My prairie gentian is very mature (at least ten years old) and now it needs to be moved. It's overgrown with periwinkle, which is very much a weed in my garden.

I've lifted it, but I can't seem to remove the periwinkle, which has rooted between and among the gentian's stems.

I've thought of washing all the soil off the root ball to make removal easier, but I'm worried this may give the plant too much of a shock.

I may be able to divide it, but I don't know whether prairie gentian would survive it.

Any suggestions would be very much appreciated.