From our roots' perspective, water is life. It is the medium through which we transport essential nutrients from the soil up through our stems to our leaves. When the soil becomes too dry, this crucial transport system fails. The tiny root hairs, responsible for water uptake, begin to desiccate and die. Without a steady supply of water, the cells at the leaf margins—the farthest points from the roots—are the first to suffer. They lose turgor pressure, collapse, and die, resulting in the dry, brown, crispy edges you observe. This is our most desperate distress signal for a drink.
While you provide us with nutrients through fertilizer, too much of a good thing is toxic. Synthetic fertilizers are essentially mineral salts. When applied too frequently or in too high a concentration, these salts accumulate in the soil. This creates a hostile environment for our root system. The high salt concentration actually pulls water out of our roots through osmosis, reversing the natural flow. This is chemical burn. The resulting drought stress, despite damp soil, manifests as browning and crisping on our leaf margins and tips as the salts are drawn to these endpoints.
Our leaves are constantly breathing, a process called transpiration where water vapor escapes through microscopic pores called stomata. Many begonias, especially the Rex and other fancy-leafed types, are native to humid jungle floors. In the dry air of a modern home, particularly near heating vents or air conditioners, the rate of water loss from our leaves can exceed the rate at which our roots can draw it up. This causes the cells at the leaf edges, which are most vulnerable, to dry out and die. The crispy brown edge is a direct result of this atmospheric drought.
You may not see it, but the health of our leaves is a direct reflection of the health of our roots, hidden in the soil. If we are suffering from root rot due to chronically waterlogged, oxygen-deprived soil, our roots are dying and decaying. A compromised root system is utterly incapable of absorbing the water needed to supply the leaves, even if the potting mix is wet. Conversely, if we are severely pot-bound, with a dense, circling root mass, water may flow around the roots and out the drainage hole without being absorbed. In both cases, the leaves experience a water deficit, and the edges brown and crisp as a consequence.
Our beautiful, often delicate leaves are adapted to dappled sunlight under a forest canopy. When placed in direct, intense sunlight—especially through a hot window—the light energy overwhelms our photosynthetic systems. The intense heat and radiation literally cook the sensitive tissues, causing rapid water loss and cellular death. This sunscald appears as bleached, brown, and crispy patches, often starting on the edges and tips of the leaves that are most exposed to the light. It is a severe burn from which we cannot recover.
