You wouldn't want to take a division out of the ground for transplanting right now because it's shooting season. However, a stabilized division in a pot can be transplanted most any time as long as the weather is mild and you take care not to mess with the root ball or break any emerging shoots. Unlike many plants, you do not loosen up the roots of a bamboo when de-potting for transplant. Just dig a hole, remove the pot, and plunk the root ball in the hole and fill in. I live in Zone 8 and generally plant out March-October, but I've even done it in the winter months as long as we're not having freezing weather.