Ryan Burkitt wrote: Are culms the soft new shoots that pop up in the spring?
"A culm is the aerial (above-ground) stem of a grass or sedge." So I usually call the "shoots" just that until they're above the height that the bunnies, deer and geese tend to chew on them.
Bamboo culms (hollow stalks) grow to their maximum height the first season. Their side branches may continue to grow higher later, but the actual length of the central core won't change, and if something stops it at any spot in that initial grow period, that will be its final height.
So that initial growth from shoot to full sized culm happens quickly over a short period. This is why the plant needs to have good energy stores to do so. The better those stores, the more and larger culms you will get. This is why it may be challenged when a small part is transplanted. I have successfully transplanted my bamboo, but some of it has taken a long time to get going.
So if you're transplanting, if you shorten the vertical height, you're removing the plant's ability to make sugar and limiting its height until it can produce new shoots to grow into new culms.
I consider myself an amateur at this stuff, but I have had some success with transplanting P. dulcis and P. nigra. My P. nigra has been blooming, but I have not seen any actual seeds forming. It may need a cross pollinator, or I may not know what I'm looking at, but so far my main plant and my potted version are still alive. Blooming normally kills the mother plant, but I have read of exceptions.