Saguaros Against the SunriseA stand of Saguaros are outlined against a beautiful, Tucson sunrise.

The majestic Saguaro Cactus, (Carneglea gigantean), is a living symbol of the Southwest and its bloom is the state flower of Arizona.  Exceedingly slow-growing—a ten-year old plant is less than 2 inches tall!  In adulthood it will tower over all the other plants of the Sonoran Desert and reach about 45 feet. All the growth occurs at the tips.  By 70 years old a Saguaro can reach 6.5 feet and begin producing flowers.  By 95-100 years a Saguaro Cactus can reach 15-16 feet and may start to produce its first arm.  It reaches adulthood at about 125 years, and may live to be up to 200 years old.  Saguaros don’t always produce arms, and when they do not, the Saguaro is called a spear.  Arm buds normally emerge first at the greatest trunk or stem diameter.  Since flowers are produced at the tip of each arm or stem, growing new arms increases the number of flowers and fruits, the likelihood of reproducing, and serves to ensure the survival of the species. 

Saguaro Pair

 This thriving, dual stemmed Saguaro with a tremendous number of arms is likely 125-200 years old.

Occasionally the Saguaro takes a different form called a Cristate Saguaro.  Theories for the Saguaro growing a crested or otherwise unusual shape vary between a genetic anomaly, freezing, or a lightning strike somehow affecting the growth pattern.  Whatever the reason, Cristate Saguaros are sure to draw attention because although every Saguaro is different, Cristate Saguaros are rare and have very atypical growth patterns.

        The Cristate Saguaro Cactus on the left exhibits a fan shaped pattern with many arms.  The Saguaro on the right is displaying typical growth.

Saguaros have a waxy skin that helps to prevent water loss, gives protection against predators, and protects against sunburn and freezing temperatures.  Snow does fall in the Sonoran Desert but is generally short-lived in the Saguaro habitat.  Saguaros can not tolerate temperatures below 23 degrees for longer than about 29 hours before freeze damage occurs, which can harm the arms of the cactus, causing them to drop downward.

Winter in the Sonoran DesertA winter snowfall remains on the cactus at Sabino Canyon at sunrise.
The interior is filled with a tissue which holds water like a sponge.  To accommodate the potentially large influx of absorbed water, the pleats in the plant’s structure allow the cactus to expand like an accordion.  When the desert is dry, the Saguaro uses its stored water to survive and the pleats contract.

Tufts of 2" spines are distributed evenly on every pleat on the main trunk and on every arm.

Since the majority of a Saguaro is made up of water, an adult cactus can weigh 4-6 tons or more. The enormous weight is supported by a circular skeleton of interconnected, woody ribs which correspond to the number of pleats on the outside of the plant.  New ribs form by forking as the Saguaro grows, and the corresponding pleats follow the same pattern. The rib skeleton remains standing long after the cactus has died. 

A Gilded Flicker perches on the ribs of a dead Saguaro cactus.

Along each vertical rib on the waxy exterior are 2” straight (not hooked)  tufted spines with a felt like material in the center that provides shade and protection from freezing temperatures.  The spines are also a barrier from hungry animals like mule deer, jack rabbits or big horned sheep who will resort to eating the fleshy Saguaro exterior in times of drought or limited food supply.   

Saguaro Cactus SpinesSaguaro cactus spines measuring 2 inches long line the ribs of a Saguaro Cactus at Pinnacle Peak in Scottsdale, AZ

Cross Section of a Saguaro CactusCross section of a Saguaro Cactus

Beginning at the outside edge, the parts of the cactus are the spines, epidermis (green, waxy layer), cortex or pulp, ribs, and pitch (inside the ribs). 

The Saguaro has one main tap root that can extend 2-5 feet underground to provide stability and absorb water from underground sources.  In addition, it has a network of hairy roots that extend out from the base of the cactus as far as the Saguaro is tall.  These roots specialize in water absorption and are only 3-5” below the ground.  They take advantage of any rain that falls in the desert, even when there is fast runoff due to water and snow melt over the highly compacted desert soil.  The superficial roots also wrap around rocks to provide stability, which is important, given the height, weight, and mountainous conditions in which the Saguaro grows.  These roots can absorb up to 200 gallons of water during a rainfall, and help to withstand winds and overly saturated soil that can topple a water-laden Saguaro.

Saguaro flowers bloom at night during May and June and stay open until the following midday but not all of the creamy white, three inch flowers with yellow centers bloom at once.  Each cactus averages about 4 open flowers a day for about 30 days.  The primary night pollinator is the long-nosed bat who is attracted by the sweet scent of nectar and is perfectly suited given the high position of the substantial flowers that can withstand the bat’s weight.  Daytime pollinators include honey bees, white winged doves, a variety of hummingbirds, orioles, woodpeckers, gilded flickers, verdins and house finches.  As soon as the blossom is pollinated, it starts to set fruit. 

