This is a common question we hear from homeowners. Unfortunately, there is no guaranteed way to fully control wild bermuda grass in St. Augustine lawns.. Bermuda grass spreads in 3 ways -
1) seed germination - Seeds that are spread from actively growing Bermuda in or around your lawn.
2) Stolons - These are the "runners" that grow above ground in the same way that St. Augustine grass speads to fill in bare areas.
3) Rhizomes - These are the underground runners that grow like Zoysia grass does, spreading underground like a "Torpedo" and popping up in another area of the lawn a couple inches or even feet away.
Pre emergent products are made to prevent the #1 way Bermuda spreads, seed germination. Pre-Emeergent means before the seeds "emerge" or germinate. It will control 75-80% of Bermuda seed from germinating when it is active. However, if the lawn has established bermuda grass in it or neighboring to it, pre-emergent herbicides will not stop the active stolons or rhizomes from spreading.
Pre-Emergent weed controls will help keep seed germination lower when applied at the proper time (spring and fall, at the correct temperatures). Pre-emergent herbicide typically is not a practical approach in completely controlling bermuda grass due to the #2 and #3 ways they spread.
St. Augustine is also very susceptible to pre and post emergents whereas bermuda is very tolerant of them, so there needs to be caution used to not damage the healthy turf you want to salvage wehn trying to control the weeds yous do not. Some active ingredients of pre emergents that help with seed germination of bermuda are trifuralin, oryzalin, and pendimenthalin.
Spraying Non-selective weed controls on the bermuda and St. Augustine, to follow with resodding typically is the chosen method of control. Typically, due to neighboring lawns and left over unkilled rhizomes, this even leads back to having some bermuda in the lawn. The best bermuda control recommendation is here on one of our blogs: