The giant saguaro (Carnegia gigantea) is one of the most conspicuous plants of the desert Southwest. It is a familiar sight in many a Western movie and almost every cartoon depicting a desert theme, though its natural distribution is almost completely confined to southern Arizona and western Sonora, Mexico. This is one of the few species in the Cactaceae that have been extensively studied, and its basic ecology is now well understood.

Much of the fundamental research was published in a trilogy of articles by Steenbergh and Lowe that describe, among many other things, growth rates, maturity, and longevity. Growth rate is controlled mainly by the water storage capacity (i.e., size) of the plant and the amount of water available in the environment. The latter factor is governed by rainfall and soil texture. Saguaros, regardless of age, mature (flower for the first time) when they are about eight feet (2.4 m) tall.

Because a tiny cactus can store little water, a seedling can continue growing for a very limited time after the soil dries out. The larger it is, the more water it contains and thus the longer into the dry periods it can continue to grow. So an average saguaro in the wild takes almost a decade to attain its first inch (2.5 cm) in height, and after 30 years it is only two feet (0.6 m) tall. But the next two feet take only another 10 years, and plants six feet (1.8 m) tall are growing a relatively impressive four inches (10 cm) a year.

In the driest of Steenbergh and Lowe's study sites (Organpipe Cactus National Monument, average annual rainfall 9 inches or 228 mm), saguaros take an average of 75 years to attain maturity from seed. In the wettest site (Rincon Mountains east of Tucson, Arizona, 16 inches/400 mm rainfall), they reach maturity in only 40 years. In the Tucson Mountains on the west side of Tucson (12 inches/300 mm) maturity is reached in 55 years.

Saguaros respond to supplemental water and fertilizer with even faster growth than in nature. Nurseries can produce six-inch-tall (15.2 cm) seedlings under greenhouse conditions in three years. I obtained a batch of such seedlings in 1979 and planted them in "sandy soil on the west side of Tucson, where they should take about 55 years to flower on their own. They were irrigated twice weekly from April through October and fertilized about once a month. The first plant flowered 17 years after planting, or 20 years from seed.

The saguaro is neither the fastest- nor slowest- growing of the giant columnar cacti. Cardon (Pachycereus pringlei) and hecho (P. pecten-aboriginum) can grow nearly two feet (0.6 m) a year but cannot mature in Tucson because they are extremely frost-tender. Trichocereus terscheckii may be the champion of the hardy species. After growing fairly slowly to three feet (0.9 m) tall, plants are capable of adding 18 inches (46 cm) a year. After 17 years in the ground from six-inch seedlings, my largest is 11 feet (3.4 m) tall and has several branches from one to six feet long. It flowered when it was seven feet (2.1 m) tall after 14 years in the ground. At the slow end of the scale are two Trichocereus tarijensis var. poco that are only five and seven feet (1.5 and 2.1 m) tall at 17 years from six-inch seedlings; they have not yet flowered. Organpipe cacti (Stenocereus thurberi) and senita (Lophocereus schottii) are intermediate between these extremes in growth rate, though neither flowers until quite large.

[Our Saguaro reached 8 feet tall sometime last year, and now has flower buds - editor]