Effect of Emergence Time on Growth and Fecundity of Redroot Pigweed (Amaranthus retroflexus) and Slender Amaranth (Amaranthus viridis): Emerging Problem Weeds in Australian Summer Crops

Abstract

Redroot pigweed (Amaranthus retroflexus L.) and Slender amaranth (Amaranthus viridis L.) are considered emerging problematic weeds in summer crops in Australia. An outdoor pot experiment was conducted to examine the effects of planting time of two populations of A. retroflexus and A. viridis at the research farm of the University of Queensland, Australia. Both species were planted every month from October to January (2017-18 and 2018-19), and their growth and seed production was recorded. Although both weeds matured at a similar number of growing degree days (GDDs), these weeds required a different number of days to complete their life cycle within each planting date. The growth period was reduced, and flowering occurred sooner as both species experienced cooler temperatures and shorter daylight hours. Compared to other planting times, both species exhibited increased height, biomass, and seed production for the October-sown plants, and these parameters were reduced by delaying the planting time. The shoot and root biomass of A. retroflexus and A. viridis (averaged over both populations) was reduced by more than 70% and 65%, respectively, when planted in January, in comparison to planting in October. When planted in October, A. retroflexus and A. viridis produced 11,350 and 5,780 seeds per plant, but these were reduced to 770 and 365 seeds per plant in planting date January, respectively. Although the growth and fecundity of these species were dependent on planting time, these weeds could emerge throughout the late spring to summer growing season (October to March) in southeast Australia and produce a significant number of seeds. The results showed that when these species emerged in the late spring (October), they grew vigorously and produced more biomass, in comparison with the other planting dates. Therefore, any early weed management practice for these species could be beneficial for minimizing the subsequent cost and inputs towards their control.