PLANT OF THE WEEK
Dr. T. Ombrello - UCC Biology Department
Common names: Saguaro, Sahuaro, Giant Cactus
Scientific name: Carnegiea gigantea (formerly Cereus giganteus)
Explanation of scientific name:
Carnegiea - named in honor of Andrew Carnegie (1835-1919), American industrialist and philanthropist. In 1903 the Desert Botanical Laboratory of The Carnegie Institution was established at Tucson, Arizona. In 1908 the Saguaro cactus was moved by plant taxonomists from the genus Cereus to a newly established genus, Carnegiea. The Saguaro is the only member of this genus.
There are almost 2000 species of cacti found in the Americas, yet by far one is best known as the iconic image of the deserts of western North America. The Saguaro (pronounced “sa-WAH-ro”) has a distinctive silhouette that most people instantly associate with the beautiful but harsh Sonoran Desert of Arizona, Mexico, and a small portion of California. The Saguaro’s habitat can be hot and dry. Daily summer temperatures often rise over 100 degrees F, and the annual rainfall is usually less than 12 inches. Saguaros cannot tolerate prolonged temperatures below freezing, and that is why their range does not extend into the deserts further north.
The Saguaro is considered to be one of the largest cacti in the world, and is the largest cactus in the United States. It can attain heights in excess of 50 feet with trunk diameters approaching 2 feet. They can weigh over 8 tons and live up to 200 years. Their growth pattern can best be described as slow but sure. Germinating from a tiny black seed, Saguaros can take up to a decade to reach 1 inch in height. In 25 years one may be a foot tall, and take as much as 50 years to reach 3 feet. At about 75 years it will produce its first branch (arm) and will be over 150 years old when it reaches its full height. Saguaros do not produce annual rings of growth in their trunks as trees do. So, determining the exact age of any one individual is difficult, and is often just an educated guess.
The root system of a Saguaro is quite shallow, but extensive. The roots often spread as far from the trunk as the cactus is tall. This root pattern not only anchors the plant, but provides plenty of surface area to absorb water from even a light rainfall. When precipitation is plentiful mature Saguaros have been known to absorb over 700 gallons of water in 10 days. The water can then sustain the plant during the frequent prolonged periods of drought. The trunks and branches of Saguaros swell and shrink like bellows when water is gained or lost. The vertical pleats become more obvious as water is depleted in dry weather. The stored water exists in a gel form in the trunk, and will not drain out if the plant is injured. Stories of tapping Saguaros as a source of water for people are just myths.
The leaves of Saguaros are modified into spines, offering the stems obvious protection from animals and some protection from intense sunlight and drying winds. The green branches and the trunk take on the role of photosynthesis. Their minimal surface area limits water loss through transpiration but can carry on sufficient photosynthesis to support the plant’s slow growth. While typically circular in cross-section, flattened stems are occasionally produced on Saguaros. Known as fasciated or crested growth forms, these unusual stems capture the attention of humans. Saguaros that develop them are especially prized, but they appear on only one out of every 200,000 individuals. Virtually any plant can produce a fasciated branch, and an article on this odd but fascinating growth form can be found on the author’s webpage.
Saguaros typically produce their first flowers after they reach 6 feet in height. The large, fragrant, white flowers bloom in the spring and can be pollinated by bats, birds, and bees. The resultant red fruits ripen in the summer and open to release the numerous seeds within. Healthy plants yield many thousands of seeds annually, adding up to numerous millions over a lifetime. Of course in an area with a stable population of Saguaros, on average only one seed will germinate and then grow to maturity. Young Saguaros succumb to the harsh conditions of their habitat, including consumption by animals that can get past their protective spines, unfavorable weather, and trampling. If a seed germinates beneath a tree or large shrub, its chances of survival improve under the protective canopy of what are called “nurse” plants. Palo Verde and Mesquite trees often serve in this role.
Saguaros are intimately connected to the other members of the desert community. The flowers provide nutritious nectar and pollen to their pollinators. The sugar-rich fruits are used for food by many animals, including people. Native Americans even grinded the seeds into a flour. Birds nest on their branches and even in their trunks. Gila Woodpeckers and Gilded Flickers excavate cavities into the trunks to raise their young. Once abandoned, other animals of all kinds move in. Inside these cavities it can be 20 degrees F cooler in the summer and 20 degrees F warmer in the winter. When large Saguaros die their soft tissues degrade revealing an internal woody skeleton that may remain standing for many years. Native Americans utilized this wood in construction.
In Arizona the beloved Saguaro is the State Flower and the species is afforded protection by state law. The Saguaro is a “salvage-restricted protected native plant.” A permit is needed to move one, and each plant moved must carry an official seal. Poachers, traffickers, and vandals can be fined up to $100,000. Large Saguaros do not transplant readily, and many do not survive moving. Even with the protection they are afforded, the future of Saguaros is not assured. They can be destroyed on private property, and many are as people sprawl into their environment. The biggest threat they face is loss of habitat. National Parks within their range are important sanctuaries.
Saguaro cacti in the Sonoran Desert
near Tucson, Arizona
A Saguaro cactus with a developing “arm.”
This does not occur until the plant
approaches 75 years of age.
The “skeleton” of a long dead
About 1 in every 200,000 Saguaro cacti
develops a fasciated or crested stem.
See my Plant of the Week entitled “Fasciated Plants”
for more information on this unusual plant growth form.