Well if you remember anything from high school biology, it’s the chlorophyll which is the key to converting the sunlight into chemical energy.
Okay, but what makes it “green” as in environmentally friendly?
To answer this you have you compare the woody material from bamboo with other comparable hard materials, like plastics or tree wood. In general, bamboo grows quickly, "like a weed" you could say, and bamboo canes can be harvested after several years.
A pine tree typically needs a dozen or so years to grow and mature before it can be harvested, whereas hardwood trees, like oak trees, typically need dozens of years.
Moreover, harvesting a cane of bamboo does not normally entail the death of the plant, as new shoots can continue to grow up from the roots the plant, without the need for additional planting.
The key to bamboo's rapid growth and regeneration is its rhizome root structures that run under the ground. In fact, bamboo propagates almost exclusively through spreading its root system and sending out new shoots. Most species of bamboo rarely flower and thus rarely produce any seeds. In fact, most common species only flower about once every 50 years.
Bamboo can have either "clumping" or "running" rhizomes. Running rhizomes, as the name suggests, spread out horizontally over a wider area more quickly. This can be useful for farmers with large acreages but can spell trouble for residential gardeners. Out of control bamboo in a garden can damage sidewalks and patios and lead to disputes with neighbors.
Bamboo does require considerable water resources but otherwise is relatively easy to cultivate. It requires minimal management and not very susceptible to pests, thus requiring little to no pesticides and insecticides.
Simply put, bamboo has a short growth cycle, it matures quickly after being planted, and thus can be harvested more frequently than trees. Add to this the linear shape of the canes and the fact that they only get so wide (6 inch diameters for giant bamboos), and you have a relatively easy to harvest plant with a high proportion of usable material. Growing bamboo for it's woody material is therefore a rather efficient and sustainable use of land. As an added plus, bamboo plants per acre extract up to 35% more carbon dioxide from the air than most trees.