When a plant develops unusual bumps on its leaves, it indicates the plant is reacting to an external or internal stimulus. These growths are either a defense mechanism or a physiological malfunction where the plant’s cells grow abnormally. While alarming, these bumps are often a cosmetic issue that does not threaten the overall health of a mature plant. Understanding the precise cause is the first step toward effective management.
Primary Cause: The Formation of Plant Galls
The most frequent cause of green bumps is the formation of a plant gall, an abnormal swelling of plant tissue caused by a biological irritant. Galls are induced when an organism, often an insect or a mite, introduces chemical compounds that alter natural plant growth hormones. This manipulation forces the plant to create a protective structure that provides food and shelter for the invader’s developing offspring.
The primary culprits are tiny creatures such as gall wasps, gall midges, and eriophyid mites. Gall wasps (family Cynipidae) are known for forming woody, spherical growths on oak trees, like the oak apple gall. Gall midges, which are small flies, can cause blister galls on maple leaves. Eriophyid mites, which are microscopic arachnids, typically cause finger-like or pouch-like galls, such as the maple bladder gall, or a felt-like mass called erineum.
Galls form almost exclusively on new, actively growing plant tissues in the spring, as mature cells are not sensitive to the stimuli. The specific shape and location of the gall are characteristic of the organism and the host plant. In most cases, galls are merely unsightly and do not cause serious, long-term harm to the health of a large, established tree or shrub.
Secondary Causes: Environmental and Physiological Issues
Not all bumps are caused by a biological invader; some result from internal water stress known as edema. Edema is a physiological disorder occurring when a plant’s roots absorb water faster than the leaves can release it through transpiration. This imbalance leads to an excessive buildup of water pressure within the leaf cells, often on the underside.
The excess pressure causes cells to swell and rupture, forming water-soaked spots that become raised, warty swellings. These bumps eventually dry out, turning tan or brown and developing a corky texture. Edema is often triggered by environmental conditions like excessively moist soil, high humidity, cool air temperatures, and low light intensity, which suppress the plant’s ability to transpire water effectively.
This type of bump is distinct from galls because it relates to poor environmental management rather than a pest. Edema is most frequently observed in container-grown plants, greenhouses, and during prolonged cloudy weather. While the corky scarring is permanent, the plant typically recovers once the environmental conditions are corrected.
Identifying the Culprit and Assessing Plant Health
Differentiating between a gall and edema requires a close visual inspection. Galls are usually firm, distinct, and may contain the developing insect or mite inside. Older galls may show a small exit hole where the adult pest emerged. The presence of a spherical or nipple-like structure on a specific host plant, such as hackberry or oak, strongly suggests a gall-forming organism.
Bumps caused by edema typically appear as blister-like, translucent swellings on the leaf underside before turning into a tan, corky patch. Edema is not limited to a single point but may occur randomly or along leaf veins, and it will not contain an insect larva or an exit hole.
Determining the severity involves assessing the overall plant vigor. If only a few leaves are affected, the problem is largely cosmetic. However, widespread coverage or involvement of stems and main branches may affect the plant’s long-term health.
If the plant is small, or if significant defoliation or branch dieback is occurring, professional consultation may be beneficial. Arborists or local agricultural extension offices can provide definitive identification and help determine if the growth is a harmless gall, simple edema, or a sign of a more aggressive bacterial or fungal infection, such as crown gall.
Management and Prevention Strategies
Management strategies are highly dependent on the cause of the bumps, with the most practical approach often being prevention and cultural control. For galls, chemical control is generally ineffective because the gall structure protects the pest inside once it has formed. Focus instead on improving plant health and removing the source of the infestation.
If feasible, pruning out and destroying affected leaves or small twigs before the gall-makers emerge in late winter or early spring can reduce the population. For certain pests, a dormant application of horticultural oil just before bud break can smother overwintering mites and insects. Since most galls do not harm the plant, the best approach is often to tolerate the presence of the growths.
For edema, management focuses on adjusting the plant’s environment to promote effective water release. This includes reducing the frequency of watering, especially during periods of cool or cloudy weather, and ensuring the soil has excellent drainage. Increasing air circulation around the plant and, for indoor plants, providing adequate light and slightly warmer temperatures will help increase transpiration and prevent future outbreaks.