Turner, F. A. and Van Acker, R. C. 2013. In situ emergence timing of large and small crabgrass in residential turfgrass of southern Ontario. Can. J. Plant Sci. 93: 503-509. Large, Digitaria sanguinalis (L.) Scop., and small, Digitaria ischaemum (Schreb.) ex Muhl., crabgrass are problem weeds in turfgrass. Due to an increasing number of cosmetic pesticide bans in Canada there is a need to better understand the biology and ecology of crabgrass in order to develop and hone management approaches. The assessment of crabgrass recruitment timing is particularly relevant to its management, including the timing of alternative herbicide applications. This study focused on determining the emergence timing of established populations of large and small crabgrass in typical residential turfgrass stands in southern Ontario. Small crabgrass emerged earlier than large crabgrass at 346 and 515 growing degree days (GDD), respectively. In typical southern Ontario lawns both small and large crabgrass emerge after cool-season turfgrass has established and emergence continues late into the summer. For example, even within the last 2 wk of July we observed over 700 seedlings m^-2 of large crabgrass emerging in some observation plots. This study also confirmed that small crabgrass emerges earlier than large crabgrass. There was a greater difference in emergence timing between species rather than among sites, suggesting that it is important to differentiate between species when timing management approaches. The late and prolonged emergence of crabgrass makes residential lawns that are not well maintained susceptible to infestation for a long portion of the growing season. This study also demonstrated that cumulative GDD may be a reliable measure for tracking crabgrass emergence suggesting, that it could be used as a tool for management, including the application of alternative herbicides. This study reinforces the importance of maintaining healthy and dense turf stands throughout the season as a deterrent to crabgrass infestations.