The Saguaro Cactus
Saguaro cacti, Carnegiea gigantea, only grow in the Sonoran Desert. However, they do not grow in all parts of the Sonoran Desert.
The range of the saguaro is limited by freezing temperatures in winter. Saguaros are also limited by elevation. They are generally found growing from
sea level to approximately 4,000 feet in elevation. Saguaros growing higher than 4,000 feet are usually found on south facing slopes where freezing temperatures are less likely to occur or are shorter in duration.
Saguaros are a very slow growing cactus. In Saguaro National Park, studies indicate that a saguaro grows between 1 and 1.5 inches in the first
eight years of its life. These tiny, young saguaros are very hard to find as they grow under the protection of a 'nurse tree', most often a palo verde, ironwood or mesquite tree. As the saguaro continues to grow, its much older nurse tree may die. Some scientists believe that competition
from the saguaro may lead to the death of the nurse tree by taking water and nutrients from the soil in the immediate area.
As a saguaro begins to age, growth rates vary depending on climate, precipitation and location. We do know that the period of greatest growth in a saguaro cactus is from unbranched to branched adult.
At Saguaro National Park, branches normally begin to appear when a saguaro reaches 50 to 70 years of age. In areas of lower precipitation, it may take up to 100 years before arms appear. When a saguaro reaches 35 years of age it begins to produce flowers. Though normally found at the terminal end of the main trunk and arms, flowers may also occur down the sides of the plant. Though, additional arms allow the saguaro to produce extra flowers. Flowers will continue to be produced throughout a saguaro’s lifetime.
An adult saguaro is generally considered to be about 125 years of age. It may weigh 6 tons or more and be as tall as 50 feet. The average life span of a saguaro is probably 150 - 175 years of age. However, biologists believe that some plants may live over 200 years.
From Flower buds to a cactus Saguaro buds begin to appear in mid-April around the top of the main trunk and arms.
The buds open into large white flowers from late April through early June. Each flower opens in the middle of the night, and closes the following day. Saguaro flowers are pollinated by nectar feeding bats, birds and insects.
After a saguaro flower has been pollinated, it begins its transformation into a fruit. Complete ripening takes approximately 30 to 40 days. Dried flowers often remain attached to the ripening fruit.
Near the end of the ripening period, the bright red fruits begin to split open. Inside are approximately 2000 tiny black seeds. Both the flesh and seeds are consumed by a variety of desert animals - when the fruit and seeds are eaten by a coyote or cactus wren, the seeds pass through their digestive unharmed and are distributed throughout the desert. However, if the seeds are eaten by a dove or quail, they will be completely consumed in the digestive system.
It is estimated that a saguaro can produce some 40 million seeds during its lifetime. However, few will survive to become a seedling. Fewer still will become an adult. The low survival rate of seedlings is due to drought, prolonged freezing and animals eating them.
If a saguaro seed falls in the right place, and at the right time, it may germinate and begin its long life. Only a few of the seeds a saguaro produces in its lifetime will germinate and grow to maturity.
For the best chance of survival, saguaro seedlings need the protective cover of a "nurse plant". The shade and additional moisture under the nurse plant is crucial to the survival of young saguaros.
In Saguaro National Park, saguaros begin to grow arms at about 65 to 75 years of age.