Natural History

The Saguaro is the largest cactus in the United States and the most famous of the Sonoran Desert flora. Saguaros typically grow to heights of 40 ft, though one cactus was measured at 78 ft. The saguaro has a cylindrical stem with pleated ribs which are lined with clusters of stiff spines (supposedly the spines that grow above 8 ft are more flexible). Saguaros are most profuse in southern Arizona, though some may also be found in western Sonora and along the Southern California border. They typically grow on rocky slopes and in well drained flat lands below 4000' in elevation. 
 

The saguaro has a thick waxy skin that restricts loss of moisture. The outer surface is covered with pleats, which allow the stem to expand during water uptake, preventing the cactus from bursting. A mature saguaro can soak up 200 gallons of water during a single rain storm. Areoles (the pads which produce spines) are spaced at 1" intervals along the pleats. The spines serve to protect the cactus from herbivores and provide some protection from the sun. Just beneath the skin is a thin layer of chlorophyll containing cells in which photosynthesis takes place. Beneath that layer, and making up the bulk of the cactus, is water storage tissue. A saguaro is typically more than 90% water. Water is needed for plant survival, but also plays an important role in heat regulation. The water within the cactus heats slowly throughout the day (preventing the cactus from cooking), then releases its heat at night, keeping the cactus warm. At the center of the saguaro is a woody skeletal structure which runs the length of the main stem and branches into the arms. The saguaro has a main root stem that extends downward about 2 feet. The remainder of the root system is very shallow (usually no more than 4 inches below the surface) and extends radially away from the plant to a distance about as far as the plant is tall.

Saguaros produce flowers in May or June once they have achieved a height of about 8 feet. The 3" white blossoms form on the ends of the stem and arms. They open at night and remain open part of the following day. It is commonly accepted that the flowers are pollinated by bats, however, recent studies suggest that bees and doves play a larger roll in pollination than bats do. In June and July the flowers mature into 3" long fruits which contain up to 2000 tiny seeds. The fruits are eaten by many birds, mammals, and insects. Seed dispersal is primarily performed by birds, which are unable to digest the seeds. If a saguaro seed finds itself in a favorable place to grow it may form a seedling. Seedlings grow very slowly during the first few years and are susceptible to herbivores and the elements. Successful seedlings usually can be found growing underneath a nurse plant, such as a palo verde or creosote bush, which provides shade and protection for the young plant. Even with this protection, seedlings require several consecutive years of milder and wetter than average weather to survive. Because these conditions may occur only a few times a century it is not uncommon to find large groves of saguaros that are very close in size and age. The speed with which a saguaro grows depends on the amount of rainfall, the size of the plant and the type of soil. A typical saguaro may take 10 years to reach  1 1/2 inches in height and 30 years to reach 2 feet. They grow somewhat quicker after that and grow to a height of 8 feet in about 55 years.
 

The plant begins to grow arms when it is between 50 and 100 years in age. The number and size of the arms is correlated with soil and rainfall. Some saguaros have been observed with up to 50 arms, some never grow any. Arms always grow upwards, those that droop towards the ground are probably the result of frost damage.

Numerous animals rely on saguaros for food and shelter. Birds such as the Gila Woodpecker and Gilded Flicker excavate nest holes in the stems. The saguaro protects itself from disease by sealing the hole with scar tissue, which becomes dry and woody. These holes make excellent nesting sites and seem to cause the plant little damage. Old nests sites may later become residences for Elf Owls, Finches, Flycatchers or Martins. Larger birds such as Hawks build nests in the angles between the stem and arms. Saguaros can live for 150 to 200 years in age, however, even a fallen saguaro can become a home for many small animals such as snakes, rodents or invertebrates.

Native Americans have found many uses for the saguaro as well. The woody ribs are used for fences and combined with mud and grasses to make homes. Saguaro fruit is an important seasonal food and is also used to make a ceremonial wine. Seeds are also eaten and are consumed plain or dried and ground to make a flour.