each year until the plant approaches its particular species limits of height and diameter. Many tropical bamboo species die at or near freezing temperatures, while some of the hardier temperate bamboos can survive temperatures as low as . Some of the hardiest bamboo species can be grown in USDA plant hardiness zone 5, although they typically defoliate and may even lose all above-ground growth, yet the rhizomes survive and send up shoots again the next spring. In milder climates, such as USDA zone 7 and above, most bamboo remain fully leafed out and green year-round. Mass flowering Bamboos seldom and unpredictably flower and the frequency of flowering varies greatly from species to species. Once flowering takes place, a plant declines and often dies entirely. In fact, many species only flower at intervals as long as 65 or 120 years. These taxa exhibit mass flowering (or gregarious flowering), with all plants in a particular 'cohort' flowering over a several-year period. Any plant derived through clonal propagation from this cohort will also flower regardless of whether it has been planted in a different location. The longest mass flowering interval known is 120 years, and it is for the species Phyllostachys bambusoides (Sieb. & Zucc.). In this species, all plants of the same stock flower at the same time, regardless of differences in geographic locations or climatic conditions, and then the bamboo dies. The commercially important bamboo Guadua, or Cana brava (Guadua angustifolia) bloomed for the first time in recorded history in 1971, suggesting a blooming interval well in excess of 130 years. The lack of environmental impact on the time of flowering indicates the presence of some sort of "alarm clock" in each cell of the plant which signals the diversion of all energy to flower production and the cessation of vegetative growth. This mechanism, as well as the evolutionary cause behind it, is still largely a mystery. Invasive species Some bamboo species are acknowledged as having high potential for becoming invasive species. A study commissioned by International Bamboo and Rattan Organisation, found that invasive species typically are varieties that spread via rhizomes rather than by clumping, as most commercially viable woody bamboos do. Certain bamboos have become problematic, such as the Phyllostachys species of bamboo which are considered invasive and illegal to sell or propagate in some areas of the US. There are approximately 61 species of Phyllostachys. Animal diet Soft