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 on 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 to 2018 and 2018 to 2019), and their growth and seed production were recorded. Although both weeds matured at a similar number of growing degree days (GDD), they required a different number of days to complete their life cycles depending on planting date. The growth period was reduced and flowering occurred sooner as both species experienced cooler temperatures and shorter daylight hours. Both species exhibited increased height, biomass, and seed production for the October-sown plants compared with other planting times, 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 plant−1, but these were reduced to 770 and 365 seeds plant−1 for the January planting date, 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 could 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 energy inputs toward their control.

中文翻译：

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出苗时间对红根苋 (Amaranthus retroflexus) 和细苋 (Amaranthus viridis) 生长和繁殖力的影响：澳大利亚夏季作物中新出现的问题杂草

红根藜 (苋菜L.) 和细长的苋菜 (苋菜L.) 被认为是澳大利亚夏季作物中出现的问题杂草。进行了户外盆栽试验，以检验种植时间对两个种群的影响。A. 反曲和A. viridis在澳大利亚昆士兰大学的研究农场。10 月至 1 月（2017 年至 2018 年和 2018 年至 2019 年）每个月种植这两个物种，并记录它们的生长和种子产量。尽管两种杂草在相似的生长期天数 (GDD) 下成熟，但它们需要不同的天数来完成其生命周期，具体取决于种植日期。由于两个物种都经历了较冷的温度和较短的日照时间，因此生长期缩短了，开花发生得更快。与其他种植时间相比，10 月份播种的植物的高度、生物量和种子产量均有所增加，而这些参数通过延迟种植时间而降低。枝条和根的生物量A. 反曲和A. viridis与 10 月份相比，1 月份种植时（两个种群的平均值）分别减少了 70% 和 65% 以上。十月播种时，A. 反曲和A. viridis生产了 11,350 和 5,780 颗种子厂-1, 但这些被减少到 770 和 365 种子植物-1分别为一月份的种植日期。虽然这些物种的生长和繁殖力取决于种植时间，但这些杂草可能会在澳大利亚东南部的整个春末至夏季生长季节（10 月至 3 月）出现，并能产生大量种子。结果表明，当这些物种在晚春（10月）出现时，与其他种植日期相比，它们生长旺盛，产生的生物量更多。因此，对这些物种的任何早期杂草管理实践都可能有利于最大限度地减少随后的控制成本和能源投入。