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The effects of leaches from dry leaves of 71 ground cover plant species on lettuce were tested at the first screening. The inhibitory effects on radicle and hypocotyl elongations of lettuce varied with the different species of cover plants that were used. Eight species of Oxalis showed strong inhibitions (4–27% of untreated control on radicle elongation). Inhibitory activities of seven species of cover plants on three weed species, live amaranth (Amaranthus lividus), southern crabgrass (Digitaria ciliaris) and common lambsquarters (Chenopodium album), were tested at the second screening. Moss pink (Phlox subulata), trefoil (Oxalis brasiliensis), red spiderlily (Lycoris radiata), creeping thyme (Thymus serpyllum), European pennyroyal (Mentha pulegium), roman chamomile (Chamaemelum nobile) and star‐of‐Bethlehem (Ornithogalum umbellatum) were selected as donor plants because of their high inhibitory effects on lettuce growth and their usefulness as ornamental ground cover plants. Effects of leaches from dry leaves and exudates from the roots of these species were assayed on agar. Radicle elongations of all tested weed species were inhibited by leaches from trefoil and red spiderlily (8–31% and 14–24% of untreated control, respectively) and exudates from moss pink, trefoil and creeping thyme (11–43%, 31–74% and 22–67% of untreated control, respectively).

Laboratory and field experiments were conducted to evaluate the usefulness of Oxalis spp. as allelopathic ground-cover plants for weed management. Some Oxalis spp. have previously been reported to possess strong allelopathic activities but few studies have been conducted on their activities in fields. This study aimed to investigate allelopathic activities and the possibility of weed suppression in five species of common Oxalis: shamrock oxalis (Oxalis articulata Savigny), Bowie's woodsorrel (Oxalis bowiei Lindl.), trefoil (Oxalis brasiliensis Lodd. ex Knowl. et West.), lucky clover (Oxalis deppei Lodd. ex Sweet) and Oxalis hirta L. The effects of the leachates from dry leaves and the exudates from living roots of these plant species were tested in laboratory experiments. The leachates from O. articulata, O. bowiei, O. deppei and O. hirta and the exudates from O. deppei caused >84% inhibition of the radicle elongation of lettuce seedlings, but no effect was observed on the seed germination of lettuce. In the field experiment, O. deppei significantly reduced the weed population in July. A significant relationship was observed between the weed population and the percentage ground coverage of Oxalis spp. In contrast to the weed population, a significant relationship was observed between the weed above-ground biomass and the allelopathic activity of exudates from Oxalis spp.

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