What do you mean, so it curves away from the grove. It'll probably just have another rhizome attack your rubro by this fall and your rubro will start its own war. Is it still connected to the mother plant? There is always a chance something could die from disturbance, but have 7 ft long rhizome chunk has alot of energy stored up in it and should shoot. Also, when did you move it, it should be close to shooting right now, which could negatively hurt it.

Hi!

I've had experience digging up my p. nigra henon shoots..I may be crazy but it seems like the newer shoots that I have transplanted seem to do better than the older ones. If you want the culm to live and thrive by transplanting it, you want to wait until they harden off a bit and finish growing. I transplanted some errant p nigra henon culms (first year growth) in the summer last year, they actually seem to still be alive too. I take a shovel, dig up the dirt until I see the runner, and then take a hand pruner and clip the runner. Makes it infinitely easier to dig up too. I then keep them watered and I also cut them down a bit to keep them from getting stressed and losing water due to evaporation. If you know it's going to be cloudy during some days in the summer, it's good to transplant then, most plants do better than if the days are sunny so they don't crisp.
Hope this helped! :)

I kept the rhizome attached to the mother plant, dug up the terminal 7 feet, dug a new trench for it, and re-planted it so that it will continue growth in a different direction instead of growing directly into the rubro. I did this about the first week of April. I figured doing this would "encourage" that particular rhizome to spread in the new direction. If I had simply pruned the last foot or two off of it, then it would have branched and continued to grow directly into my rubro. I understand that both plants will eventually continue to send countless rhizomes into each other. My only concern was whether or not this particular rhizome will survive or even shoot for me after being dug up in early spring and replanted.

Yeah, it'll survive. Even if you had cut it off the mother plant it would've most likely survived and shooted.