Germination and seed persistence of Amaranthus retroflexus and Amaranthus viridis: Two emerging weeds in Australian cotton and other summer crops

Redroot pigweed ( Amaranthus retroflexus L.) and slender amaranth ( Amaranthus viridis L.) are becoming problematic weeds in summer crops, including cotton in Australia. A series of laboratory and field experiments were performed to examine the germination ecology, and seed persistence of two populations of A . retroflexus and A . viridis collected from the Goondiwindi and Gatton regions of Australia. Both populations of A . retroflexus and A . viridis behaved similarly to different environmental conditions. Initial dormancy was observed in fresh seeds of both species; however, germination reached maximum after an after-ripening period of two months at room temperature. Light was not a mandatory prerequisite for germination of both species as they could germinate under complete darkness. Although both species showed very low germination at the alternating day/night temperature of 15/5 C, these species germinated more than 40% between ranges of 25/15 C to 35/25 C. Maximum germination of A . retroflexus (93%) and A . viridis (86%) was observed at 35/25 C and 30/20, respectively. Germination of A . retroflexus and A . viridis was completely inhibited at osmotic potentials of -1.0 and -0.6 MPa, respectively. No germination was observed in both species at the sodium chloride concentration of 200 mM. A . retroflexus seedling emergence (87%) was maximum from the seeds buried at 1 cm while the maximum germination of A . viridis (72%) was observed at the soil surface. No seedling emergence was observed from a burial depth of 8 cm for both species. In both species, seed persistence increased with increasing burial depth. At 24 months after seed placement, seed depletion ranged from 75% (10 cm depth) to 94% (soil surface) for A . retroflexus , and ranged from 79% to 94% for A . viridis , respectively. Information gained from this study will contribute to an integrated control programs for A . retroflexu s and A . viridis .