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your right the rootd do grow sideways, but they are very easy to split of from the main plant, I did this with my black bamboo last year and just stuck them in a spent compost bag with some soil. They have survived the winter and are looking really well. A great way to get more plants and if you have no where to plant them then consider selling.
I would would plant the way you are considering. If they do start to get a bit cramped you can lift the pot and cut through the root and hay presto even more free plants.
Hope this helps
The programme you need to watch is last Fridays Gardeners World. Have to say I have a Black Bwboo planted directly into the soil with no barrier around it. Planted around 3 years ago it looks happy enough but hasn't overgrown in any way. However if you're concerned it might be best to keep it within your old dustbins. G.World suggests they like a lot of moisture and advise putting grass cuttings around the base as mulch.
Black Bamboo need full sun (to go black), moist soil and to be kept out of strong wind. I have mine without a barrier and have no signs of running yet. Heavier the soil the less likely they are to run.