Habitat
The icon of the Sonoran Desert. If the hike is in the Sonoran desert, it will have Saguaros, even if we haven't associated them on the dynamic map.
Flowers
Large creamy white. 4" - 6" wide. clusters at the end of the columns/arms. Not all of the flowers on a single Saguaro bloom at the same time. Instead, over a period of a month or more, only a few of the up to 200 flowers open each night, secreting nectar into their tubes, and awaiting pollination. These flowers close about noon the following day, never to open again. If fertilization has occurred, fruit will begin to form immediately. The saguaro does not bloom until it is around fifty years old The saguaro produces a fruit that can hold up to two thousand seeds. Curved arms on saguaro cacti are a result of freeze damage. When a particularly sensitive cactus freezes, the arms sometimes droop downwards and then as the plant recovers the arms grow upwards again, creating a bend.
Description
Saguaros are slow growing, taking up to 75 years to develop a side shoot. Some specimens may live for more than 200 years; the champion saguaro grows in Maricopa County, Arizona and is 13.8 m tall and has a girth of 3.1 m - it was injured as a result of the Cave Creek Complex fire
Comments
Saguaro is the Tohono Oodham name. Crested Saguaros are rare.

From Arizona-Sonora Desert Museum website:
A ten yr old plant might only be 1.5 inches tall. Three-foot tall saguaro is around 40 years old. 25-foot tall saguaro is around 100 years old. The largest plants, with more than 5 arms, are estimated to be 200 years old.
An average old Saguaro would have 5 arms and be about 30 feet tall.

When rain is plentiful and the saguaro is fully hydrated it can weigh between 3200-4800 pounds. A fully hydrated saguaro contains 90% water and a large plant weighs about 80 pounds per foot.

Most of the saguaros roots are only 4-6 inches deep and radiate out as far from the plant as it is tall. There is one deep root, or tap root that extends down into the ground more than 2 feet.

Although the the Sonoran Desert experiences both winter and summer rains, it is thought that the Saguaro obtains most of its moisture during the summer rainy season. When water is absorbed, the outer pulp of the Saguaro can expand like an accordion, increasing the diameter of the stem and, in this way, can increase its weight by up to a ton.

The drooping arms seen on many old saguaros is a result of wilting after frost damage. The growing tips will turn upwards in time. There is a myth that arms are produced so as to balance the plants, but research shows arm-sprouting to be random. Many saguaros can be found with several arms all on the same side of the main stem.

The lower trunks of old saguaros lose their spines and develop dark, corky bark.

The chief agent of mortality of mature saguaros in the Arizona Upland is freezing temperatures. The saguaro is a tropical cactus with limited frost tolerance, and it reaches the northern, coldest limit of its range in Arizona Upland.

hard freezes damage or kill the smallest and largest saguaros, as well as other tropical elements of the flora. Most often these freezes kill a portion of the outer layer of saguaro tissue, which forms a brown scab tissue.

A frost-damaged saguaro may survive for another decade or even longer, but eventually it weakens until it can no longer resist infection. Bacterial rot caused by Erwinia cacticida turns the flesh of weakened plants into an odoriferous black liquid.

After the saguaro dies its woody ribs can be used to build roofs, fences, and parts of furniture. The holes that birds nested in or "saguaro boots" can be found among the dead saguaros. Native Americans used these as water containers long before the canteen was available.

http://tucsoncitizen.com/wryheat/2011/06/23/saguaro-cactus-icon-of-the-sonoran-desert/:
Many large saguaros contain holes excavated by Gila Woodpeckers and Gilded Flickers. The birds go in and down, removing the fleshy part of the cactus. The cactus produces scar tissue, calus, which quickly becomes very hard and impervious to bacterial infection. This material, often in the rough shape of a boot, survives after the cactus dies and rots away. Woodpeckers generally excavate a new hole each year, leaving the hole for other cavity-nesting birds.

a link that talks about the saguaro with great pictures: http://www.kuriositas.com/2012/01/saguaro-cactus-and-its-greedy-guests.html

Other interesting notes: a saguaro often weighs less than an aspirin at age five and it may take about 10 years to get just an inch and a half tall, about the size of your thumb!