repotting/dividing Golden Bamboo in containers
i've got six large containers of golden bamboo on my back patio. they're all about 8' tall. after 1 1/2 years in their current containers, i'm worried that they'll soon be rootbound. so far, the plants look great, with no discoloration, a good new culm grown in spring and fall. but i'm pretty sure at least one of the containers is in need of dividing/repotting to give the bamboo more space to spread out. i have a couple of questions:
when is the best time of year to do this division/repotting? spring? or now, fall? (as i said, all the containers have some good new culms coming up, their fall growth spurts.) i was thinking that if i did some now, the smaller bamboo with extra soil in bigger containers might stand a better change of overwintering. (last winter was very mild in nyc)
what's the best way to divide the plants? i was planning on tipping the plants on their sides, shimmying them out of the pots, then, cutting them in half and repotting each half in separate pots. what tool is best suited to cutting the roots in half for best division? what's the best way to divide the plant?
many thanks for all responses.