Saguaro cacti, Carnegies 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. 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 protections of a "nurse tree", most often a palo verde 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. 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 may 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.
How do Saguaros Reproduce?
In late April through early June, the tops of the saguaro's trunk and arms sprout a profusion of large, creamy white flowers. Individual flowers open at night and close the following afternoon. To develop into fruits, they must be pollinated within this time frame. Pollination is carried out by nectar feeding bats, birds, and insects. After the flower has been pollinated, it closes up and dries out as the fruit begins to develop and bulge at the base of the flower.
Each fruit contains about 2,000 tiny black seeds. When the fruit and seeds are eaten by a coyote or cactus wren, the seeds pass through their digestive system unharmed and are distributed throughout the desert. However, if the seeds are eaten by a dove or quail, they will be completely consumed in the digestive system.
It is estimated that a saguaro can produce some 40 million seeds during its lifetime. However, few will survive to become a seedling. Fewer still will become an adult. The low survival rate or seedlings can be attributed to drought, prolonged freezing, or animals eating them.
Why do Saguaros Have Pleats?
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 allow the flesh to soak up water, expanding like an accordion. Conversely, when the desert is dry, the saguaro uses its stored water and the pleats contact.
Because the majority of a saguaro is made up of water, an adult may weigh 6 tons of 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 outsider of the plant. As the saguaro grows, the ribs will occasionally form and the corresponding pleat will also fork at the same place.
Sometimes saguaros have the chance to soak up a larger than usual amount of water- such as after a large summer rainstorm. The ground can become very sock after such a storm and with the added weight of the new water, the heavy cactus sometimes falls over or loses an arm.
Why are Some Saguaros Crested?
Even when saguaro cacti grow in their normal form, they rarely grow symmetrically. Saguaros sometimes grow in odd or misshapen forms. The growing tip occasionally produces a fan-like form which is referred to as crested or cristate. They are considered to be somewhat rare. 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 lightening strike or freeze damage. At this point we simply do not know what causes this rare, crested form.
Do Animals Use the Saguaro?
Saguaro cacti are host to a great variety of animals. The gilded flicker and Gila woodpecker excavate nest cavities inside the saguaro's pulpy flesh. When a woodpecker abandons a cavity, elf owls, finches, and other birds move in.
Large birds, like the red-tailed hawk, also use the saguaro for nesting and hunting platforms. Their stick nests are constructed among the arms of a large saguaro. In turn, ravens and great horned owls may take over an abandoned hawk nest.
Saguaro cacti also provide a valuable source of food for animals. In early summer saguaro flowers provide nectar and pollen for bats that in turn pollinate the flowers. In mid-summer, ripening fruit provides moisture and an energy-rich food for birds, bats, mammals, reptiles, and insects during a time of scarcity.
In drier areas of the Sonoran Desert, park rats, jackrabbits, mule deer, and big horn sheep will also eat the young saguaro's flesh when other water sources are not available.
Do Humans Use the Saguaro?
Archeological evidence indicates that the Salado people used the saguaro in their daily lives. The strong, woody ribs were gathered and used in roof construction in both the Upper and Lower Cliff Dwellings.
Additionally, saguaro ribs were used to collect saguaro fruits, which grow high up on the plant. Several ribs were tired together with a cross piece at the end. These long poles were used to knock ripe fruit down from the top of the plant. It would then be gathered to eat. In the present day Tohono O'odam culture, the saguaro is a sacred plant, to be given utmost respect. The calendar is based on the cycles of the saguaro, and includes ceremonies involving the sago such as one that involves the making of wine from saguaro fruits.
The Tohono O'odam continue to gather saguaro fruit just as their ancestors, the Ancient Sonoran Desert People, did. They use the sweet fruits to make the ceremonial wind as well as jelly and candies. They also use the high protein saguaro seeds as chicken feed.
Are There Threats to the Saguaro?
The saguaro is a long-lived cactus, most affected by long-term climate cycles of frost and drought. Reports of a saguaro "disease" popularized almost fifty years ago, but saguaros are not subject to blights. The saguaro is a common plant in the Sonoran Desert and is not an endangered species.
Without question, the biggest threat to the saguaro is our rapidly expanding human population. With this influx of people has come about threat to the saguaro- exotic plants. Exotic plants almost always out-compete native plants for the limited resources of water and nutrients. Exotic plants have also led to an increase in wildfires in the desert, which harm or kill native vegetation, including the saguaro.
Saguaros and other cactus are not adapted to a fire regime, as fires did not regularly affect them in the past. Prior to the introduction of exotic grasses for cattle feed and landscaping, the native desert grasses were sparse and did not carry a fire far. Therefore a lightening strike would have caused only a small patch to burn before running out of fuel.
Other threats to the saguaro include vandalism, attempted transplant or theft of the cactus. The saguaro is one of many plants in Arizona protected by the Native Plant Protection Act, and within national park lands, the removal of any plant is illegal. Also, all land in the state is owned by somebody whether it is federal, state, tribal, or private.
To remove native plants from any lands, permission must be granted by the landowner and a permit must be obtained from the Arizona Department of Agriculture. Homeowners wishing to move a saguaro (or any protected native plant) on their own property may also do so by obtaining the appropriate permit from the Arizona Department of Agriculture. (No affiliation with the National Park Service)
Keep in mind that saguaros less than about four feet tall have the best chance of surviving a transplant. Saguaros can be difficult to transplant successfully, requiring special care and also irrigation. Consult a plant expert for advice.