I have a very old white heather, how hard can I prune it ?
You can only prune very lightly in early spring. Other than that I do not recommend pruning this. Pruning lightly on an older plant will likely help regenerate it though. Do not take more than a third of the plant at a time. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/groundcover/false-heather/growing-mexican-heather-plants.htm