A catkin is a slim, fuzzy, dangling cluster of tiny flowers that grows on certain trees like willows, birches, and oaks. The name comes from the cluster's resemblance to a cat's tail: soft, fuzzy, and hanging down.

Catkins usually appear in early spring before the tree's leaves emerge. If you've ever seen what looks like fuzzy little worms hanging from tree branches in March or April, you were probably looking at catkins. They might be yellowish, greenish, or purplish depending on the tree species.

These flower clusters play an important role in tree reproduction. Most catkins release clouds of pollen into the wind, which travels to other catkins and helps trees produce seeds. A single catkin can release millions of pollen grains. Many trees produce two types of catkins: some release pollen while others catch it to make seeds.

After releasing their pollen, catkins typically fall to the ground, where you might see them scattered on sidewalks and lawns like small, fuzzy caterpillars. People with pollen allergies often notice when catkin season arrives because those millions of pollen grains floating through the air can make them sneeze.