Answer: As you have seen, zoysia is a slow grower. So if the Bermuda grass has a chance to germinate and take hold, it could outcompete with the zoysia. Because they are both grass species, herbicides intended for use on broad leaf weeds will not work. Also, pre-emergent herbicides, that prevent weed seeds from germinating, won't be of use because the seeds have already begun growing.
This is a tough one, and I'm sorry that I can't help. You might consider contacting a local lawn care professional and see what they recommend,
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