A simple model of canopy expansion and senescence with accumulated thermal time from sowing was used to describe differences in canopy development and potential size across a range of sites in the UK between 1992 and 1995. The principal model inputs were nitrogen, temperature and sowing date. The model was calibrated across six sites within the major wheat growing areas using canopy data collected during the 1992/93 and 1993/94 growing seasons and was validated using data collected in the 1994/95 season. The model was able to predict the time course of green area index (GAI) over a season with an r2[ges ]0·87 for the five English sites, and an r2=0·68 for the single Scottish site. This model may prove to be a useful approach to forecasting the potential canopy size, based upon an estimate or measure of the total nitrogen available to the crop (both from applied fertilizer and the soil).