Question: I have brown scarring on the backs of my lower leaves. Why?
Answer: This is a question we received by phone. We didn’t immediately have an answer until he happened to mention that he grew in clay pots, and also wondered why the bottom row of leaves was bent over the pot rims. The culprits, of course, were the clay pots but, since we hadn’t grown violets in these in nearly 30 years, it didn’t quickly come to mind. Besides their weight, expense, and difficulty to clean (and keep clean), scarring of the petioles and undersides of leaves can be a problem presented by clay pots. Being porous, the pot absorbs the water (and whatever is added to it). The water will evaporated and the pot will dry, but some of the fertilizer remains within the clay, most noticeably on the pot rim–like a ring on a bathtub. These accumulate over time and can burn, or “scar” the undersides of leaves and their petioles when they come in contact with the pot rim. This can be more of a problem for African violets, which produce leaves in a flat rosette, than for plants that grow more upright.
A solution for those who prefer to grow violets in clay pots is to create a barrier between the leaves and the pot rim. One simple solution is to cut thin strips of aluminum foil and fold them over the pot rim. Using a glue gun, you might also apply a thin bead of glue around the pot rim–or apply a bead of silicone of bath-tub sealant. Another is to cut thin nylon tubing (like the kind found in tropical fish stores), lengthwise, then secure them to the pot rim. Some exhibitors use the latter method even with plastic pots, since it helps protect the lower leaves of large showplants from the sharp rims on plastic pots (“rolled”, or rounded, rims are better for this reason).
That said, clay pots are attractive and can make plants look even more so. Just be aware of their shortcomings and adjust your culture accordingly. Because they are porous, soil will dry much more quickly than when potted in plastic. Water is evaporated only from the soil surface with plastic pots, but from soil and pot surface with clay pots. This means that plants with small root systems, like miniatures, will dry very quickly in clay pots and will need to be watered quite frequently. Also, very light, porous, soilless mixes that work so well with plastic pots may dry too quickly in clay pots. A heavier mix that retains more water may work better when using clay pots, unless one plants to keep the plant and soil constantly wet. Finally, if you want the advantages of plastic pots, but the appearance of clay, you might simply grow in plastic and slip plant and pot into a slightly larger clay pot!