How Does a Saguaro Cactus Grow?

Sprouting Seed

• A Saguaro cactus sprouts from a shiny black seed the size of a pinhead. A Saguaro cactus produces tens of thousands of seeds each season. Only a handful find a hospitable place to grow. Saguaro seeds need a sheltered spot to germinate, such as under a "nurse tree" like mesquite or by large rocks. The shelter shields the seeds from summer and winter temperature extremes and from the birds and rodents that eat the seeds.

Slow Growth

• Growth is slow, with a seedling only about a quarter-inch tall in the first year. It will take about 15 years to reach a foot in height. At about 30 years and 3 feet tall, a Saguaro will produce flowers and fruit. By age 50, a Saguaro may reach 7 feet in height. A Saguaro starts to grow its characteristic arms around age 75 and, by age 100, may reach 25 feet. It will be fully mature at age 150 and reach 50 feet in height with more than a dozen arms. A full-grown, fully hydrated Saguaro can weigh more than 2 tons.

Flowers & Fruit

• Saguaros flower from April through June. Each plant has dozens of flower buds. Each sweet-scented flower opens in bloom a few hours after sunset and wilts by the next afternoon. But each night sees several flowers blooming at once. The flowers attract birds, bats and insects seeking their sweet nectar. As these creatures travel from flower to flower to feed, they transport pollen that fertilizes other flowers as they go. Saguaro fruit ripens in June and July, with much falling to the ground. The sweet pulp attracts animals, birds and insects that feed on the fruit and ingest the seeds. Each fruit contains hundreds of seeds, many of which are excreted in feces someplace far away.

Conserving Water

• The Saguaro is designed to conserve water. It has strong woody ribs that hold the plant upright. Between the ribs, accordion-like pleats allow the Saguaro to expand as it takes in water. Spongy, gelatin-like flesh inside the plant stores the water the plant takes in through its widespread but shallow root system centered on a tap root over 2 feet deep. The shallow roots extend as far from the trunk as the cactus is tall. In a good rain the plant may take in 200 gallons of water, enough to last a year. It has no leaves to transpire water. Instead, its green trunk and branches make its food through photosynthesis. The spines keep animals from eating the plant and provide shade and a shield from drying winds. Its waxy skin helps hold in water.