"Cal." stands for caliper; caliper measures the thickness at the base of the trunk of a tree. When a tree is said to have a 1/4" cal., that means the base of the trunk is 1/4" thick.
Enter your 5-digit ZIP code to find your Plant Hardiness Zone.
What is a Hardiness Zone?
A hardiness zone is a geographic area defined by average annual minimum winter temperatures, helping gardeners determine which plants can survive in their region. Developed by the USDA, the 13 numerical zones indicate cold hardiness—for example, a plant "hardy to zone 10" tolerates minimum temperatures of 30 to 40 °F (−1.1 to 4.4 °C).