|i live in Abilene, TX. It's dry here. 2/3 of the yard is st. augustine and the other 1/3 is a|
|I'd be inclined to remove the mystery grass and either lay sod over the bare area or take sprigs or plugs from the existing St. Augustine and plant them in the bare area. Installing sod will give you the fastest results and since it's already well established you won't have problems with weeds popping up.
Probably the least expensive solution is to spread top soil over the low spots to help level the yard. You can spread it right over any grass that is growing in these areas. The grass will eventually grow through the topsoil. Spread about an inch of soil over the grass. Once the grass grows through the topsoil and looks healthy you can spread another inch of topsoil over it. Repeat until the low area is level with the rest of the yard. In 4-6 weeks your lawn should look great. Best wishes with your lawn!