Although the plants commonly called bamboo have woody, trunklike stems, they are not trees. They are, in fact, grasses, and they are an extremely diverse group, with more than 1,400 species in about 70 genera.
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Culms, Nodes and Rhizomes
The tall, vertical stems of a bamboo plant are called culms. In most species of bamboo, the culms are hollow, and they're divided into segments by intermittent solid divisions called nodes; the part of a culm between two nodes is called an internode. Leaves and branches grow outward from the culms at their nodes.
A bamboo plant expands over time via the growth of underground shoots called rhizomes. New culms grow from the rhizomes and push upward, and buds on the rhizomes also form new rhizomes, all of which gradually makes the plant bigger and causes it to spread.
Growth and Life Cycle
As culms develop, they grow vertically, but they don't expand in diameter the way that tree trunks do. Instead, they remain the same diameter from the time they're new shoots to the time they reach their full height, and they attain their entire mature height in a single growing season. In spring, new shoots emerge, and old culms drop their leaves and grow new ones.
Because culms typically grow very quickly, bamboo can be particularly useful in a garden as a screen or hedge.
Most bamboo species flower only once during their lifetimes, and the plants typically die after they flower. The flowering cycle is usually very long, however, and flowering may occur only after 60 to 130 years of growth.
Some bamboo species spread quickly via rhizomes that grow horizontally under the soil surface, producing both new culms that grow upward and new rhizomes that continue the horizontal spread. Those species are called running bamboos. Their growth habit can cause problems in a garden because the spreading rhizomes are difficult to contain and can quickly invade areas where they're not welcome.
Methods for controlling the spread of a running bamboo include growing the plant in a raised bed, digging a trench around the plant and installing a below-ground barrier that blocks the growth of rhizomes.
Species that are called clumping bamboos spread slowly because their rhizomes grow primarily in an upward direction to produce new culms. New rhizomes develop around the perimeter of this kind of bamboo plant, forming a clump. The relatively compact form of clumping bamboos, along with their slow spread, make them less problematic than more invasive types.
Many bamboo species are tropical plants that can't withstand cold weather, but some varieties, such as those in the Fargesia genus, are hardier and can grow in cooler climates. The 'Rufa' variety (Fargesia dracocephala ‘Rufa’), for example, is an 8-foot-tall clumping variety that is hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9.
Less hardy species include black bamboo (Phyllostachys nigra), a 30-foot-tall running variety hardy in USDA zones 7 through 10, and Himalayan Blue Mountain bamboo (Borinda boliana), a 30-foot-tall, blue-caned, clumping variety that is also hardy in USDA zones 7 through 10.