Interaction effects of Allium spp. residues, concentrations and soil temperature on seed germination of four weedy plant species

Soil in laboratory microcosm experiments was amended with dried and milled crop residues of onion (Allium cepa L. cv. Mission) or garlic (A. sativa L. cv. California Early). The amendments, along with the additional factors of amendment concentration (0, 1 or 3% (w/w)) and soil temperature (23 or 39 °C), were evaluated with respect to germination of seeds of the weedy annual plants Echinochloa crus-galli (L.) Beauv. (barnyardgrass), Portulaca oleracea L. (common purslane), Sisymbrium irio L. (London rocket) and Solanum nigrum L. (black nightshade). Deleterious effects of seed exposure to 39 °C versus 23 °C were demonstrated in ECHCG, POROL, and SSYIR; and to increasing amendment concentration and the [concentration × temperature] interaction in all four plant species tested (P < 0.05). The effect of garlic versus onion amendment was significant only in SOLNI, where garlic residues demonstrated more herbistatic and/or herbicidal activity. The results of these experiments showed that the garlic and onion plant residues were capable of causing significant reductions in seed germination during their decomposition in soil, but only at elevated (39 °C versus 23 °C) soil temperature. At the agricultural level, activity of these residues could be exploited as a component of integrated weed management using appropriate crop sequencing, and may extend utility of soil heating treatments (e.g. solarization) for weed control to cooler climates or seasons, or for shorter treatment durations.