@cats1, I don't think an underground pipe would make a plant more susceptible to cold damage. In one of my daylily beds there is a heavy metal pipe that holds an old satellite dish for a clematis to grow on. The pipe is about 3 feet into the ground with daylilies and clematis growing all around it, I've never noticed any difference in their growth or a tendency to fail to survive our zone 4 winters even with the top portion exposed to lower temps than the underground section. I would be more concerned about what your pipe is for, water, sewer, gas...and if you want to be digging so close.
This is an image taken in early spring showing the pipe in the bed with the daylilies and the clematis that grows on it.
