Saguaro
Carnegiea gigantea

Family: Cactaceae

Probably the most famous cactus in the world, the saguaro is the Lion of the Sonoran Desert. Plants grow to 40’ tall and have many “arms”. White flowers appear on the tops of the stems like crowns in summer. Following the flowers are green skinned fruits that bust open to reveal sweet, red, edible flesh with large black seeds.

Saguaro roots can grow as much as 100’ from the plant! Besides its woody ribs, described below, the roots are an essential element to keeping these giants upright, as well as quickly responding to rains and absorbing as much water as they can when water infiltrates the soil.

It used to be thought that saguaros lived many hundreds of years, because their growth was measured on regular years. But in wet years saguaros can put on more growth. It is now believed they can live up to 200 years, often less. Plants usually grow as singular stems for the first 6 or 7 decades of their life. At that time they begin to produce “arms”.

Photo by Steve Jones, iNaturalist
Carnegia gigantea on SEINET

The specialized morphology of the saguaro allows it to withstand many years of drought—they store water in their tissues, and their anatomy allows for expansion or contraction between the ribs, depending on how turgid with water their tissue are. Their succulent nature, and method of photosynthesis also helps them—they only open their stomata (tiny microscopic openings that allow the escape of gasses and water vapor) when it cools off at night and they will loose less moisture than if they were to open their stomata at night. Sadly, in recent years saguaros have struggled—not with drought, which they are equipped to handle, but with the extended periods of very hot temperatures where the nights do not cool off. Because they usually wait for cooler night temperatures to open their stomata for release of gasses and vapor, extended periods of high night temperatures have caused saguaros to literally bake, as their stomata don’t open to release water vapor, which cools the plant down. The saguaros literally don’t get a chance to cool off and they literally get cooked.

Another threat for saguaros is the introduction of invasive species like Buffelgrass (Cenchrus ciliaris), a grass from Africa that was introduced to feed cattle, but has escaped cultivation, and has spread about the low desert. Buffelgrass is not like our native grasses, which lack density and don’t easily carry fire. Buffelgrass burns hot and long, and many of our native plants, especially cacti like saguaros, have not evolved with fire, and do not recover from fires, which previously were rare in the low desert.

This cactus is found exclusively in the Sonoran Desert. The most important factors for growth are water and temperature. If the elevation is too high, the cold weather and frost can kill the saguaro. It is thought that the saguaro obtains most of its moisture during the summer rainy season, so monsoon rains are especially important to the saguaro.

The saguaro is a keystone species: which is a species that is very interactive with its ecology (other organisms in the environment)—the presence of the saguaro is important to many, many organisms; like the primary pollinators, the Lesser Long-nosed Bat (Leptonycteris yerbabuenae), the white winged dove (Zenaida asiatica), and various species of bees which feeds on the nectar of the flower. Other diurnal pollinators are birds such as Costa's hummingbird, the black-chinned hummingbird, the broad-billed hummingbird, the hooded oriole, Scott's oriole, the Gila woodpecker, the gilded flicker, the verdin, and the house finch according to studies that examined the relative contributions of diurnal pollinators.

Saguaro seeds are small and short-lived. Although they germinate easily, predation and lack of moisture prevent all but about 1% of seeds from successful germination. Seeds must wait 12–14 months before germination; lack of water during this period drastically reduces seedling survival. The existence of nurse plants is critical to seedling establishment. Foothill palo verde trees (Parkinsonia microphylla) and triangle bursage (Ambrosia deltoidea) represent important nurse species. They act by regulating temperature extremes, increasing soil nutrients (by holding organic material to the ground), and reducing evapotranspiration, among others. While nurse plants reduce summer temperatures on the ground, they are even more important in protecting young saguaros from extended frosts which limit the range of saguaros.

Photo of Lesser Long-nosed Bat Leptonycteris yerbabuenae by mamole on iNaturalist

Interestingly enough, one of the primary pollinators, the white winged dove (Zenaida asiatica), also consumes the fruit so much that few seeds escape consumption. But they also play a key role in seed distribution as not all the seeds will be eaten. For desert white-winged doves, 60% or more of their diet is saguaro-based. Their breeding cycle coincides with that of the saguaro blooming. Certain species of Sonoran Desert ants also depend heavily on saguaro fruits.

Photo of White-winged Dove (Zenaida asiatica) by Andy Bridges, iNatualist

Gila woodpeckers and gilded flickers create holes in the cactus to make nests, which are later used by other birds, such as elf owls, purple martins, rosy-faced lovebirds, several species of tyrant flycatchers and wrens, and house finches. The resulting nest cavity is deep, and the parents and young are entirely hidden from view. The saguaro creates callus tissue on the wound. When the saguaro dies and its soft flesh rots, the callus remains as a so-called saguaro boot, which is used by some peoples for storage.

The O'odham word for saguaro is Ha:sañ. The name "Saguaro" is a Hispanized version of the O'odham word.

Wherever the Saguaro occurs, it is important to the people who reside there. The indigenous people respectfully use saguaro for food—the fruits being one of the most important and sacred foods in the Sonoran Desert. Fruits are eaten fresh, dried, syrups, fermented into drink, and countless other preparations. The seeds are also eaten either raw, or processed into a meal (ground up into a flour).

Inside these enormous cacti are large woody rips that give the saguaro its balance and strength. For such a heavy, upright plant, it is remarkable that more aren’t blown over by our torrential summer storms with their high-speed winds. The ribs (with the help of the extensive roots) are part of the architecture that holds them upright. These ribs are often found after saguaros have died, and the flesh has rotted away. They have been used for various purposes—tools, construction of shelter, and more.

Its scientific name is given in honor of Andrew Carnegie. In 1933, Saguaro National Park, near Tucson, Arizona, was designated to help protect this species and its habitat. There is only one species of Carnegia (the genus is monotypic). The species name, gigantea, obviously, refers to the size of this large cactus.

Found on rocky or gravelly soils on slopes, rocky ridges, outwash fans, canyons, and benches from 500-3,500’ in southeast California (just a handful of individuals are found there) southern Arizona, and Sonora, Mexico.