The two general patterns for the growth of bamboo are "clumping", and "running", with short and long underground rhizomes, respectively. Clumping bamboo species tend to spread slowly, as the growth pattern of the rhizomes is to simply expand the root mass gradually, similar to ornamental grasses. "Running" bamboos, though, need to be controlled during cultivation because of their potential for aggressive behavior. They spread mainly through their rhizomes, which can spread widely underground and send up new culms to break through the surface. Running bamboo species are highly variable in their tendency to spread; this is related to both the species and the soil and climate conditions. Some can send out runners of several metres a year, while others can stay in the same general area for long periods. If neglected, over time, they can cause problems by moving into adjacent areas.