We have some Sinobambusa tootsik albostriata (a running bamboo) in galvanized livestock tanks (AKA horse troughs) in our side yard. This past season (after about 3-4 years of healthy growth), many of the lower leaves have yellowed, with many of them falling off. This has reduced the screening quality of the bamboo. I'm trying to determine if this is a cyclical phenomenon (e.g., we would get more green leaf lower with the new culms which are coming up), if it's just the nature of bamboo to lose lower leaves as it develops (i.e., the bamboo only grows leaves that will get sun exposure, namely the uppermost part of the plant, which now shades the lower), or if the bamboo is unhealthy in some way.

These pictures were taken in May, but the plant hasn't changed much since then (although new culms are coming out of the ground, so I assume the plant overall may be OK). I add compost to these planters once per year, and they are watered via drip irrigation (although I'm not sure if they're getting too much or too little).

Any thoughts?

Here is a link that might be useful: Pics of both our bamboo and same species at Foothill College Bamboo Garden