March and September have been recommended for bamboos generally. Of course, some in this region, with its comparatively mild climate will chop them apart whenever they have the opportunity.
A local wholesale nurseryman told me that he divides his black bamboo in April. Check with you local nursery, I would expect that the time would be similar between Aldergrove and Victoria, both being Pacific Northwest.
I divide Phyllostachys in in late March or April, after any risk of freezing temperatures are minimal, but before they start throwing out shoots. I have also divided them in the fall, usually September/October. The fall dividing is combined with rhizome pruning. Both seasons are good in south western BC because of the abundance of rainfall. The divisions are pretty tough as long as they come with good sized root balls. But the one thing which will kill them early on is drying out. For that reason, I have avoided dividing them in the summer months - generally June through August are "no go" months for me. I don't divide them in the winter. Winter brings with it another set of problems. As previously mentioned, the cold is not an important factor in mild south western BC, but high winds, heavy rain fall and snow falls make it difficult to anchor a new division securely in the ground.
