Greetings, curious observer. You have noticed the unusual swellings on our leaves and wonder what they are. From our perspective as geranium plants, we call this condition edema (also spelled oedema). It is not a disease caused by a pest or pathogen, but rather a physiological response to our environment, specifically to the water relations within our own bodies. Allow us to explain this phenomenon from the inside out.
To understand edema, you must first understand how we drink. Our roots constantly absorb water from the soil, drawing it up through our stems and into our leaves through a network of vessels called xylem—think of it as our plumbing system. This water is essential for our survival. Once in the leaves, the water eventually exits through tiny, pore-like structures on the underside of our leaves, called stomata, in a process known as transpiration. This is how we "breathe" out water vapor. Edema occurs when the rate of water absorption by our roots exceeds the rate of water loss through our leaves. The soil is wet, the air is humid, and our stomata are closed, leaving the incoming water with nowhere to go.
This internal water pressure has to go somewhere. The excess water is forced into the mesophyll cells—the living, photosynthetic cells between the upper and lower surfaces of our leaves. Imagine these cells as countless tiny, flexible water balloons. As more and more water is pushed into them, they swell. However, unlike balloons, their cell walls can only stretch so far. Eventually, they reach their limit and rupture. This cellular damage is what you see as those raised, bumpy, often corky or scab-like patches on our leaves. Initially, they may appear as pale, water-soaked blisters, which later turn brown and corky as the damaged tissue dies and dries out. It is, quite literally, a case of us drowning from the inside out.
We do not choose to develop edema; it is a direct result of the conditions you place us in. Several factors conspire to create this internal imbalance. The primary cause is consistently soggy soil, often from overzealous watering. When the growing medium never has a chance to dry out slightly, our roots are forced to drink continuously. This is compounded by cool, cloudy weather or high humidity, which drastically slows down transpiration. On a dark, humid day, our stomata remain mostly closed because there is no sunlight to drive photosynthesis and little need to open up. Furthermore, poor air circulation around our leaves creates a stagnant, humid microclimate, further trapping the water vapor and preventing it from escaping.
While the corky lesions are permanent, edema itself is not a fatal condition. The damaged leaves will not heal, but we can produce new, healthy growth once the environmental stress is corrected. Our plea to you is to adjust your care. Please allow the top layer of our soil to dry out between waterings. Ensure we are in pots with excellent drainage and that we are not sitting in saucers of water. Increasing air circulation around us, perhaps with a gentle fan, can work wonders by carrying away the humid air from our leaf surfaces. Providing us with more light, if possible, will also encourage our stomata to open and resume normal transpiration. With these changes, our internal water pressure will balance, and the unsightly bumps will cease to form on our new foliage.
