There isn't one specific word for when trees grow their leaves back. However, depending on the context, you could use a few different terms:

* Leafing out: This is the most common term and describes the process of trees developing new leaves in the spring.

* Bud break: This refers to the point when buds on the tree begin to open and release new leaves.

* Sprouting: This is a more general term that can refer to any plant growing new shoots or leaves.

* Reafforestation: This term is used when an area of trees that has been cleared is replanted.

So, the most accurate term depends on what you want to emphasize. If you are talking about the process of trees growing new leaves after a period of dormancy, "leafing out" is the most common and appropriate term.