late season in UK would be about now, right? I know my Ph. are rhizoming like crazy in S. Texas.

be brave: lay down your 2-gal pots and gently wiggle out the root balls to see how root bound they are now. It won't hurt the bamboo. This is known as "first-hand direct experience" and is the best, richest way to learn. :)

If the rhizomes are already circling the pots (and shooting up the sides and out as whipshoots), then I'd move up to 5-gal. If not, then it's your call.

if you're in zone 8b, why not put it in the ground now?

I'm not ready to commit it to the ground right now. It's building up good root mass in the containers, and will do super well when it gets in the ground.

rfgpitt,

I do not have room for these plants at the moment. I plan to house them in a raised bed which will be constructed early October. If this goes according to plan I will plant them at that stage. My question was borne of the potential need for a plan B in case of delays.

Unautre,

Thanks for the input. I think I will just go ahead and pot them on into larger containers now.If I have to plant them out come early October I do not think it will be a problem.