The Largest Cactus in the United States

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Reaching 40 feet tall, the Saguaro Cactus (Carnegiea gigantea) is the largest cactus native to the United States. The tallest saguaro cactus ever measured reached a height of almost 78 feet.

Geography of Saguaro Cactus

Saguaro Cactus only grown in one region of the United States. The spiny, branch cactus is native to the Sonoran Desert. A section of the Sonoran Desert in southern Arizona, southeast California, and western Sonora, Mexico, is the only place that the Saguaro Cactus grows.

Saguaro Cactus grows from sea level to about 4000 feet in elevation in southern Arizona. Freezing temperatures in winter limit the growth of the Saguaro Cactus at higher elevations in the Sonoran Desert. Above 4,000 feet Saguaro Cactus only grown on south facing slopes.

The cactus is considered a desert indicator species and a keystone species as the plants provides food and shelter for many animals.

Ocotillo (left) and saguaro (right) are two plants found in the Sonoran Desert. Photo: National Park Service/Alice Wondrak Biel, Saguaro National Park, Public domain.
Ocotillo (left) and saguaro (right) are two plants found in the Sonoran Desert. Photo: National Park Service/Alice Wondrak Biel, Saguaro National Park, Public domain.

Saguaro National Park

In 1933, President Herbert Hoover established the Saguaro National Monument in the Rincon Mountains. In 1961, President John F Kennedy Saguaro added the added the Tucson Mountain District to the monument.

The two sections, located east and west of Tucson in Arizona, were established in 1994 as a National Park to help preserve the Sonoran Desert landscapes that support the Saguaro Cactus.

The park’s two districts are a combined area of 91,327 acres.

Satellite image with the boundary of Saguaro National Park in Arizona outlined.
Saguaro National Park is located just west of the city of Tucson, Arizona. Image: Landsat 8, NASA.

Nurse Trees

Saguaro Cactus use “nurse trees” for protection in the first years of growth. Usually fast growing trees such as palo verde, ironwood or mesquite, these nurse trees provide shelter for the very slow growing cactus. A saguaro cactus only grows about 1 to 1.5 inches in the first 8-10 years.

As the Saguaro continues to grow, scientists believe that competition for water and nutrients kills off the nurse tree.

A young columnar cactus growing with a nurse tree in the desert.
A young Saguaro Cactus growing under the protection of a green paloverde nurse tree. Photo: Alice Wondrak Biel / NPS, public domain.

Saguaro Cactus Grows Very Slowly

The massive heights of the Saguaro Cactus are only reached after a very long period of growth. Around 70 years old, Saguaro Cactus reach about six feet in height.

The iconic arms (also known as branches) of the Saguaro Cactus only start to appear when the cactus is about 95-100 years old and around 15-16 feet tall.

Around 125 years old, the Saguaro Cactus is considered an adult. A mature Saguaro Cactus may have several branches or none at all. Biologists don’t know what determines whether or not a the cactus grows branches.

Saguaro cactus are believed to live on average between 150 – 175 years although some cactus may reach 200 years old.

Shallow Root System

While the tap root of the Saguaro Cactus extends about five feet into the soil, the overall root system of the plant is very shallow. Saguaro roots are only around 3-5 inches deep, but they extend out in a radius as long as the plant is tall.

The taproot serves to access underground water in the desert.

The main roots of the cactus are covered in special hairs that serve to collect as much as 200 gallons during a rainfall. The summer monsoons that bring rain to the Sonoran Desert often last only a few minutes so the widespread net of a Saguaro’s root systems acts to quickly absorb any water before it runs off.

The water collected is then stored in the cactus to provide hydration during dry periods in the desert.

The extension root system also acts to support the cactus, keeping it anchored and upright during the winds that often accompany rainstorms.

Flowering Season

The Saguaro Cactus flowers during late spring into early summer.

Multiple species of bats serve as pollinators of the Saguaro Cactus by feeding on the nectar produced by the white flowers. The bats also eat the fruit produced and disperse the seeds.

A cactus with a white flower blooming.
A Saguaro Cactus in bloom. Photo: Bob Szaro, USGS, public domain.

Cactus Provide Refuge for Animals

Saguaro Cactus providing nesting sites for the Gila Woodpecker (Melanerpes uropygialis). The woodpeckers burrow out cavities in the cactus. The nesting cavities provide a safe place from predators and the cactus provides a warmer space during the colder months.

Abandoned cavities are used by elf owls, screech owls, purple martins, finches and sparrows.

Other birds, such as Harris’s Hawk (Parabuteo unicinctus) will build nests in the arms of the Saguaro Cactus.

Three juvenile hawks site in a nest built in the arm of a cactus.
Harris’s Hawk Nest in Saguaro Cactus. Photo: NPS, public domain.

Saguaro Cactus also provide a safe space for bobcats fleeing predators. The thick-padded paws of bobcats (Lynx rufus) allow them to climb cactus without injury from the prickly spines that can grow up to three inches (7 centimeters) long.

The tall cactus also provides a safe platform for the bobcats to view the terrain.

A bobcat sitting in the arm of a tall cactus in the desert.
Bobcats will climb Saguaro Cactus to escape from predators. The tall plants also provide a vantage point for the felines. Organ Pipe Cactus National Monument. NPS photo, public domain.

References

Saguaro cactus. (2016, January 6). U.S. National Park Service. https://www.nps.gov/orpi/learn/nature/saguaro-cactus.htm

Saguaro cactus fact sheet. (n.d.). Arizona-Sonora Desert Museum, Tucson, AZ. https://www.desertmuseum.org/kids/oz/long-fact-sheets/Saguaro%20Cactus.php

Where saguaros grow. (n.d.). NPS.gov (U.S. National Park Service). https://www.nps.gov/sagu/learn/nature/location.htm

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