| The first pic of the second set of pics, is a rhizome of a running bamboo, which it would have to be to get that large in your zone. However, the second pic looks like a clumper, such as one of the large bambusas, but if it's all the same plant, then you probably have a phyllostachys, but which one....? Newly emerging shoots can be a very big help if you can post some close-up pics when they emerge in the Spring. This is one of the best ways to identify a species of bamboo. When you say 5 3/4" in diameter, you do mean diameter and not circumference, right? How long and wide are the individual leaves? On the culms, on alternating sides of each internode, is there a groove?(The nodes being the ring(or part) where the limbs grow from). The internodes being the stretch in between the nodes. Any additional pics would help. Kt | 
