I have quite a few bamboo in large containers and over time they get very congested , the best way to deal with this is to divide them, that said they are quite difficult I take them out of the old oil drums and with a decent mattock I divide them, a old saw and an axe also comes in useful, once divided you can repot them and give them some decent soil, I like to use a mix of good loam and home made oak leaf compost, I chuck in every spring a couple of handfuls of chicken pellets and leave them to it, just keep them watered well during long dry spells and in heavy snowfall I knock off the snow to stop them bending.