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Supplement for Family Go Guide

The Amazing Expanding Saguaros
(a supplemental activity from the Family Go Guide!)

You have learned that saguaro roots draw in enormous amounts of water which is stored in the absorbent inner tissues of the cactus. But did you know you can actually measure the change in the circumference (that is the distance around) of a saguaro as it swells out after it "drinks" in summer rains?

Begin this activity in June before the summer rains fall. This will give you a measurement of saguaro circumference after months of drought. Once the summer rains begin, remeasure the saguaros throughout July as they swell with moisture. This exercise works best with saguaros less than 8 feet tall because the expansion area of taller saguaros is too high to reach.  Adult help is suggested in wrapping the string around the saguaro. Those spines do get in the way.


  1. Choose two or three saguaros in your neighborhood or along one of your favorite trails. In your notebook identify them with letters or numbers so you know which one you are measuring each time. You will have to remember which saguaro is A, B, C or 1, 2, 3.
  2. Very carefully wrap a string around the thickest part of the saguaro, taking care to place the string up against the saguaro skin. This is a bit tricky, but important. If spines hold your string away from the skin you will not get an accurate measurement. Be patient and keep from getting poked.
  3. Use the marker to make a mark on the string where the end touches it. Then make a tiny mark on the saguaro skin to indicate where your measurement point is.
  4. Remove the string and measure the length from the end of the string to your black mark. This tells you the circumference of the saguaro stem. Record the measurement in your notebook along with the date and the identity of the saguaro (A, B, C or 1, 2, 3).
  5. Repeat the measuring process with the other saguaros you have chosen.
  6. After the summer rains begin remeasure the saguaros. Place your string at the black mark so you measure at the same place each time. With each measurement take notes of circumference, date, and saguaro identity.
  7. Do this a few times throughout the summer. Make a chart of the measurements you have recorded. Have the saguaros expanded after the rains have fallen?

Your chart may look something like this.

Date Saguaro A Saguaro B Saguaro C
June 15      
July 10      
July 25      
August 10      

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