I tilled an area about 3 feet from a large Crape Myrtle (all the way around), about 6 inches down. I did indeed get a lot of small roots (like pencil thick) and a couple large ones (about wrist thick) that I had to chop and remove. Will some of these be replaced by the tree? I really hope I didnt kill the tree.
*EDIT: it's been 6yrs (Nov 2022) and the tree is still standing strong!
I'm afraid only time will tell - generally, its never a good idea to cut through any roots from a tree, though a little disturbance of fine (less than half the thickness of a pencil) roots in a single area doesn't usually cause too much trouble. However, that is not what you're describing at all - it appears you have cut through some much larger 4-6 inch roots, and through less thick roots all round the tree. If you live in an arid region and the soil is very dry, you could certainly increase frequency of watering, but that depends to some extent on the season wherever you are as to whether that will make much difference. Otherwise, there is nothing you can do to offset the damage, what's done is done - the tree will either recover, or it will die.
We do the exact same thing when moving a tree to a new location. Cutting the roots helps a tree to produce new feeder roots within a rootball that is manageable. Very basic way to move any plant, shrub or tree. Just don't do it again. And water often and deeply.
I'd go get some mycorrhizae at a plant store to help the roots left operate better. Mycorrhizae is a common fungus that works with roots in a symbiotic way. Whenever I transplanted any tree, shrub, I'd always apply some of this fungus. And even moving 20' Rhododendrons in the middle of summer I've not had a single death yet.
Do allow the soil to dry out before watering deeply again. You could also get some 3 or 4 inch PVC, drill holes every 6" beginning with the rim. Insert it at an angle within the drip line (draw a line to the ground from the perimeter of the canopy, make a circle around the base of your tree. Within that circle is an upside down version of the canopy. Not as deep as the depth of the canopy, roots are shallow. When you water, also put water in the pipe. Roots are pretty much within the top 6" of soil and not much farther. This will encourage deeper root growth to get at the moisture down deeper. When the soil dries out at the top 2 inches of soil, water deeply again. Forget fertilizer or any other additives until spring.
Something else you should do is lightly prune by thinning not heading. Reduce the canopy that needs lots of water. This will lessen stress of your tree. Need to reduce canopy when you cut roots.
Why are you tilling?
Crepe myrtles are unstoppable, it will survive fine. I dug six about half the size of the one shown. The church did not want them. I put the spade in around them to cut roots and pulled them out with rope and car. They still had roots so I thought of another location to plant/bury them. Bottom line, they all grew well and require regular pruning to prevent rubbing on the building. Plus suckers came up in the original location requiring cutting. I do not recall the season but they were growing, not dormant.
