Defoliation is the loss or removal of leaves from trees and plants.

In nature, defoliation happens when insects eat leaves, when diseases attack plants, or during autumn when deciduous trees naturally shed their foliage. A caterpillar infestation can cause severe defoliation of an oak forest, leaving branches bare in midsummer. Drought or extreme weather can also trigger defoliation as stressed plants drop their leaves to survive.

Humans sometimes cause deliberate defoliation. Farmers might defoliate cotton plants before harvest to make picking easier. Gardeners remove some leaves from tomato plants to help fruit ripen.

The opposite process, when plants grow new leaves, is called foliation or leafing out. While temporary defoliation rarely kills healthy trees (they usually grow new leaves the next season), repeated or severe defoliation can weaken or destroy plants by preventing them from making food through photosynthesis. Scientists study defoliation patterns to understand forest health, climate change effects, and insect population cycles.