They are oak galls. They are houses for certain developing insects, usually wasps. The insect injects a hormone into the plant that causes the gall to form around the larval insect. It takes 1 to 2 years for most gall insects to mature, and they usually "hatch" in the spring. Some galls can be up to 2 inches, and some infestations can weigh down branches. But, most of the time galls are hardly noticed. Galls are difficult to control because spraying must be done at the time adult insects are laying their eggs.

on deciduous oak... obviously ... they do not last 2 years ... as they are leaf issues ...

as far as i understand .... they are completely cosmetic and not worth being concerned about or treating ... as with a vast majority of leaf issues ...

but ... i do not live in evergreen oak territory.. so i cant address that ...

ken

I never pay attention to the leaf galls on my live oaks. Live oaks push off most of their leaves as their new flush are coming in so it just seems that they are evergreen because the leaves are always on but the leaves do come off routinely. I found one round of galls somewhat ornamental in a away.

They don’t look quite like the typical oak galls I see around here. Are they light weight or dense and solid in feel? If they are light, they are likely galls.

I cut one open. Here are a few more pictures. The outside is a thin hard shell and the inside is nut-like. They basically look like they are made of acorn material. They weren't very hard to cut.

We did have galls in spring that were the size of a cherry, light weight, pale green, with a fuzzy thing in the middle. These are not similar.

Thanks! Looks most like the photo of "oak leaf gall caused by midges." I'm satisfied with that answer unless someone has more detail to share.

I find this interesting, but also can't look at too many gall photos or it makes me kind of itchy!