Geranium Oedema — What Are Those Bumps on My Pelargonium Leaves?

If you have noticed small, raised, corky or water-soaked bumps appearing on the underside — and sometimes the upper surface — of your pelargonium leaves, you are looking at oedema (sometimes spelled edema). It is one of the most commonly misdiagnosed conditions in geraniums because it can look alarmingly like a disease or pest attack. In fact, it is a physiological disorder, not an infection, and it cannot spread from plant to plant. Understanding what causes it is the key to preventing it.

What Causes Oedema?

Oedema occurs when a plant absorbs water faster through its roots than it can release through its leaves. This happens most commonly when the compost is wet and warm while the air around the leaves is cool, humid, and still. Under these conditions the leaf cells fill with water to the point of bursting, forming small, blister-like swellings. As the ruptured cells dry out they become corky, brown, and rough to the touch. It is most common in overwintered pelargoniums grown in heated glasshouses or on indoor windowsills during winter and early spring.

Is My Plant Damaged?

Existing oedema marks are permanent — once the leaf cells have burst and corked, that tissue will not recover. However, the damage is purely cosmetic and does not reduce the plant's ability to grow or flower. Affected leaves can be removed if their appearance bothers you, but there is no need to do so from a health perspective. Removing the underlying cause is far more important than removing the marked leaves. New growth produced under correct conditions will be completely free of oedema.

How to Reduce and Prevent Oedema

The principal fix is to bring watering and leaf transpiration back into balance. Water less frequently — allow the compost to approach dryness before watering again, particularly during cooler, darker winter months when the plant's water demand is low. Improve ventilation around the plant so that water vapour can escape from the leaves more easily. Move plants to a brighter, warmer position where they can transpire more actively. In a greenhouse, increase ventilation and reduce humidity by opening vents during the day even in winter.

Compost and Pot Considerations

Heavy, moisture-retentive compost is a contributing factor. If you are growing pelargoniums in a dense, peat-heavy mix, consider repotting into a more open, gritty compost with 20–30% perlite added. Terracotta pots are beneficial because their porous walls allow moisture to evaporate from the sides, reducing the risk of waterlogged compost. Ensure drainage holes are clear and never stand pots in saucers of water for extended periods.

Which Plants Are Most Susceptible?

Ivy-leaved pelargoniums and certain zonal varieties are more prone to oedema than others. Scented-leaf pelargoniums tend to be more tolerant. Hardy geraniums grown in the ground rarely show oedema because outdoor conditions naturally provide the airflow and soil drainage that prevent the disorder. If a particular variety repeatedly develops oedema in your conditions, try growing it in a brighter, drier spot or switch to a less susceptible cultivar.