Answer: No, things don't sound too good for your rose bush. If just the stems were turning black, your plant might have a chance of recovering, but if the graft union is also turning black, it's pretty much a no-go. Even if the roots were still alive, they would send out shoots of a wild rose, not the hybrid rose you purchased. I think the best thing to do is remove and replace your rose bush. Roses like rich, well draining soil, ample water, and lots of sunshine. Amend the planting area with organic matter prior to planting your new rose bush. Organic matter such as compost will help the soil drain quickly, yet retain just the right amount of water to make the roots happy. Water deeply once each week and make sure your rose bush gets at least 6 hours of sunshine every day.
Better luck with your new rose bush!
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