Saguaro Cactus

The Saguaro Cactus (Carnegiea gigantea)

Pronounced “sah-wah-roh”, the saguaro is the icon of the American west. Images of these cacti are seen all through our lives as a symbol of the Sonoran Desert, and a trip to this desert is not complete without a examining of one of these famous desert cacti. These huge green columnar cactus have fascinated nearly every person who has seen one. To the local Tohono O’Odham people, the saguaro cactus are even more important: these giant cactus are not plants to the Tohono O’Odham, but a different type of humanity, and are viewed as respected members of the Tohono O’Odham Tribe.

While the saguaro cactus has become a symbol of the American West, the saguaro cactus will only grow in the Sonoran Desert. As a desert indicator species, the range of the saguaro cactus is limited to southern Arizona. Saguaro cactus will grow from sea level to about 4000 feet in elevation. The farther north and higher in elevation you go, the saguaro cactus will restrict their growth to the warmer, south facing slopes. Saguaro cactus are plentiful within Organ Pipe Cactus National Monument. Impressive “saguaro forests” can be seen along the Ajo Mountain Drive.

Nurse Plants

Just as we humans need nursing and caring for when we're young, many young cacti begin their lives under the protection of a nurse plant. While one saguaro can produce an estimated 20-40 million seeds in its lifetime, only a few seeds will actually sprout, and even fewer grow to maturity. Nurse plants protect the tiny, delicate cactus seedlings from temperature extremes and sunburn by providing shade. Slowly decaying leaf litter in the area of these plants provide nutrients and hide the young plants from hungry animals seeking a juicy bite of cactus salad.

This protected, humid environment at the base of nurse plants help the saguaro to be a tall, mighty cactus. Unfortunately for the nurse plants, as the cacti grow larger, they require more nutrients and end up competing with the nurse plant for precious desert water, and the nurse plant often winds up losing the competition with the thirsty cacti, losing their life in the process.

Saguaro Growth

The saguaro cactus is the largest cactus in the United States. The saguaro cactus grows as a column at a very slow rate, with all growth occurring at the tip, or top of the cactus. It can take 10 years for a saguaro cactus to reach 1 inch in height, but these mighty cacti eventually grow to reach an average height of 40 feet, and the tallest saguaro ever measured towered over 78 feet into the air! By 70 years of age, a saguaro cactus can reach 6 and a half feet tall, and will finally start to produce their first flowers. By 95-100 years in age, a saguaro cactus can reach a height of 15-16 feet, and could start to produce its first arm. By 200 years old, the saguaro cactus has reached its full height, reaching upwards of 45 feet tall. Some saguaros have been seen with dozens of arms, while other cactus never produce a single one. Why this happens remains one of the desert’s mysteries.

The saguaro cactus is a master of desert survival. Every aspect of this plant is specifically designed to thrive in the sometimes harsh Sonoran Desert. The skin of the saguaro cactus covered with a thick waxy coating that waterproofs the plant, and reduces water lost to the air through transpiration. The skin of the cactus is covered with hard spines and flexible bristles as a method of defending the water stored inside.

How Old Is that Saguaro?

No one knows for certain. Estimating the age of a cactus is difficult. There are no annual growth rings as there are in trees; rainfall, soil conditions, and exposure to sunligh all influence the rate of growth for a saguaro.

Long-term scientific studies, photographic records and other data aid researchers in estimating the age of saguaros. Age and height relationships will vary in other parts of the monument and in other parts of the Sonoran Desert. For instance, in a wetter environment such as on the east side of Saguaro National Park in Tucson, growth rate is faster: a cactus one inch tall may only be six years old instead of 10. It may reach a full height of 46 feet in merely 173 years! Environment plays an important role in the growth of these mighty giants!

One Thirsty Cactus

The root system of a saguaro cactus is impressive: the cactus will send a large, single taproot straight down into the soil to a depth of about 5 feet, which gives it access to water that is stored deep underground. The saguaro sends out a massive maze-like array of roots close to the surface as well to catch water near the surface.

Very little water is instantly used; instead, most of the water collected ends up being stored within the cactus to use during periods of drought. The interior of the cactus is filled with a sponge-like tissue which is used to hold the water. As more and more water gets stored, the skin of the cactus begins to expand, making room for even more storage. As a result, the saguaro cactus can become quite heavy as more and more water is stored. At full capacity, a foot of saguaro cactus can weigh upwards of 90 pounds, and a full height saguaro can weigh over a ton!

Saguaro Fruit

Saguaro cactus will produce flowers during late spring into early summer, and can produce tens of millions of seeds in its lifetime. In average years, the flowering occurs between April and June. The flowers are a milky white, and emit a sweet nectar which attracts multiple species of bats. These bats feast on the flower nectar, and in process act as pollinators for the saguaro cactus. As the cactus produce fruit, the bats will start to eat the fruits, and in turn help spread the saguaro seeds across the desert. Only a few of these sees actually sprout, and even fewer grow to maturity.