Rods are available in the following styles: 3/8" Round, 3/8" Hex, and 7/16" Hex. All the rods are made from high strength alloy steel. Using the 3/8" Round rod as a basis for comparison, the 3/8" Hex rod is about 20% stiffer than the 3/8" Round rod. The 7/16" Hex rod is approximately twice as stiff as the 3/8" Round rod.
As the size of the tip increases (from 3/8" to 7/16"), additional effort will be required to push the tool into the ground.