Answer: Based on your decription I am not certain what is happening, but I think it might be related to soil moisture levels. Bamboos are grasses and grow best in soil that is evenly moist during the growing season. (It should also be well drained so the plant is never sitting in saturated soil, but ideally it would not dry out. The goal would be damp like a wrung out sponge.) Using an organic mulch will help keep the soil moist. They also need adequate fertility, preferably provided in slow release form by compost and mulch breaking down over time to feed the soil. (The American Bamboo Society cautions in particular against using a weed and feed type product on bamboo, I mention this in case yours is growing in proximity to a lawn that is being treated that way.) Finally, this is a plant for a full sun location, it will grow more slowly in too much shade.
The plant has to build up its reserves in order to send up new culms each spring. The healthier the plant, the bigger and better the new growth will be. The culms appear and then grow very fast so they need ample water and nutrients during that time frame. If the plant is stressed while the culms are appearing,or if the culms are physically damaged, they may die back as a result.
I hope this helps you trouble shoot. If you don't think dry soil would have influenced it, you might want to also consult with your local county extension to try to diagnose the problem.
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