From a plant's physiological standpoint, edema is a disorder, not a disease caused by a pathogen like a fungus or bacterium. It is a stress response resulting from an imbalance in water relations within the geranium's tissues. Essentially, the roots are absorbing water from the soil at a rate faster than the leaves can transpire (release) it into the surrounding air. This internal water pressure, known as turgor pressure, builds up to a critical point within the mesophyll cells (the inner tissue of the leaf). The excessive pressure causes these cells to expand and rupture. The subsequent healing process forms corky, scar-like tissue on the underside of the leaves, which appears as the raised, bumpy patches you observe. This is the plant's attempt to seal the damaged areas and prevent further water loss or pathogen entry.
The root of the edema condition lies in the environmental factors that disrupt the plant's natural transpiration process. The two main culprits are excessive soil moisture and high atmospheric humidity. When the soil is kept consistently wet or poorly drained, the roots are encouraged to uptake water continuously. Concurrently, if the air around the plant is stagnant and humid—common in overcrowded greenhouses or indoor settings with poor air circulation—the leaves cannot effectively release water vapor. Cool, cloudy weather or nighttime conditions exacerbate the issue, as transpiration rates naturally drop significantly while root activity may continue, creating a perfect storm for internal water pressure buildup.
Treating edema involves adjusting the plant's environment to restore the balance between water uptake and loss. First, reassess your watering practices. Allow the top layer of soil to dry out thoroughly before watering again. Geraniums prefer a cycle of thorough watering followed by a period of drought. Ensure the pot has excellent drainage and never let the plant sit in a saucer of standing water. Second, immediately improve air circulation around the plant. This can be achieved by spacing plants further apart, using a small fan on a low setting to gently move air, or simply moving the plant to a breezier location. Enhanced airflow promotes transpiration, helping the plant process the water its roots absorb.
Prevention is the most effective long-term strategy. Monitor weather and seasonal changes closely; water geraniums much less frequently during prolonged periods of cool, cloudy weather. In indoor or greenhouse settings, maintain lower humidity levels through ventilation and avoid overcrowding plants. Choose a well-draining, aerated potting mix specifically formulated for container plants, as heavy, moisture-retentive soils will perpetuate the problem. While the existing bumpy leaves will not revert to their smooth state, new growth will emerge healthy if the environmental conditions are corrected. You can prune away the most severely affected leaves to improve the plant's overall appearance and redirect its energy.
