Answer: Vinca can usually take the heat, but they are annuals and they do die out when their lifecycle is finished. This can vary by as much as 3-4 weeks between plants, but eventually they will all die out. What might be hurrying them along is the reflected heat from the rock mulch. It absorbs and reflects the heat and in the last few weeks things have been pretty hot in Casa Grand. Add the wind and the rain and it may be too much for your vincas. You might try growing portulaca in the bed to see if it does any better. It seems to thrive in the desert heat. Best wishes with your garden bed.
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