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Iconic background star of many an old Western, Carnegiea gigantea or the saguaro is probably the best-known cactus of all.

The saguaro is endemic to the Sonoran Desert, growing as far north as southern Arizona at elevations of between sea level to approximately 4,000 feet. It will not take freezing temperatures so at the northern end of its range it may be found mainly on south-facing slopes.

Carnegiea gigantea is undoubtedly the largest cactus in the US. At a height of around 40-45 feet (12-14 metres) when mature, it is not ideally suited to the average greenhouse. It is certainly slow-growing though. It will start its life under the protection of a nurse tree, and it can take 8-10 years for a seeding to reach 1–1.5inches (2-3cms) in height. As the saguaro grows the nurse tree will often die, possibly through competition with the saguaro for nutrients and water.

 1120 Fig 1 Saguaro forest

Fig. 1 Saguaro forest in Sonora. Photo: Tomas Castelazo
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It can begin to flower at around 35 years old, and may start to produce arms from around 50 years old, depending on the growing conditions. The average age of a mature saguaro is probably 150-175 years although it is believed that some plants are over 200 years old. A mature plant could well weigh 6 tons or more.

The root system is also impressive. The saguaro produces a single taproot that will penetrate to a depth of around five feet (1.5metres). In addition it will send out a massive array of roots within three inches or so of the surface. Thus the saguaro can access water from deep underground and make use of any rain which falls around it.

 1120 Fig 2 Flowers

Fig. 2 Saguaro flowers near Scottsdale, Arizona. Photo: Dan Hiris
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The saguaro’s large white flowers open at night and remain open the following day to about mid-afternoon. They are pollinated mainly by bats although they are also visited by birds and insects during the day time. One saguaro can produce tens of thousands of seeds in a year, and perhaps as many as 40million in its lifetime, but very few of these will grow to adulthood.

Sheila Cude
 

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