Giant Saguaros: From Tiny Seeds to Towering Titans
When we think of saguaros, we usually marvel at the big ones, the 20 and 40-foot tall gains that have lived for over a century, enormous cacti that make the Sonoran Desert such a unique landscape. But a saguaro begins, as do all living things, very small. A single saguaro seed is not much bigger than the size of the period at the end of this sentence, glossy and black. Each tiny seed contains all of the information necessary to grow into a forty-foot tall towering titan, if it is fortunate to land in the right place and not get eaten.
I often search for the smallest saguaro I can find while hiking the desert preserves in and around Phoenix, especially the pea-sized seedlings that have a pair of succulent leaves topped by a soft tuft of spines and two threadlike rootlets that cling to the soil. Birds, mice and squirrels harvest most of these tender morsels before they reach a month old, so the chances are slim to find one at this early stage. The statistics are astonishing but grim: *An average healthy, mature saguaro can produce more than 150 fruits in one season. *Each fruit may contain more than 200 seeds. *Thus, one saguaro can produce over 300,000 seeds each year. *Over a century of reproductive life, a single large saguaro may bear more than 30 million seeds. *Out of all those seeds, only one out of a thousand will end up in conditions where there is enough moisture to enable it to sprout. *Ultimately, only one out of several million seeds will become a full-grown cactus in the wild due to predation of fruit, seeds and you cacti by many different creatures, including humans. Freezing, drought, and flash floods also take their toll on young cacti. In the shelter of a "nurse tree" or shrub, usually a Palo Verde or Mesquite, a young saguaros chances are much higher. Shade an moisture will prevent it from getting dehydrated during those tender years. Dispersal by birds that eat saguaro fruit is the most effective for seeds to land in suitable germination and establishment sites, as they deposit them in droppings while perched on tree limbs. Saguaros generally out-live their nurse plants, but they do not parasitize them in any way. There is no simple formula by which to estimate the age of a saguaro because they do not have growth rings like woody trees. Aging a saguaro by height depends on location, especially the amount of rainfall they receive. Data from measurements taken of saguaros at three locations in the wild show that at 100 years, height can vary from between 15 and 30 feet. A 1 to 3 inch tall wild saguaro is about 10 years old. At about 25 years, a wild saguaro will be 2 to 3 feet tall. Two hundred years is a long life for these giant cacti.
Late April to early June is the season for saguaro blossoms. the official state flower of Arizona. Each flower blooms for only one night, opening after dusk and remaining open until early afternoon the next day. A new set of flowers blooms for up to six weeks every night, attracting numerous pollen and nectar feeders, including long-nosed bats by night, doves and many species of insects during the day. Most of these creatures also serve as pollinators for the saguaro.
Wild saguaros begin producing flowers and fruit at about 50 years, but this ranges from 30-75 years, depending on available moisture. Flowering age corresponds to between 6 and 8 feet tall. Saguaros begin producing "arms" after 50-70 years, which also increases flower production, since flower buds emerge at the top of each arm. A mature saguaro with many arms can weigh up to 9 tons: a hydrated stem weighs about 80 pounds per foot, so a 30-foot tall single-stemmed saguaro may weigh about 2400 pounds. The tallest recorded specimen once lived near Cave Creek, AZ, reported at 78 feet tall before it died in 1986. The current record for the tallest living saguaro is a dwarf in comparison at only 45.3 feet tall and a circumference of ten feet. No matter what their size, any saguaro you encounter is a miracle of desert survival.
Since May 2005, I have written a natural history column for a neighborhood newsletter published by Jennifer Moore called "Our Big Backyard: Natural History of the Deem Hills," and since 2010, "A Suburban Naturalist."All of these columns are reproduced here. Deem Hills is a City of Phoenix Desert Preserve located just north of Happy Valley Road in north central Phoenix, and just west of I-17. The Preserve and adjacent hills comprise over 900 acres of open space with several miles of hiking trails. For more perspectives from the Deem Hills and suburban Phoenix, also check out my companion blog, Kat Tracks.
As a naturalist with a special interest in botany and plant ecology, some of my greatest passions are to explore, photograph and write about native plants. If I can spark an awareness or interest in natural history in others along the way, that is my greatest reward.