Geography and distribution
The Sonoran Desert is the only place where the iconic saguaro cactus, Carnegiea gigantea, occurs naturally. This unique habitat, which is home to many other characteristic plants and animals, is shared by the United States (Arizona, California) and Mexico (Baja California, Sonora). The saguaro cactus ranges north to the edge of the Hualapai Mountains in Arizona and along the Colorado River in south-eastern California. The largest saguaro populations occur in the state of Sonora (Mexico) where its distribution reaches its southernmost point around Ciudad Obregón, the second-largest Sonoran city.
A member of the cactus family (Cactaceae), Carnegiea gigantea is a tall, tree-like cactus. It is columnar in form, the stem and branches tall and thin, like pillars, and grows up to 16 m tall. The ribbed stem (trunk) can reach a diameter of 75 cm. It is the largest columnar cactus native to the United States but is extremely slowly-growing, reaching only 0.6 cm tall after two years. It flowers once it has reached about 30–35 years of age and a height of about 2.0 m. The first branches, which grow out from the sides of the stem, only appear after it has reached a height of 4–5 m and an age of 50–70 years.
The saguaro is a stem succulent, meaning it stores water in its trunk and branches as an adaptation to its dry environment. It has a deep anchoring taproot (the main root that goes straight down into the soil) up to 1 m long and an extensive, shallow, lateral root system (roots that spread outwards from the main taproot) that allows it to take up water quickly after the infrequent rains typical of the Sonoran Desert. During this time the trunks swell considerably.
The trunk bears dense, strong, thick, spines up to 3.8 cm long on 12–30 prominent, vertical, ribs. The white flowers appear just below the top of the stem between April and June. They are 8.5–12.5 cm long and 5–6 cm in diameter. The flowers open during the night and are primarily pollinated by bats that feed on the abundant nectar. However, they do remain open for some time in the morning when other animals, such as bees and birds (doves), visit the flowers and further aid in pollination. The edible, red, fleshy fruits ripen from May to July and are 5.0–7.5 cm long and 2.5–4.4 cm in diameter.
Threats and conservation
Although not considered endangered, the saguaro cactus is protected under the Arizona Native Plant Law. Illegal collecting of plants is a concern around cities, but the most significant impact on their populations is posed by urban development, especially near Phoenix and Tucson. Nowadays, however, developers have a duty to move or protect saguaros that would otherwise be destroyed during building operations.
Saguaro fruits are highly prized among Native Americans. The delicious fruits have a juicy, red flesh with lots of tiny, black, nutty-tasting seeds (up to 2,000 per fruit). They were a staple food of the Tohono O’odham (formerly known as the Papago) and Pima Indians and were either eaten fresh or turned into juice, syrup, jam, wine (for the rain-making ceremony) or vinegar. The seeds are used as chicken food or ground into a flour to prepare a cake.
The internal ‘woody ribs’ (which form an inter-connected ring-like skeleton) of dead stems provided building materials (especially roof beams) and firewood, and were also used as splints for broken bones.
The Tohono O’odham organised their traditional calendar around the saguaro’s annual fruiting cycle and considered the species to be so important that plants were regarded as fellow humans who should not be hurt.
Food for wildlife
The flowers, fruits and seeds of the saguaro are an important food source for Sonoran wildlife, such as the collared peccary, long-nosed bats, rodents, western white-winged doves, woodpeckers and insects. Saguaros provide nesting habitats for birds and small mammals. When gila woodpeckers (Melanerpes uropygialis) and gilded flickers (Colaptes chrysoides) dig their nests into the plant’s flesh it produces a hard callus lining to the cavity that seals it off from the surrounding living tissue. Once a saguaro has died and the soft flesh has rotted away, the hard lining of these bird nesting cavities remains intact, forming curious looking container-like structures that stick out among the remains of the dead plant. These so-called ‘saguaro boots’ were used as containers by North American Indians.
Saguaro cacti are grown in desert gardens as ornamentals.
Millennium Seed Bank: Saving seeds
The Millennium Seed Bank Partnership aims to save plant life worldwide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.
Stored seeds of Carnegiea gigantea behave in an orthodox manner (meaning the seeds will survive the drying and freezing process), and one collection is held in Kew’s Millennium Seed Bank based at Wakehurst in West Sussex.
This species at Kew
Saguaro cactus can be seen growing in the Princess of Wales Conservatory at Kew. It is also grown behind-the-scenes in the Tropical Nursery.
Pressed and dried specimens of Carnegiea gigantea are held in Kew’s Herbarium, where they are available to researchers by appointment. The details of some of these specimens can be seen online in the Herbarium Catalogue.
Specimens of the wood, stem, rib and fruits are held in Kew’s Economic Botany Collection in the Sir Joseph Banks Building, where they are available to researchers by appointment.