Plants use under ground stems to multiply their numbers by asexual reproduction and to survive from one year to the next, usually over a period of dormancy. Some plants produce stems modified to store energy and preserve a location of potential growth to survive a cold or dry period which normally is a period of inactive growth, and when that period is over the plants resume new growth from the underground stems. Being underground protects the stems from the elements during the dormancy period, such as freezing and thawing in winter or extreme heat and drought in summer or fire. They can also protect plants from heavy grazing pressure from animals, the plant might be eaten to the ground but new growth can occur from below ground stem that can not be reached by the herbivores. A number of plants, including "weed"y species, use underground stems to spread and colonize large areas, since the stems do not have to be supported or strong, less energy and resources are needed to produce these stems and often these plants have more mass under ground than above ground.