The objective of the present study was to analyze and compare the allelopathic activities of weedy and grain amaranths. For this purpose, the seeds of garden cress (Lepidium sativum L.) were germinated on filter paper moistened with aqueous extracts of Amaranthus retroflexus L. and Amaranthus cruentus L. cv. ‘G6’. The extracts were prepared from fresh roots, stems, leaves, and inflorescence with seeds (1: 2 w/v in water) and used, either undiluted (100%) or at varying concentrations (75, 50, and 25%). Although, all the extracts delayed germination, the leaf extracts of both species and the inflorescence extracts of grain amaranth, proved the more powerful. The root and stem extracts of grain amaranth reduced germination significantly only when used undiluted, whereas none of the extracts at 25% concentration, except that of the pigweed leaves, affected root elongation. However, in some cases, root elongation was even stimulated. Compared to the pigweed amaranth, the grain species exerted a stronger inhibitory effect on the germination process, and root elongation.
Key words: Allelopathy, germination rate, germination speed, grain amaranth, Lepidium sativum, pigweed, root elongation.