Carnegia gigantea (Englem.) Brit & Rose

by Mead Zaccagni Mier, Native Plants Class 2003
Common names: Saguaro, Giant Cactus.
Family: Cactaceae
Synonymy: Cereus giganteus
Etymology: The Genus name comes from Andrew Carnegie (1835-1919), an internationally renowned philanthropist. The common name, Saguaro, is an Indian name (1).

Identification
Growth form: Considered a tree/ subshrub, the Saguaro is the largest cactus in the US, commonly up to 12 m tall and with extremes of 23.8 m (5). It takes a bowling pin shape for about the first 35 years until it reached around 2.5 meters. At about 65 years it has achieved its maximum number of ribs and begins to branch. It can grow up to 50 arms (4).
Roots: Saguaro has a large tap root reaching 60 cm deep with smaller shallow roots, less than 10 cm deep, reaching as far out wide as the cactus is tall (3).
Stem: Fleshy tissue for water storage with ribs in center (4,6). Green photosynthetic exterior with waxy skin that pleats in accordion shape. Pleats expand for water uptake creating stems that can weigh 80 pounds per foot (3,4).
Leaves: Leaves are modified into clusters of white or pink spines located every 2.5 cm on the outer ribs (2). The lower 2.4 m have hard spines while above that point they are more flexible and bristly. There are up to 30 spines per areole and spines are up to 5 cm long (3).
Inflorescence/flowers: The Saguaro flower is the Arizona state flower. Petals are spirally arranged and white. Flowers are 8 cm in diameter, clustered on ends of arms, require cross pollination, and have the aroma of “ripe melons” (3, 2). Flowers are pollinated by bats and insects(3, 6).
Fruit: Saguaros propagate by seed. Juicy, red, oval-shaped fruits ripen one month after flowers bloom (2). When ripe, the fruit splits into 4 sections revealing over 2000 tiny, black seeds (2, 3).
Similar species: Carnegia is the only species in its genus (5). There are 5 other species of Cereus, the past genus for this species, in Arizona, none nearly as large as the Saguaro (2).

Ecology
Life history: Perennial (5)
Native/introduced: The distribution of the Saguaro defines the boundaries of the Sonoran Desert. Its range spans from western Sonora, Mexico to southern Arizona and California. It is a tropically derived plant with relatively recent evolutionary origin in Arizona (3, 4, 5)
Photosynthetic pathway: CAM
Phenology: Saguaros are long lived, averaging life spans of 85 years, but can live up to 200 years (2). Most deaths occur during the first year’s establishment. Flowers bloom in May to June just before monsoons, opening at night and staying open until midday the next day (3, 4).
Distribution: The elevation Carnegia gigantia is found at is 183 m to 1100 m on rocky slopes and well drained flats (2). The greatest abundance of Saguaro occurs in the Arizona Upland. In more northerly ranges, Saguaro is frequently found on southern slopes. Its native range borders tropics in warm arid deserts. Saguaros are cold intolerant, vulnerable to fire in the more mountainous habitats, and also vulnerable to wind and lightning (3,4 ).

Uses
Wildlife: Wood rats, bighorn sheep, and jackrabbits eat the stem tissue. Birds of prey use the Saguaro for hunting perches and nesting between arms. Other birds eat and help to spread the seed. Birds also nest in holes in the stems, called boots, which were previously dug out by woodpeckers and flickers (3, 4)
Human: Saguaros are protected by the US federal government and are conserved in Saguaro National Park. The Tohono O’odam consider the Saguaro another form of humanity. The Saguaro is a part of many of their creation stories as well as the basis for the seasons of their calendar year. Natives have traditionally used the Saguaro boots for storage, the fruits for food and wine, the seeds for flour, and the ribs for fences and houses (3).

References
1. Charters, Michael L. 2003. California Plant Names, Word Meanings and Name Derivations. www.calflora.net/botanicalnames/index. Sierra Madre, CA.

2. Epple, Anne Orth. And L.E. Epple. 1995. A Field Guide to the Plants of Arizona. The Globe Pequot Press, Connecticut.

3. Phillips, Steven S and Patricia Wentworth Comus, Ed. 2000:A Natural History of the Sonoran Desert. Arizona-Sonora Desert Museum Press, Tucson, AZ.

4. Stenbergh, Warren F. and Charles H. Lowe. 1983. Ecology of the Saguaro III: Growth and Demography. Scientific Monograph Series, Number 7, US Department of the Interior, National Park Service, Washingaton, D.C.

5. USDA, NRCS. 2002. The PLANTS Database, Version 3.5 (http://plants.usda.gov). National Plant Data Center, Baton Roughe, LA 70874-4490 USA

6. Usher, George. 1996. A Dictionary of Botany. Constable & Company Ltd, London