Root Pruning, Crepe Myrtle Tree
We have a Crepe Myrtle tree that was planted about 4 feet from the corner of the concrete front porch of a home we purchased about 2 years ago; home was built in 2005 and tree was probably planted about that time. In preparing to lay some stepping stones from porch to yard, we discovered a root with Dia 2¼ inches growing back toward/under the porch, which we want to "prune", to prevent future damage to the porch.
The tree, appearing healthy & guessing to be at least 10 years old, is now about 15 feet tall with a base = Dia 10 inches as six trunks, with three about Dia 2 and three about Dia 3½.
(1) Can we prune the root now or should we wait until this winter? (2) Any risk with harming this tree?
First things first, root damage is about the most common tree killer there is. Since roots are often out of sight we tend to behave as though anything we do to them is not as meaningful to the tree as cutting off branches. Loss of root system means the tree experiences drought, even if it's raining. Every structural root is attached to literal miles of fine root system collecting moisture necessary for the tree's existence.
That said, there are trees that handle root loss better than others. Crape Myrtle is one of those.
Before severing the root, check the root crown of the tree and determine that the tree has a healthy spread of structural roots and isn't dependent on this one lone root to provide the biggest share of water.
If the tree doesn't have lots of other structural roots, you might want to reconsider the tree. Is it worth keeping if it is going to damage the porch?
If the tree has decent roots, then sever the root cleanly. Use a pruning saw and make the cut as perpendicular as you can. You want the cut surface to be as small as possible.
Once you sever the root, also look into a root barrier to keep the tree's future root system from growing under. Root barriers are everthing from sheet metals to plastics to spunbonded fabrics with embedded herbicides.
A side note, if the root is growing under concrete that means there are resources there for the root to use. You need to check up and see if there is water, or leakage that is allowing the soil under concrete to remain moist enough to support living roots. Could be a problem you don't know about there!
Please let me know if you have further questions.