Q: Last winter, I cut all of the English ivy vines off my hardwoods at the base
of the trees. The vines survived all year without withering; indeed they seem
to be still thriving. I am certain I’ve completely severed them all. 

A: I don’t know why ivy would survive this long. It is not parasitic so the ivy
gets no nourishment from the tree. The little rootlets that attach the vine to the
tree can absorb rainwater. I guess it’s possible your vines got enough water
through that means to survive until now. But I am certain of this: if an ivy vine
has no contact with the ground, it WILL die eventually.