Help, my Oak tree has weird bumps and growths!

While sitting in our hot tub this morning I noticed that the black oaks around us have several walnut-sized green masses on their stems. 

During my 30 year horticulture career, I had many calls and questions about abnormal growths on oak and other trees. These abnormal growths, called galls, can be very disturbing to the people whose plants are affected. Fortunately, most galls affect only the appearance of the trees and are not detrimental to plant health.

Galls are a plant's response to insects, mites, bacteria, fungi, or nematodes. Galls are actually created by the plants themselves in response to some stimulus from the invading organism. The plant may create the gall in response to the invaders feeding or the critter might "trick" the plant into forming a home for it to live in.

Each gall-inducing insect or mite chooses a specific plant to associate with. Identification of the gall maker is based on the gall it produces. Insects on trees produce over 2,000 types of galls. The majority of these insect galls are produced by wasps, and over 700 of these wasp-produced galls occur on oak trees.

Wool Sower Galls on burr oak

Over the years, I've seen several types of galls on oaks in my yard. One year the burr oak tree in my yard had these very interesting wool sower galls, which are tan, spongy, and pong-pong ball sized. It was fun to pull the gall apart to look for the ant-sized adult wasp inside. Don't worry, they don't sting.

Oak apple gall is also caused by several species of gall wasps. It consists of large, dry galls attached to the midrib or petiole of a leaf. As the galls mature they become papery. The single larva in each "apple" is inside a small and very hard seedlike cell. No, they are not edible!

Leaf gall on oak

Although leaf galls may be unsightly, most do little or no damage to the host plant. However, the horned and gouty oak stem galls can be debilitating, even killing younger trees. It too is caused by a wasp, but this one causes woody masses on stems that reach two inches or more. Over it's one to three year life cycle, this wasp's gall can choke off and kill the ends branches and twigs. The pin oak is a primary host for the horned oak gall.

There is no effective chemical control for galls. The gouty and horned oak galls can be removed by pruning, and the debris should be destroyed. The other galls mentioned do not remain on the tree from year to year so don't have to be pruned away. They may or may not return the following year.

Enjoy your oak trees!

I really like this factsheet on Oak Problems, including galls, from the University of Illinois Plant Clinic (found under Publications: Reports on Plant Disease).