I have Spectabilis planted in a raised bed that we filled with a mix of manure, sawdust, leaves, top soil and sand so it is a pretty loose, fertile mix. Each fall I remove any culms & rhizomes that are growing beyond the small space allocated for it, and this year only 1 culm was out of bounds. I carefully removed the culm and attached rhizomes which were approximately 18-20 feet long - all of these in the photo were branches of the rhizome that the culm came from. Jim is standing on the grove side of the culm where I snipped the rhizomes at the edge of the mini-grove which is barely visible above the banana leaf in the background.
I crammed the culm with 4 attached rhizome branches, each a few feet long into a 15 gallon pot, it held an entire 2 cubic feet bag of potting soil as well. It will be interesting to see what comes up in the pot next spring.
Right after I dug this our new bamboo friends Jim & Lisa from Michigan Bamboo came down for a visit and Jim was kind enough to help me with this photo. I culled this part of the bed last year so these plus another 6-8 rhizomes not attached to this culm but equal in length had grown this summer.
Sorry, I wish to post the photo in this message but do not know how to do this but here is a link.
