Backyard Gardener: Is there plant warfare going on?
Mexican sunflower (Tithonia rotundifolia) planted to provide color, attract pollinators (lower left) and add interest to the vegetable garden growing between two low tunnels (upper left). It is 11 feet tall. Unaffected lettuce is growing more than five feet away from Mexican sunflower (upper right) and lettuce potentially affected by allelopathy is growing within five feet of Mexican sunflower (lower right). All lettuce shown was planted on Aug. 21, 2020 and photographed on Oct. 12, 2020. (Jeff Schalau, University of Arizona/Courtesy)