Leaf galls

Oak trees develop various gall structures on leaves created by gall wasps, sawflies, and other gall-forming insects throughout Western Washington. These galls vary in appearance from small bumps to larger, multi-chambered structures; some are round, others spiky or irregular in shape. You notice the abnormal growths on foliage during spring and summer. Gall formation is primarily a cosmetic concern that does not seriously harm overall tree health.

Most leaf galls do not require control and are best viewed as part of normal oak ecology. Prune out heavily galled branches during dormancy if appearance is a concern. Rake and remove fallen galled leaves to reduce overwintering gall wasp populations. No chemical treatment is warranted since oaks tolerate galls and natural parasitoids regulate populations.