Saguaros have a relatively long life span. They take up to 75 years to develop a side arm. 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. It grows in Maricopa County, Arizona, and is 13.8 meters (45.3 ft) tall with a girth of 3.1 meters (10 ft). These cacti can grow anywhere from 15 to 50 feet. They grow slowly from seed, and not at all from cuttings. Whenever it rains, saguaros soak up the rainwater. The cactus will visibly expand, holding in the rainwater. It conserves the water and slowly consumes it.
The spines on saguaro having a height less than 2 meters grow rapidly, up to a millimeter per day. When held up to the light or bisected, alternating light and dark bands transverse to the long axis of spines can be seen. These transverse bands have been correlated to daily growth. In columnar cacti, spines almost always grow in aureoles which originate at the apex of the plant. Individual spine growth reaches mature size in the first season and then cease to grow. Areoles are moved to the side and the apex continues to grow upwards. Thus, the older spines are towards the base of a columnar cactus and newer spines are near the apex. Current studies are underway to examine the relationship of carbon and isotope ratios in the tissues of spines to the past climate and photosynthetic history of the plant.