What are those bumps?

Moist conditions after a dry spell can cause oedema. Photo/Illustration: provided by M.F. Heimann (reprinted from Compendium of Flowering Potted Plants, 1995, American Phytopathological Society, St. Paul, Minn.)

Oedema is mostly seen in plants in greenhouses and other indoor environments in late winter, but it can occur in outdoor plants as well. It’s likely to develop during cool, cloudy, muggy weather and when moist soil conditions follow a dry spell. The blisters form when the plant’s cells swell and burst from taking on water more rapidly than they can release it. The process of releasing water as vapor, called transpiration, is inhibited in highly humid environments.

Oedema will not spread to other plant tissue, but the lesions will remain unsightly. Affected tissue will often dry and fall away, especially on thin-leaved plants, leaving a hole in its place. In most cases, the plants will develop healthy new leaves and flowers as more favorable conditions prevail. Removing affected leaves and petals can improve the plants’ overall appearance.

To prevent oedema, avoid extremes in watering practices and check the soil for moisture needs before you water. Increase air circulation, reduce humidity, and move the plant or prune nearby plants to increase the exposure to light. Using fans or dehumidifiers indoors will also help.