In northern ecosystems, the onset and growth of spring photosynthesis may have an important influence on the annual carbon (C) budget, yet the controls have not been clearly identified, especially for peatlands. We used a 5-year set of daily carbon dioxide (CO2) exchange measurements derived from an eddy covariance tower located at Mer Bleue, an ombrotrophic bog near Ottawa, Canada, from March to May [day-of-year (DOY) 60–150], 1999–2003. We used half-hourly measured net ecosystem exchange minus modelled ecosystem respiration to estimate daily photosynthesis, as gross ecosystem production (GEP). The onset of GEP in each year was closely related to the thinning and disappearance of the snow cover, occurring between DOY 86 and 101. GEP increased during the spring, reaching 10-day average values of between 5 and 9 g CO2 m−2 day−1 by the end of May. This increase was initially associated with moss activity (Sphagnum and Polytrichum), followed by the evergreen shrubs. Peat temperatures in the rooting zone (10–20 cm depth) and increases in shrub leaf nitrogen and chlorophyll a concentrations contributed to this rapid increase in GEP. Examination of moderate-resolution imaging spectroradiometer (MODIS) images over several years revealed that the temporal resolution (16-day composites) was inadequate to capture the onset of GEP but estimates of gross primary productivity and photosynthesis from MODIS 8-day composites for the most part followed the pattern and magnitude of CO2 exchange observed at the tower.