Answer: Bermuda is the warm season grass of choice in Phoenix and it is typically overseeded with a cool season grass such as perennial rye each October. When the weather cools the bermuda goes dormant and the rye greens up. Then the rye browns out in April or May and the bermuda greens up. You can overseed with bermuda now but it may take an entire season for your turf to green up on schedule. I don't know what ugly old grass is coming through but the safest option is to dig it out. Any grass killer you use will affect both the bermuda and the cool season grasses. Good luck with your lawn.
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