Q: This year I tried to grow some gladioli in a spot where I had lantana last year. The results were tortured looking. After researching on the Web I find that lantana is allelopathic but I could not find how long this effect stays in the soil. Could you tell me please?
A: Dr. Mark Czarnota, University of Georgia weed specialist, has studied the phenomena of plants that chemically interfere with other plants nearby (allelopathy). He comments that although the allelopathy of black walnut trees is well-known, the exact mechanism is not well-understood. Sunflower farmers claim that sunflowers are allelopathic but some scientists suggest that development of fungal antibodies and the increase of soil disease organisms could also play a part.
Lantana is a noxious weed in Australia and India, due in part to its documented allelopathic effects. If lantana could survive our winter it might even become a kudzu-like pest in Georgia. Fortunately, allelopathic chemicals are not very persistent. Once a plant dies, the chemicals it once exuded gradually disappear. I have no research to back me up but my guess is that the allelopathy of lantana does not last more than a few months. Your gladiolas might have had a virus that caused them to be malformed.