| Summary: | <i>Amaranthus retroflexus</i> L. (Amaranthaceae), Redroot pigweed, is native to North America, but has become a weed of agriculture worldwide. Previous research into competition with food crops found it significantly reduces yields. Additionally, taxonomy, biomass allocation, physiological responses to light intensity, water stress, elevated CO<sub>2</sub>, and herbicide resistance have been investigated. To extend other research findings, we investigated growth and biomass yield in response to (i) soil moisture stress, and (ii) drought and elevated CO<sub>2</sub>. Additionally, we investigated seed germination rates following exposure to three elevated temperatures for two different time periods. Overall, moisture stress reduced plant height, stem diameter, and number of leaves. Elevated CO<sub>2</sub> (700 ppm) appeared to reduce negative impacts of drought on biomass productivity. Heating seeds at 120 °C and above for either 180 or 300 s significantly reduced germination rate. These results inform an understanding of potential responses of <i>A. retroflexus</i> to future climate change and will be used to predict future occurrence of this weed. The finding that exposing seeds to high temperatures retards germination suggests fire could be used to prevent seed germination from soil seed banks, particularly in no-till situations, and therefore may be used to address infestations or prevent further spread of this weed.
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