Like Pat said. It looks like from the picture, that you have some green culms, (the long leaf like sheaths which wrap around the shaft itself.) which means that it is immature . Like he said if it starts to wrinkle length ways, it is too young. Look for shoots that have dry, light greyish brown culms, which is what the older shoots have. As for length, cut them long, and then when dry, and ready to use, cut them to length. You can spine them, and cut them. As for filling in the ends, if they have decent thickness, you really don't need to fill them, unless they have really thin walls. You can cut the notch for the arrow head, insert your arrow head, and glue it with whatever method, and wrap about an inch from the arrow head, and glue that also, and make a smooth transition from the head to the shaft with the glue, pitch, etc. Same for the notch, cut it, about 3/4 of an inch, or a very little less,above a node, if you don't have a node where you have cut it, just cut the notch, and wrap just below the notch, and glue it also. Wrap at least a half to 3/4/ of an inch below the cut. And mark the stiff side before cutting the knock notch. You want the stiff side against the bow. If you want to get some of that boo in the peoples yard, stop and ask them. The worse they can say is no. Of course they can adlib a bit with it also. But just stop, and explain that you saw the boo in their, yard, and you have been looking for some, and tell them why, and ask if they mind if you cut some, or if you could dig up some root stock, for transplanting. Just ask. Doesn't hurt to ask.
I doubt that they would be interested in Primitive archery, but you might make an N.A. style arrow, and give to them, or even a Japanese style arrow, or just your own style arrow, and show them what you do with the bamboo.
Don't be proud, go up and knock on the door. Introduce your self, tell them you have an unruly step red headed step child, and you would like to have a couple dozen of those bamboo stalks, to adjust the child's way of thinking, sort of a tune up, so to speak.
Seriously, just ask.
Wayne