Alliaria petiolata, an invasive biennial plant, actively grows and achieves high maximum rates of photosynthesis in early spring during its second growing season, when many indigenous ground layer species are still dormant and high levels of irradiance reach the forest floor. Alliaria petiolata also extends its growing season into the summer through stem elongation and production of new leaves that are adapted to ambient irradiance levels, unlike native deciduous forest spring ephemeral species or summer forbs. Photosynthetic rates for two populations of A. petiolata in a forest ground layer were measured in summer and fall of the plant's first growing season (1998) and in spring of the second growing season (1999). During the second growing season, estimates of native ground layer cover were made at 3–4 wk intervals from early April to late May. Maximum photosynthetic rates (A_Max) were positively correlated (r² = 0.791, df = 3, P = 0.0436) with mid-day irradiance reaching the forest ground layer and were negatively correlated (r² = 0.911, df = 3, P = 0.0116) with percent ground layer cover. The highest A_Max (mean ± SE) occurred on 6 April (17.8 ± 0.6 μmol CO₂ m⁻² s⁻¹) and then declined during May, being 8.8 ± 0.3 μmol CO₂ m⁻² s⁻¹ and 4.7 ± 0.4 μmol CO₂ m⁻² s⁻¹ on 2 and 24 May, respectively. Alliaria petiolata's pre-adaptation to achieve maximum rates of photosynthesis before the active growth of many native ground layer species when irradiance reaching the ground layer is high, and temperature and moisture conditions are favorable for the species, contributes to its successful spread and eventual domination of forest ground layers.