Pierre Mancenillier wrote:
A friend of mine is offering me to remove from his
garden a few rhizomes with their culms.
CUT
The culms are fairly "large" about 20 ft / 6 m high
and 1 inch / 2.5 cm in diameter, possibly a little more.
CUT
Assuming that everything is OK (soil, sun, moisture,
mulch etc.) what size culms can I expect from these
rhizomes nest year?

This depends entirely on how much rhyzome and intact root you are able
to preserve when you cut and dig them.

If you leave almost no root and rhyzome, it will likely die. In this
case, if it doesn't die, the plant can actually return to health and
leaf out, but never put up ANY culm again. (If all rhyzome buds are
removed, it can not generate new ones)

If you have only a 4 inch section of rhyzome, you might expect anywhere
from a spindly little shoot up to a 1/2 inch shoot that reaches 8 ft.
tall. It depends somewhat on luck, but also how much sun it can soak up
and recover this year.

If you manage to get 12 to 18 inches of rhyzome with a lot of root
growing off of it, you might very well get a much bigger culm next
year.

Also, you should try to trim the top in proportion to the amount of
root you get... If you are not able to get a lot of rhyzome and root,
then cut the culm shorter... If you get a lot of root, try to keep a
lot of the culm so it can have a lot of leaf mass to soak up sun over
the coming year. Then, next fall after it has it's new healthy culms
leafed out and producing, you can cut the original culm off entirely if
you think it looks ugly.

(Ideally, all trimming of culm for appearance should be done in late
fall, so the plant can get full use out of all of the leaves during the
summer months when they are most important.)

Cheers!