You want to divide it when it is dormant, which is typically the winter months for most bamboo. While F. nitida is a fairly hardy bamboo that might survive in you climate Zone, I'm not sure that what you got is going to survive. The reason people don't sell bare root bamboo is that the survival rate is very low and it will take years for the plant to size up.

People looking for cheap bamboo often buy it on eBay where someone who knows little about bamboo will sell them some rhizome stripped of dirt for cheap shipping, and the buyer ends up with some expensive dead sticks in a few weeks. I'm not saying it never works, but it's usually not a path to success for someone who doesn't have experience.

Bamboo is sold in pots because the common way to propagate it is to dig up a division with an intact root ball, and then to re-pot it and stabilize it for 3-6 months to make sure it survives. Then it takes another year or two before you get new, above-ground growth. If you take a small root ball, or mess with the root ball by removing dirt, you greatly decrease your chances for success.

Thanks for the info. I will keep my expectations low.