Answer: Most roses are grafted so that one variety makes up the top of the bush and a different variety, usually hardier, makes up the rootstock of the plant. It does sound like the canes that are flowering on your plant originated from the rootstock. This can happen when roses are planted too deep or when the top is killed during the winter and yet the hardier rootstock survives.
There's nothing you can do to revive your John Davis rose. If you replace the shrub, be sure to mulch the plant after the ground freezes in the fall. Apply a one-foot-thick layer around the plant so that the graft union is covered. You can lay the canes on the ground under mulch or leave them in place and wrap them in burlap or another insulation material.
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