Why did the gardener love math class?

This joke plants a smile with some clever wordplay, specifically a pun. Puns are a type of wordplay that use a word or phrase that has multiple meanings or sounds similar to another word, creating a humorous double entendre. In this case, the humor blossoms from the word "dividing."

In gardening, "dividing plants" is a real and important technique. It involves digging up a plant and separating its clump into two or more smaller sections, each capable of growing into a new individual plant. This is done to propagate new plants, improve plant health by preventing overcrowding, and rejuvenate older, less vigorous plants. Gardeners literally divide plants to make more of them, much like how one "divides" numbers in math class. The joke playfully connects these two very different meanings of the same word.