Plant galls: which are harmful, which aren't

Galls are abnormal growths of plant tissue triggered by insects, mites, nematodes, fungi, or bacteria. When a plant's growth-regulating hormones are hijacked by a foreign organism, cells divide and enlarge in patterns that produce distinctive structures. The result can look alarming -- balls.

Galls are abnormal growths of plant tissue triggered by insects, mites, nematodes, fungi, or bacteria. When a plant's growth-regulating hormones are hijacked by a foreign organism, cells divide and enlarge in patterns that produce distinctive structures. The result can look alarming — balls hanging from oak twigs, red spindles on maple leaves, spongy masses on stems — but in most cases, galls cause minimal harm to established trees and shrubs.

The most important thing most gardeners don't know: the majority of common galls on trees require no treatment.

How galls form

Per Penn State Extension, gall formation is initiated when a small insect, mite, or pathogen stimulates plant cells to divide and differentiate abnormally. The gall provides shelter and food for the insect larva or mite colony inside. The gall's structure — its shape, texture, and location — is largely genetically determined by the insect or pathogen that caused it, not by the plant. This is why the same gall form appears repeatedly on the same host plant species.