow them and don't stress out so much.

One last word on your Bermudagrass-it is considered one of the worst invasive weeds in the South, so it's just as well you like it, you're probably going to keep on having it.