Answer: The foliage on bamboo is constantly renewed, with the oldest foliage dying off and being replaced by new, so what you are seeing may be normal -- although the yellowing is usually seen in spring and the growth spurt generally happens in the spring to summer.
You can trim off the discolored parts if you wish. You can also prune back side branch tips and control top growth somewhat if you need to, cut just above a node. This will cause it to grow denser. In outdoor stands, the older thinner culms are pruned out occasionally to keep the grove somewhat open as well. You may need to do this if your container is becoming crowded.
Or, you may need to repot. Bamboo in a container is usually repotted each year for the first few years until the maximum manageable container size is reached (say 30 gallons or larger), after that it will have to be divided annually to allow it space to grow and stay healthy.
Keep in mind that bamboo likes heat and humidity and grows best with evenly moist soil and ample nitrogen -- it is basically a grass. The soil should always be moist (not sopping wet) and never dried out. Assuming it is in a large enough container, you might try fertilizing with a complete water soluble, high nitrogen fertilizer such as 20-10-10 or similar proportions per the label directions, perhaps alternating with a kelp-based product (provides ample micronutrients) and be sure to water adequately during the growing season. Also make sure your container is large enough that it does not dry out excessively quickly.
If it is outdoors on a paved surface that reflects heat, you could also try raising it up a few inches to allow air to circulate beneath it, this would help reduce heat stress on the roots. Good luck with your bamboo.
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