Question
i have 6 pin oak/water oak trees in my yard. all about 24" in diameter. i have notice hard ball type growth on some lower branches. they are green and look like they have brown needles in then. this growth completely encircles the branch which seems to die off. what is this growth and is it harmful to my trees? my neighbor has it in his tree also.
thank you,
Jim

Answer
These are called horned oak galls.  The horned and gouty oak galls may be seriously injurious to pin and red oak, respectively. Heavily galled pin oaks may be disfigured and weakened. Some have died from the debilitating effects of the gall maker, a small cynipid wasp.

The horned oak gall maker is a member of the wasp group of insects. However, it does not sting people. The tiny wasps develop in large ball-shaped twig swellings, and the majority emerge during early May. All the adults appear to be female. They lay eggs on the underside of newly formed oak leaves. These eggs develop into tiny galls along the veins of the oak leaf causing little damage to the leaves. The insects in the vein galls mature and emerge as wasps around mid-July. They are much smaller than their parents. Both males and females occur in this brood. These wasps then mate and the females soon lay eggs in oak twigs. It is not known how soon these eggs hatch or when the twig galls being to develop. While it is possible that the gall begins to form the same year the eggs are laid, it is more likely that they will not develop until the following year. Larvae may be found in twig galls but do not complete their development and emerge as adults until two years from the time the egg was laid.

No insecticidal control chemicals can be recommended at this time. Where possible, cut out the galls before the horns form and destroy them.
Here is a web link with pictures. http://www.tulsamastergardeners.org/plant_pathology/webs/hornedoak.html