Bamboo troubles

I have a bamboo stalk in a soda bottle 2/3 full of sandy stuff. Val gave it to me a few years ago, and I've had it in my grad office, and now my work office, ,ever since. Hasn't grown a lot, I think the sand isn't such a good base. But just a week ago, a new leaf, possibly branch/stem, appeared at the side of the bottle, popping up out of the sand. And it's been consistently growing. I don't know what to do, cause it's gonna run out of space in there, there's no way it'll be able to come up through the bottleneck. At the same time, I don't want to try to stop it's growth. And I can't really break the bottle to make a bigger opening.