Allelopathic effects of selected medicinal plants on Amaranthrus retroflexus and Chenopodium murale

Thirty medicinal plant species were screened for their herbicidal activities against Amaranthus retrofexus and Chenopodium murale in laboratory and glasshouse experiments. Aqueous extracts of Alhagi maurorum, Capparis spinosa, Citrullus colocynthis, Lavandula officinalis, Origanum syriacum, Rhus coriaria, Ricinus communis, Rosmarinus officinalis and Teucrium polium were highly toxic to both weed species. Shoot, leaf, or stem extracts either prevented seed germination or reduced seedling growth of the two weed species and the inhibitory effect increased with increased extract volume. In contrast, stem extract of O. syriacum and low extract volumes of certain species stimulated the weed growth. Foliage leachates or suspected volatile compounds of many of the above-mentioned plants inhibited germination and growth of both weeds. Addition of 2 g/kg dried shoot material of L. officinalis or R. officinalis to the potted soil mixture was highly toxic to weed germination and growth, and a higher rate (16 g/kg) of C. spinosa, R coriaria, and T polium severely reduced weed growth. Similar effects on both weeds were obtained when shoot extracts of L. officinalis and C. spinosa were added to the soil.