                                                                                                   Ladder-Backed Woodpecker 

            Broad-billed Hummingbird 

If the fruit of the Saguaro is eaten by a coyote or cactus wren, the seeds pass through their digestive tract unharmed and are distributed throughout the desert in scat.  But if the seeds are eaten by a dove or quail, they will be completely absorbed in their digestive system.  Few seeds will survive to become a seedling, and fewer still will become an adult Saguaro.  The low survival rate is attributed to drought, prolonged freezing, animals eating or stomping on the young seedlings and loss of nurse plant habitat.  The three inch oval, green fruit ripens and splits open to reveal the red, pulpy flesh, appreciated by all desert animals.  The Pima, Tohono O'odham and Papago Indians dried, fermented, canned and stored the fruits and used the seeds for flour and porridges.

Saguaro seedlings benefit from growing under “nurse trees” which are usually Mesquite, Palo Verde or Ironwood varieties.  The trees provide shade and protection from predators and freezing temperatures.  The branches of these trees also provide stability to the young Saguaros whose root system is not well developed, and often the cactus will be wrapped gently within a branch of the tree.  The Saguaro generally outlives the nurse tree, given that most Saguaros survive 1-2 centuries.

Twin Saguaros and their nurse treeTwo nearly identical Saguaros, between 50 and 100 years old grow tall in the midst of their Mesquite "nurse" tree.

A Mesquite "nurse" tree shields two nearly identical Saguaros at Sabino Canyon.

Saguaros are characterized as foundation species because they support so many other species in the ecosystem.  Despite their sharp spines, Saguaros serve as “hotels” for birds like Gila woodpeckers, which carve out nest holes in the plants.  The Saguaro “heals” the holes by creating a solid, internal casing around the cavity, which also provides a perfect nest inside the cactus.  Called cactus “boots”, these internal structures were also used by Native Americans as containers for water and food. 

Cactus BootsCactus boots line the inerior of the woodly ribs of a dead Saguaro cactus at Saguaro East National Park

An opening in the woody skeleton of a dead Saguaro reveals a number of "cactus boots" which developed after woodpeckers excavated nesting holes.

Since flickers and Gila woodpeckers create new holes every year, other birds like elf owls, house finches, fly catchers and purple martins move in to the pre-owned nest.  Red tailed hawks and other large birds nest in the intersections between the main stems and arms.  These holes do not appear to be problematic for the Saguaro given its ability to wall off external holes created by birds or insects.  Freezing temperatures, lightening or overwhelming mechanical damage are the biggest threats to a Saguaro’s survival once it has become established.  Even Saguaros struck by lightning or “decapitated” by the wind or other force can sometimes continue to grow new arms, bloom and survive.

Gila Woodpecker PeakingGila Woodpecker peaking out of its nest in an ancient Saguaro Cactus

A Gila Woodpecker peaks out of his Saguaro home.

The Saguaro’s strong architectural form, ability to survive climate extremes, remarkable variances in shape and proximity to beautiful mountain ranges and desert have earned it a special niche in the American Southwest.  It is a symbol and sentinel of the Sonoran Desert, a plant to emulate for its adaptability and strength, and a reminder that even under the harshest conditions, the desert is an interconnected network of animals, plants and other organisms that thrive in their unique location on planet earth.

Sabino Canyon Sky at Blue Hour

For more information about Saguaro Cactus, check out these webpages that I used to gather the information for this blog.

https://www.nps.gov/sagu/learn/nature/saguaro_roots.htm

https://www.nps.gov/sagu/learn/nature/how-saguaros-grow.htm

https://www.sciencefriday.com/articles/11-things-you-didnt-know-about-saguaro-cacti/

https://www.fs.fed.us/wildflowers/plant-of-the-week/carnegiea_gigantea.shtml

https://www.desertmuseum.org/kids/oz/long-fact-sheets/Saguaro%20Cactus.php

https://www.desertusa.com/cactus/saguaro-cactus.html

https://en.wikipedia.org/wiki/Saguaro

https://www.nps.gov/sagu/learn/nature/why_pleated.htm

https://www.nps.gov/tont/learn/nature/saguar

https://www.nps.gov/sagu/learn/nature/saguaro_reproduction.htm

https://www.fs.fed.us/database/feis/plants/cactus/cargig/all.html

http://npshistory.com/series/science/17/saguaro-ecology-3.pdf