The skin is smooth and waxy, the trunk and stems have stout, 2-inch spines clustered on their ribs. When water is absorbed, the outer pulp of the Saguaro can expand like an accordion, increasing the diameter of the stem, holding up to 1,500 gallons of water. It may soak up hundreds of gallons after just one rainfall!
The Saguaro can grow as tall as 50 feet, but it only grows at a rate of about 1 inch per year. The largest plants, with more than 5 arms, are estimated to be 200 years old. An average old Saguaro would have 5 arms and be about 30 feet tall.
The Saguaro has a surprisingly shallow root system. It is supported by a tap root that is only a pad about 3 feet long, as well as numerous stout roots no deeper than a foot. Smaller roots run radially to a distance equal to the height of the Saguaro.
The night-blooming flowers, which are about 3 inches wide, have many creamy-white petals around a tube about 4 inches long. Like most cacti, the buds appear on the southeastern exposure of stem tips, and flowers may completely encircle stems in a good year.
A sweet nectar accumulates in the bottom of this tube. The Saguaro can only be fertilized by cross-pollination (pollen from a different cactus). The sweet nectar, together with the color of the flower, attracts birds, bats and insects, which in acquiring the nectar, pollinate the Saguaro flower.
Not all of the flowers on a single Saguaro bloom at the same time. Instead, over a period of a month or more, (May - June) only a few of the up to 200 flowers open each night, secreting nectar into their tubes, and awaiting pollination. These flowers close about noon the following day, never to open again. If fertilization has occurred, fruit will begin to form immediately.
The 3-inch, oval, green fruit ripens just before the fall rainy season, splitting open to reveal the bright-red, pulpy flesh which all desert creatures seem to relish. This fruit was an especially important food source to Native Americans of the region who used the flesh, seeds and juice.