Based on the size of those seedlings, it looks like you started them right in the middle of winter right? I'm not planning on getting a mature division anymore because mine is already big enough for it to reasonably start producing rhizomes which will allow it to store more energy, shoot less often and get me a bigger increase in size. This plant can't clump forever.
Here's my latest picture of my moso grove with shoots just breaking the 5ft mark, but last fall, it showed signs of starting to run so it should behave differently this year. I'm also about a zone colder than you which might also hinder the upsize on my bamboos, but with this much new growth, and a warmer summer, I'll expect much better results next year which will be the 3rd year for almost all my bamboos.
Here's the damage, and the bottom picture shows the caterpillar causing it. There's often 1-3 caterpillars found in the unfolding leaves so I guess the moth, beetle, or butterfly is laying eggs right on the unfolding new leaves.
I have no idea why, but every bamboo I've placed in the ground has become noticeably darker and growing at a faster rate than the potted ones which are in 2-3 gallon pots and definitely not root-bound yet.
