The critical period for weed control (CPWC) has been used to define weed-control threshold triggers in many cropping systems. Using the CPWC to develop a weed-control threshold for broadleaf weeds that emerge later in the season would be valuable to cotton growers to enable them to schedule management of later emerging weeds to occur before crops suffer unacceptable yield losses. Field studies were conducted over two seasons from 2006 to 2008 to determine the CPWC for a broadleaf weed in cotton, using mungbean as a mimic weed. Mungbean was planted into cotton at densities of 1 to 50 plants m−2, at up to 450 growing-degree days (GDD) after crop planting, and removed at successive 200 GDD intervals after introduction, or left to compete full season. The data were fit to logistic and Gompertz curves. More complex models were developed and tested that included the time of planting and removal, weed density, height and biomass in the relationships. The CPWC models were able to predict the yield loss from later emerging weeds and together with an understanding of the expected growth rates of the weeds, the functions could be used predictively to determine the likely impact of delaying a weed-control input. This predictive element will be of value to cotton growers needing to coordinate weed-control inputs with other farm activities.