Red Oak with ‘gooseberries’


I have a volunteer Red oak that is now about 8+ feet tall. The tree sits on the east side of the house under the canopy of a 35 foot red oak, a 20 foot bass wood and another unknown tree. Near by there is also cedar and an ash. The location is breezy, elevated, well draining and the sapling gets dappled sunlight. It has been a healthy tree.
This year there are ‘gooseberry’ type growths in colour and approx. 1/2″ x 3/4″ oblong shape.

What are they and what to do?


Thank you for contacting the Toronto Master Gardeners.  It sounds like you have many lovely trees in a good location.  The “gooseberry” type growth shown in your photo may be an oak gall.  Oak galls are deformities on leaves or branches caused by gall-forming insects – often wasps.  They are formed by hormones secreted by insect larvae, causing plant tissue to grow around and protect them.  Once matured, the insect will eat its way out of the gall.  In general, galls can cause some discolouration or disfigurement to leaves and branches but generally don’t cause significant damage.  I have included a couple of references for further information.