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.
Here 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. 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.