From what I have read they can take alot of abuse, even to the roots and bounce back. But as for how much to take. I dunno

I'll bet you added that plastic border fairly recently. Either that or you cut a root while weeding. The baby formed from a severed root of the larger crape. the root system of the baby tree probably mainly radiates out away from the parent tree, so you can keep that in mind when you go to dig it up. I'd try to get at least a foot of root length (based on my guesstimate of size from the picture and assuming root structure similar to what I'd expect in this situation).

Crape myrtle root divisions are pretty easy to transplant. Just take your time and get a reasonable amount of root as you remove your baby crape. You may also want to be careful not to cut a lot more of the parent tree's roots in the process of digging up the baby. I'd give this job a rating of easy.

P.S. Be careful to get down deep enough to get a good piece of root. Depending on various factors, the bulk of the root system on some of these can be deeper than what you might expect from a seedling of similar size.

My sister dug up a crape baby at her AR home and potted it up for me to try and grow in IL. It survived the potting, the drive to IL, the winter IN THE POT, and planting in IL. We moved the next spring. I saw some growth just get started on it when we moved....I wonder if it made it. Seemed quite hardy to go through that much. Just give it a go.....

I had a crape on the south side of a shed and pulled it out, but didn't get all the roots and it is growing again like gangbusters! Another crape was in a shaded area and wasn't thriving so I just moved it into the open so it will get plenty of sun. Since the other one survived, I am hoping this one will too. I am in Zone 5, so they go dormant and are slow to start growing each Spring, but the flowers are gorgeous and blooming when many of the other plants have quit putting on their show.