Longest-lived cactus species

Determining the precise age of cacti is extremely challenging as they do not contain helpful markers such as internal growth rings, as used by dendrochronologists to age trees. Two of the largest cactus species, the cardón (Pachycereus pringlei) and the saguaro (Carnegiea gigantea), share the distinction of being the longest-lived cacti, both capable of exceeding 200 years, and in rare cases perhaps reaching 300 years old. Both of these slow-growing giants are native to North America's Sonoran Desert – the former exclusively to north-west Mexico and the latter with a range stretching from Sonora, Mexico, into Arizona and south-east California, USA. The oldest specimens of both species might grow to 15–20 metres (49–65 feet) tall.

For lack of a technique such as counting growth rings, botanists have had to devise other methods to estimate cactus age. For the most part this comes down to monitoring annual growth rate of a species and calculating a specimen's age from its size; however this isn't infallible because growth rate is affected by variables such as precipitation or other environmental factors such as wind, above/below average temperatures, insect infestation or damage by other animals or humans. Depending on the cactus type, the number of arms might also be factored in to determine age or the number of areolae (where spines/flowers/branches emerge). Some preliminary studies using radiocarbon dating are starting to explore the possibility of being able to age cacti more accurately.

The cardón (aka elephant cactus) is thought to just pip the saguaro to the title of the tallest cactus species in the wild: one specimen discovered by Marc Salak and Jeff Brown while hiking in Baja California, Mexico, in April 1995 stood 19.2 m (63 ft) tall, and there are reports of examples even exceeding 20 m (66 ft). An exceptional specimen of saguaro discovered in the Maricopa Mountains, near Gila Bend, Arizona, USA, on 17 January 1988 was measured at 17.67 m (57 ft 11.75 in); though again there are unverified reports of saguaro reaching in excess of 20 m.