Plant of the Week - Arizona's State Flower - Saguaro

Carnegiea gigantea (aka Saguaro) is the keystone plant and icon of the Sonoran Desert.  This slow growing columnar cactus is this country's largest cactus, often reaching between 40 and 60 feet in height and weighing (when fully hydrated) approximately 80 pounds per foot.  A mature Saguaro may weigh as much as 8 tons and they can live for more than 200 years. 

The plant's epidermis (skin) is 'pleated', allowing for expansion when water is available, and contraction during droughts.  The plant's thick skin is waxy and waterproof, restricting water loss only through the stomates (gas exchanging pores).

The plant is supported by an internal skeleton, a circle of woody ribs extending the length of the cactus and into its arms.

Saguaro arms always grow upward; frost may damage the tissue such that the weight of the arm causes it to droop downward.  If the arm survives, the growing tip will turn upward again. 

Beautiful Blooms

Flowers are formed on the growing tips, and following pollination, fruits will form, each containing approximately 2000 seeds.  Dispersal, rainfall, and other factors result in only about one mature plant per 40 million seeds. 

The waxy, white, 3 inch wide blossom is Arizona's State Flower.  The bloom opens at night and is pollinated by both the Mexican long-tongued and the lesser long-nosed bat.  Flowers stay open until the next afternoon, attracting birds and insects to consume nectar and pollen while also providing pollination services.

Saguaro blooms can be found any month of the year, but most emerge from May to June. White wing doves are one of the major avian species contributing to Saguaro pollination. Other birds and insects, including a variety of bees, also pollinate these large flowers.

Many bats feed on nectar and fruit. Two species, the long-nosed bat and the Mexican long-tongued bat, migrate through and live in Saguaro habitat. The Saguaro flowers supply their complete diet during migration. The bats feeding activity effectively pollinates the flowers.

Once ripe, usually in July, the red fruits split open attracting birds, insects, and other desert dwellers to consume and disperse the seeds, as well as enjoy the sweet fruit's flesh. Each cactus produces thousands of tiny black seeds that will germinate about five days after a rain.

A 'nurse plant', such as a palo verde, is a critical factor in the successful development of mature Saguaros.  So a seed needs to find its way under a tree or shrub for protection from herbivores and from desiccation.  As the plant matures, its spines deter herbivores and provide some shade for its skin.

A number of birds utilize Saguaros for roosting and nesting. Hawks, such as red-tail hawks, and doves construct thick nests on the arms, while Gila woodpeckers, cactus wrens and Gilded flickers excavate holes for nesting sites into the Saguaro's stem.

In defense the cactus quickly produces a thick material to heal and dry the wound.  This 'callus' structure usually takes the shape of a 'boot'.  When the plant dies the woody ribs that supported it in life and any 'boots' soon become visible as the softer plant tissue dries and crumbles away.

Saguaros, being only found in the Sonoran desert, and most abundant in the Arizona Uplands subdivision on south-facing slopes are sensitive to extremes in cold and dry weather.  This is one of the most protected plants in Arizona, be sure to check with all appropriate authorities before planting, moving or cutting down any Saguaro. 

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