Hey. Wonder if anyone can help me identify my bamboo'.

I live in Oregon, in Zone 7, and purchased what I thought was a 'black bamboo'. It is a clumping, graceful 'leaning' bamboo, with it's 2yr old culms a light black (almost dark chocolate) color. The shoots emerge straight up maybe 5-7' or so in the air, then the 4", somewhat oval, pointy yet gentle, green leaves apnpear in a tassel like formation. They don't seem to have the 'unwrapping' that pictures of 'black bamboo' has, and is definitely 'tassel' form, and it definitely is a clumping variety. Over 5 years, it has grown to approx 15' high, to about 4'wide if I were to try and put my arms around the base. It's quite a graceful and leaning and impressive bamboo.

Additionally, once identified, am hoping for suggestions about dividing it, as it has quite outgrown the space I originally had for it. I tried a few years ago to separate another clump I purchased and all but 4 of them died. The others have taken 3 years to come on w/ growth, all only have no more than 5 7' canes. Seems quite slow growing compared to the mother plant.

Thank you!