Visitors to Arizona's Sonoran Desert are often surprised at the size and stature of the native saguaro cactus (Carnegiea gigantea), which can rise to 60 feet tall and have many branches. The plant's white flower is Arizona's state flower. At all stages of its life cycle, this cactus demonstrates adaptations that allow it to reach its tall size in an area with hot arid summers and little rainfall. It is hardy in U.S. Department of Agriculture plant hardiness zones 9 through 11.
Although mature saguaros are prominent features of the landscape, young saguaros usually escape notice. Seedlings that survive infancy are tucked in the shade of shrubs and trees. By the time they grow through those plants, the saguaros are already decades old. Young saguaros grow slowly, with a 10-year-old saguaro perhaps only 1 1/2 inches tall. As saguaros mature, they produce more extensive root systems and more photosynthetic tissue, and they grow faster. A mature saguaro can take up water quickly, especially during the summer monsoons. A fully hydrated older saguaro can weigh 3,200 to 4,800 pounds. Mature saguaros are 125 years old, and they can live to be about 200 years old.
Both young and old saguaros have vertical ridges with stout, gray spines. The valleys between the ridges are deeper in older plants, allowing the water storage tissue beneath the skin to swell rapidly, expanding the accordion pleats in each saguaro's body. The skin is thicker in older plants, and it has a waxy coating that helps prevent water loss. Younger plants, with their thinner skin, don't tolerate full sun exposure, as do older saguaros, and the younger plants need the shading they get from their nurse trees or shrubs. A seedling saguaro sends down a long taproot, and it is the young plant's main root system. The taproot anchors the plant as it grows, and the taproot collects water. In old plants, the taproot can be 5 feet long. As older saguaros grow, they develop a more shallow system of lateral roots within the top 3 inches of soil, allowing them to absorb water quickly from even light rainfall.
Flowers and Fruit
Young saguaros' energy is devoted to gaining body mass. Saguaros bloom for the first time when they are about 35 years old. Their white, many-petaled flowers appear in late April through early June and grow in a ring around branch ends. The flowers are pollinated by bats, birds such as doves and flickers, and insects. Saguaros produce red, oval fruits that hold about 2,000 round, black seeds. The fruits split open from their upper end in July, revealing bright-red, seed-studded pulp that tastes similar to watermelon. Wildlife rely on the water-rich fruits for food, and the Tohono O’odham people use them as food and to make ceremonial wine.
Many dangers exist for young saguaros. The tender seedlings die if they happen to sprout in an area that has full sun exposure or in an exposed area where winter temperatures fall below 32 degrees Fahrenheit. If seedlings are not protected by other plants, wildlife eat them. Seeds that land on shallow or loose soil don't find good anchoring elements for the taproot, and they are washed away by summer thunderstorms. Of the 40 million seeds that a saguaro may produce during its lifetime, only a few might survive to become adult saguaros. An adult saguaro's greatest risks come from physical wounds that can develop rot, which spreads through the cactus and results in its death, and from wind during wet summers. Due to the plant's great weight after summer rains, the roots may not be able to hold the plant upright if the ground is wet and high wind occurs. In that case, the saguaro falls.
- Arizona-Sonora Desert Museum: Saguaro Cactus Fact Sheet
- OnlinePlantGuide.com: Carnegiea Gigantea, Saguaro Cactus
- U.S. Department of the Interior, National Park Service: Saguaro National Park -- The Saguaro Cactus
- National Park Service: Saguaro Cactus -- Organ Pipe Cactus National Monument
- Discovering the Desert; William G. McGinnies
- NA/AbleStock.com/Getty Images