That's a tough problem to solve. As you probably know, Bermuda grass is very invasive. Crab grass can be killed with specific herbicides and Bermuda grass can be killed with grass/weed killer, but your wildflowers will be killed as well. Hand-pulling is about the only other way to (sort of) control those beasts. Unfortunately the Bermuda grass can put out deep runners and come back from those underground runners. You can trench around the whole plot, digging perhaps 6" deep, and then bury a plastic or metal edging around the entire plot. That is what I do around all my flower beds/landscape beds (I probably have 300-400 linear feet of metal edging, but it involves and lot of time/work and can get pretty expensive. Even then you will get some Bermuda tunneling under the strip, but the invasive process is at least greatly slowed down. I use edging because I have built-up beds and the edging also helps keep the mulch in place. You can also just cut that trench using a flat/square blade shovel and leave the trench in place. A narrow bladed hoe can maintain the trench.
