How much sun does bamboo need?
Most bamboo, with a few exceptions, prefer to grow in full sun. Because of this, bamboo will produce most of its foliage where this is maximum sun exposure, which is obviously at the top of the bamboo. If the bamboo is free standing with a good amount of sun reaching the base of the bamboo, then it will also have good bushy foliage all the way down to the ground.
However, when bamboo are planted in close proximity to each other, or in shade of other trees, there will be little sun reaching the lower parts of the culms, which then causes less foliage growth at these lower levels. The bamboo will only put energy in growing foliage in places where there is good sunlight, and thus maximum possibility of photosynthesis.
Another growth characteristic caused by bamboo growing in shade is that it will grow taller that what it would otherwise grow to in full sun. Since the bamboo wants the light, it will try to get it by growing taller. A bamboo growing in shade will always grow taller than the same species grown in full sun.
These two growth characteristcs are important to understand, as a particular bamboo species can look quite diffenent depending on the amount of sun it recieves. If planted in shady areas, or close together, then they will be taller and less bushy at the bottoms. If planted in full sun and spaced well apart, then they will be shorter, with thicker bushier foliage all the way to the ground.
If you want to promote more foliage at the lower levels of the bamboo, then you can facilitate this by cutting the tops off the bamboo, thus letting more light reach the bottom. Also, simply thinning out the bamboo by removing old culms (cutting them off completely at ground level), will also help in letting more light 'into' the clump. This can be a little bit of a catch 22, since you will be removing culms (and thus reducing the amount of foliage). The additional light reaching the remainder of the bamboo culms will however more that compensate for this, with the extra foliar growth that it will encourage.