​​​​​​​Amaranthus species are a key risk to farm production, biodiversity, and agriculture economics of more than 80 countries across the world. A. retroflexus and A. viridis are among the most noxious Amaranthus species. The current information available on the biology these species in Australian agroecosystem is not enough for suitable management. Therefore, several studies were conducted to explore biology of these two weeds in controlled and field conditions at University of Queensland, Australia.A comparative study on two different Australia biotypes of A. retroflexus and A. viridis suggested that they are highly invasive in nature and both biotypes of these species are readily adaptive to changing climatic conditions and exhibit superior germination and reproductive ability. Overall, A. retroflexus and A. viridis germinated over a wide range of alternating day/night (15/5 to 35/25°C) temperatures, osmotic stress (0.0 to -0.6 MPa), salinity levels (0 to 200 mM of Sodium Chloride).Amaranthus retroflexus and A. viridis tolerated moisture stress and produced seeds at 100%, 75%, 50% and 25% soil field capacity (FC) corresponding to no, light, moderate, and high moisture stress when grown in the pots in a ventilated screen house, respectively. Leaf production by A. retroflexus and A. viridis plants was inversely proportional to the water stress. The number of leaves produced by A. retroflexus and A. viridis plants at 25% of FC was 54% and 53% fewer than that at 100% of FC, respectively. Shoot biomass of both species declined linearly with increasing water stress. The highest shoot biomass of A. retroflexus (46 g plant-1) and A. viridis (44 g plant-1) was observed at 100% FC. No differences were observed between the shoot biomass of A. retroflexus at 100% and 75% FC. At 25% of FC, the shoot biomass of A. retroflexus and A. viridis plants were reduced by 73% and 76%, respectively, compared with the shoot biomass at 100% FC.In conclusion, A. retroflexus and A. viridis have multifarious impacts on crop production. It is clear from the biology studies that invasiveness of this weed is associated with its ability to germinate over a wide range of environmental conditions and tolerate soil moisture stress through morphological and physiological adaptations. A prioritized multilateral integrated management plan must be devised to be implemented on a large scale to combat A. retroflexus and A. viridis in Australia to avoid the loss which these weeds have made in America.