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Ring Around the Flower Pot

The mystery of the African violets’ burned leaves

At a recent garden club lecture, a member of the club showed me a large African violet growing in a five-inch clay pot. The top edge of the pot was heavily encrusted with a white powder, several large bottom leaves were wilting and the stems appeared scorched. What was causing the white powder to accumulate on the rim of the pot, she wondered. Was whatever it was killing the leaves?

She said she grew the plant on a pan of gravel, which she kept moist at all times. She’d been growing the plant for 18 months to two years. That was all I needed to make my diagnosis.

When you constantly irrigate a potted plant from the bottom, capillary movement of water moves fertilizers toward the surface of the rooting medium. As the water evaporates from the surface, it leaves behind fertilizer salts. Because she was growing the plants in a porous clay pot, the water containing the fertilizer salts migrated to the top edge of the pot. When the water evaporated, it left behind a residue of fertilizer salts.

She was fertilizing the plant with fish-emulsion, but the nutrients released during the organic’s decomposition combined to form salts, easily transported by water upward to the surface for evaporation.

As more and more water soaked up to the soil’s surface and evaporated, more salts rose to the top. The accumulation of salts on the edge of the pot was sufficiently strong to burn the tissues on the stem of the leaves, causing the leaves to wilt and eventually die.

Since the plant had been growing in the pot for nearly two years, I suggested that she repot using a clean pot. If she continues to irrigate the potted plant from the bottom, I suggested that she irrigate the plant from the top at least twice monthly. Water poured in from the top forces the salts back down toward the bottom of the root ball.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.

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