The saguaro is an arborescent (tree-like) cactus species in the monotypic genus Carnegiea, which can grow to be over 20 m (70 ft) tall. It is native to the Sonoran Desert in Arizona, the Mexican State of Sonora, and the Whipple Mountains and Imperial County areas of California. Saguaros have a relatively long lifespan. They may grow their first side arm any time from 75–100 years of age, but some never grow one at all. A saguaro without arms is called a spear.

The arms are grown to increase the plant's reproductive capacity (more apices lead to more flowers and fruit). The growth rate of saguaros is strongly dependent on precipitation; saguaros in drier western Arizona grow only half as fast as those in and around Tucson, Arizona. Some specimens may live for more than 150 years; the largest known saguaro is the Champion Saguaro growing in Maricopa County, Arizona, and is 13.8 m (45.3 ft) tall with a girth of 3.1 m (10 ft). These cacti can grow from 40 to 60 ft tall. They grow slowly from seed, and never from cuttings. Whenever it rains, saguaros soak up the rainwater. The cactus will visibly expand, holding in the water. It conserves the water and slowly consumes it.