How Old Is That Saguaro?
HOW OLD IS THAT SAGUARO?
By: Tom Gatz, Garden Docent and Horticulture Aide
SHORT ANSWER: Unless it was grown from seed, and someone kept tabs on it, no one knows for sure.
The much longer answer to one of the most commonly asked question by visitors to the Desert Botanical Garden is that on average and under the natural conditions existing just west of Tucson with 10 inches of rainfall annually, a saguaro often weighs less than an aspirin at age five and it may take about 10 years to get just an inch and a half tall, about the size of your thumb! Under these natural conditions it may take 20 years to almost attain one foot in height and 30 years to reach two feet tall, but by age 40 it may be up to four feet tall, by age 50 up to seven feet tall, by 75 up to 16 feet, and by age 100 almost 25 feet tall. Throughout its range and depending upon soil and rainfall, it first blooms between 40 and 75 (average 55) years old, usually starts to grow arms when it is between 50 to 100 years of age (average 70), and it may live for perhaps 200 years or more (again, no one knows for sure). Reportedly, the 1996 Guinness Book of World Records tallest saguaro grew to nearly 59 feet before falling, at a location southwest of Phoenix. The record for the tallest cacti in the world is not held by the saguaro, but by the cardon cactus in Mexico. Dr. Mark Dimmitt at the ASDM reports that some cardons appear to be nearly as tall as the nearby 80-foot champion boojums that are as high as a seven-story building, although no one has measured them as far as he knows.
Please bear in mind that there are exceptions and qualifications to almost everything stated above. For example, regardless of age, saguaros mature and flower for the first time when about eight feet tall. In wetter areas with about 16 inches of rain (like in the Rincon Mountains east of Tucson) they reach this size and flower in only 40 years. Further west in Organ Pipe National Monument (with only nine inches of rain annually) it takes almost twice as long (an average of 75 years) to reach this size and flower. While arms can begin to grow when the saguaro is about 12 feet tall, with some eventually growing as many as 50 arms, others in very dry areas may never grow any arms at all. Water-retentive clay soil appears to result in bigger saguaros with more arms.
Under the exceptional horticultural conditions in Dan Bach’s greenhouse nursery in Tucson, he produces six-inch-tall saguaro seedlings in only three years. Planted in sand, watered twice a week, and fertilized monthly from spring through fall, Dr. Mark Dimmitt pushed some of these seedlings to reach eight feet tall and bloom in only 15 to 20 years! It’s a good bet that our pampered saguaros in the areas of the Desert Botanical Garden (and in our front yards) with lots of supplemental water also grow considerably faster than their desert cousins. Conversely, the 1.5 inch-tall display saguaros confined to their tiny pots on the kitchen sink in the Desert Botanical Garden’s Education Building may never grow much at all! So, when I show visitors these tiny saguaros, I explain that if these cacti were growing in the wild, they might be approximately 10 years old rather than taking a wild guess at their actual age in cultivation.
Nature amazes us and sometimes frustrates us when we want a definitive answer, by always providing exceptions to the hard and fast rules that we come up with. This is because successful species survive by having enough genetic diversity to be able to adapt and respond to gradually changing environmental conditions. Unfortunately, genetic diversity is sometimes curtailed by the human-caused loss of some populations that may possess the needed adaptive genes; these populations frequently occur on the periphery of a species range where conditions are often more harsh. For example, saguaros found in the higher elevations (up to 5,100 feet) of the species range in central Arizona have larger stem diameters and a greater density of spines on their growing tips than those from lower elevations, which is thought to be an adaptation to cold temperatures. At night, the growing tips of large stems are four degrees warmer than those of small stems. A good analogy would be the likelihood of your little toes getting frostbite before your big toes do!
However, humans often accelerate changes in environmental conditions faster than species can adapt, leading to endangerment or extinction. While saguaros are not endangered, fatalities have been increasing in recent years as fire is spread by nonnative grasses such as buffel grass and red brome (introduced for cattle forage). These contiguous areas of dry grass provide fuel for catastrophic fires in our desert uplands, a hazard to which our native saguaros are not adapted to survive, having evolved with scattered, native bunch grasses that didn’t spread fires as extensively as do the nonnative species. The next time you pause to admire one of these giant sentinels in the desert, wish it luck for a long life; it may need it.
Thanks to Dr. Joe McAuliffe, Dr. Mark Dimmitt, Judi Irons, Tina Wilson, Beth Brand, and Scott McMahon for helping me with this article, which is modified and reprinted from The Gathering, the newsletter for the volunteers at the Desert Botanical Garden.