These little structures on the leaf are called cone galls. They are formed in response to an aphid, the aptly named Witchhazel Cone Gall Aphid (Hormaphis hamamelidis). The female aphids shelter in the galls during their reproduction.

This is common to witchhazels and the galls do not negatively affect the overall health of the host plant. There is nothing you need to do. Just be aware that they are a usual occurrence for this plant.

Aphids have a variety of natural predators in the environment -- other insects and then birds that eat the insects. Insects are a very important food for nesting birds in particular.

Here is more information on leaf galls:

https://extension.umd.edu/resource/shade-tree-galls

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/other/galls/galls-on-trees.aspx

Christa

You're welcome.