A plant gall is an abnormal, structured growth on a plant’s leaves, stems, roots, or flowers. These growths are not random but are controlled manipulations of the plant’s cells in response to an external organism. Think of them as a non-cancerous tumor, where the plant’s development is hijacked to create a unique structure. Galls come in a vast array of shapes, sizes, and colors, from simple bumps to complex formations, with each swelling being characteristic of the organism that caused it.
The Architects of Galls
Gall formation begins when an organism, such as an insect, mite, fungus, or bacterium, triggers a reaction in the host plant. The primary architects are insects and mites, which initiate gall growth by laying eggs or feeding on plant tissue. Their saliva, venom, or other chemical secretions contain substances that mimic the plant’s growth hormones. This redirects the plant’s normal developmental processes, compelling it to create a custom-built home for the organism’s offspring.
This structure serves as both a fortress and a pantry. Inside the gall, developing larvae are shielded from predators and environmental extremes. The gall’s inner layers are rich in nutrients, providing a constant food source for the young. The plant is tricked into expending its energy to build and maintain this specialized nursery. Each gall-making organism is highly specialized, targeting a specific plant species and part of that plant to create its signature gall.
The gall-inducer introduces chemicals that cause a localized increase in the size or number of plant cells. This controlled overgrowth forms the gall’s unique shape, whether it be a simple lump, a spiky ball, or a structure resembling an apple. Once the gall begins to form, its development is directed by the organism inside. This ensures the structure meets its specific needs for shelter and sustenance.
A Gallery of Common Galls
There are many types of galls, with some of the most common including:
- Oak apple gall: Caused by certain species of gall wasp, this large, spherical growth is often found on the leaves of oak trees. It starts green and spongy but matures into a dry, papery, tan-colored ball where a single wasp larva develops in a central chamber.
- Horned oak gall: This hard, woody, and roughly spherical gall grows on the twigs of pin and red oaks. What makes this gall distinctive are the small “horns” that protrude from its surface, each being a capsule from which an adult gall wasp will eventually emerge.
- Goldenrod bunch gall: Also known as a rosette gall, this doesn’t form a swelling on the stem but causes the plant’s terminal leaves to form a dense, bushy cluster. Created by a gall midge, this leafy rosette provides a protected space for the larva to live and feed.
- Maple bladder gall: Appearing as small, bead-like growths on the upper surfaces of silver and red maple leaves, these galls are caused by an eriophyid mite. They are initially green but turn bright red and eventually black as the season progresses.
Are Plant Galls Harmful?
For most gardeners, the appearance of galls on a plant is more of a curiosity than a cause for alarm. The vast majority of galls are a cosmetic issue and do not inflict significant damage on healthy, mature plants. The plant isolates the gall, treating it as a localized abnormality. The gall itself is a plant-made structure that prevents the instigator from moving beyond its self-contained home.
The energy a plant expends to create a gall is minor and does not detract from its overall vigor. An established tree can support hundreds of leaf galls with little noticeable impact on its growth or longevity. In most situations, the best course of action is observation. Chemical treatments are not necessary or effective, as the gall protects the organism inside.
There are rare instances where galls can be problematic. A heavy infestation on a young or stressed plant can reduce its vitality by diverting too many nutrients. Certain woody galls on twigs and stems, like the horned oak gall, can cause minor branch dieback if numerous. Even in these cases, the damage is limited. Pruning galls is not recommended, as it does little to prevent future occurrences and is usually unnecessary for the plant’s health.