Successful biological invasion requires correspondence between invader functional traits and their utility in novel environments. We focused on specific phenological and ecophysiological characteristics of an herbaceous biennial, Alliaria petiolata, related to its successful invasion of deciduous forest groundlayers in eastern North America. We tested for phenological separation between A. petiolata and native groundlayer species during spring of its second year, when the plant accumulates 91% of its total biomass, and assessed importance of availability of high irradiance before tree canopy closure on growth and reproductive output. We experimentally shaded plants in situ during three intervals: Early: before native groundlayer was well developed (3 March–20 April), Mid: 21 April to tree canopy closure (18 May), and Late: after canopy closure to 29 May. We measured maximum photosynthetic rates (A_max) in early (13–14 April) and late (22–26 May) spring. Alliaria petiolata began rapid growth and reached maximum cover earlier than most native groundlayer species. Shading effect on plant growth and resource allocation to vegetative growth and reproduction varied depending upon timing and duration of shading. Comparison of treatments differing by being shaded or unshaded in only one of three intervals showed that unshaded plants consistently had significantly higher production than shaded plants only during the Early interval. Greatest A_max occurred in early spring (13–14 April), when ground layer irradiance was high. Success of A. petiolata in invading this community is likely related to phenological niche separation and temporal availability of resources not available to most native species in early spring.