Junglerice [Echinochloa colona (L.) Link.] is the most important grass weed species in Australian summer cropping systems. Although it is mainly a spring- and summer-emerging weed species, field observations suggest that E. colona is expanding its seasonality. A common garden experiment was conducted at the University of Queensland farm to examine the effect of planting dates on phenology, growth, and fecundity of eight populations of E. colona. All populations were planted every second month from September to July in 2019 to 2020 and 2020 to 2021. Echinochloa colona took the shortest time (4 to 6 d) to emerge when planted in November or January. However, the November population took the longest number of growing degree days to exhibit panicle emergence. In both years, populations differed in height and leaf, tiller, panicle, and seed production in response to planting times. Plants produced significantly greater biomass for the November planting (123 to 147 g plant^–1) followed by the January planting and then the September planting. The March planting produced the lowest biomass. In the first year, the lowest number of seeds (3,500 seeds plant^–1) was produced by the March planting; however, in the second year, similar numbers of seeds were produced by the March and July plantings. In the first year, seed production (51,000 seeds plant^–1) was greatest for the November planting; however, some populations also produced a similar number of seeds for the January planting. In the second year, significantly greater seed production (111,000 seeds plant^–1) was observed for the January planting compared with other planting dates. The aboveground biomass and seed production of E. colona were positively correlated. This study reveals variations among E. colona populations and suggests that although greater emphasis must be placed on controlling spring- and summer-emerging plants, management practices need to be extended throughout the year to control E. colona in southeastern Australia.