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Some information from the U.S. Department of the Interior National Park Service
Saguaro National Park
The Saguaro Cactus
Where do saguaros grow?
Saguaro cacti, Carnegiea gigantea, only grow in the Sonoran Desert.
However, they do not grow in all parts of the Sonoran Desert. The range of the saguaro is limited by freezing temperatures in winter.
Saguaros are also limited by elevation. They are generally found growing from sea level to approximately 4,000 feet in elevation. Saguaros growing higher than 4,000 feet are usually found on south facing slopes where freezing temperatures are less likely to occur or are shorter in duration.
How do saguaros grow?
Saguaros are a very slow growing cactus. In Saguaro National Park, studies indicate that a saguaro grows between 1 and 1.5 inches in the first eight years of its life.
These tiny, young saguaros are very hard to find as they grow under the protection of a “nurse tree”, most often a palo verde, ironwood or mesquite tree. As the saguaro continues to grow, its much older nurse tree may die. Some scientists believe that competition from the saguaro may lead to the death of the nurse tree by taking water and nutrients from the soil in the immediate area.
As a saguaro begins to age, growth rates vary depending on climate, precipitation and location. We do know that the period of greatest growth in a saguaro cactus is from unbranched to branched adult.
Here at Saguaro National Park, branches normally begin to appear when a saguaro reaches 50 to 70 years of age. In areas of lower precipitation, it may take up to 100 years before arms appear.
When a saguaro reaches 35 years of age it begins to produce flowers. Though normally found at the terminal end of the main trunk and arms, flowers may also occur down the sides of the plant. Flowers will continue to be produced throughout a saguaro’s lifetime.
An adult saguaro is generally considered to be about 125 years of age. It may weigh 6 tons or more and be as tall as 50 feet. The average life span of a saguaro is probably 150 - 175 years of age. However, biologists believe that some plants may live over 200 years.
Why are saguaros pleated?
The roots of a saguaro grow out from the plant in a radial fashion, several inches under the ground. During a heavy rain, a saguaro will absorb as much water as its root system allows.
To accommodate this potentially large influx of water, the pleats expand like an accordion. Conversely, when the desert is dry, the saguaro uses its stored water and the pleats contract.
Because the majority of a saguaro is made up of water, an adult plant may weigh 6 tons or more. This tremendous weight is supported by a circular skeleton of inter-connected, woody ribs. The number of ribs inside the plant correspond to the number of pleats on the outside of the plant. As the saguaro grows, the ribs will occasionally fork and the corresponding pleat will also fork at the same place.
Why are some saguaros crested?
Even when saguaro cacti grow in their normal form, they rarely grow symmetrically. Saguaros sometimes grow in odd or mis-shapen forms. The growing tip occasionally produces a fanlike form which is referred to as crested or cristate. Though these crested saguaros are somewhat rare, over 25 live within the boundaries of the park. Biologists disagree as to why some saguaros grow in this unusual form. Some speculate that it is a genetic mutation. Others say it is the result of a lightning strike or freeze damage. At this point we simply do not know what causes this rare, crested form.