Floerkea proserpinacoides (Limnanthaceae) is a spring ephemeral annual species that grows in deciduous forests throughout eastern North America. Seeds germinate from late November to December, although the first leaf emerges only from late March to early April. Growth begins in early April at the onset of favourable temperatures, following snowmelt, and continues through mid-June. Senescence coincides with increasing air temperature and decreasing light level as a result of canopy closure. In this paper, we present the results of a growth chamber study designed to determine the effect of light level on growth, biomass allocation and reproduction of F. proserpinacoides. The study consists of two parts: in a first experiment, plants were grown at five constant photosynthetic photon fluence rates (PPFR: 90, 180, 360, 540 or 900 μmol m−2 s−1), and in a second experiment, PPFR was reduced from 900 μmol m−2 s−1 to 180 μmol m−2 s−1 after 0, 14, 21, 28 or 35 d of growth. Relative humidity, temperature, nutrient and water supply were kept constant in a hydroponic sand culture experiment. Total biomass, leaf mass and leaf area increased with increasing PPFR up to 540 μmol m−2 s−1. Plants grown at the highest (900 μmol m−2 s−1) and the lowest (90 μmol m−2 s−1) PPFR had a substantially lower biomass by the end of the 35-d growth period than plants grown at intermediate PPFRs (360 or 540 μmol m−2 s−1). Despite differences in total biomass, there were no significant differences in seed production among treatments. The mean relative growth rate (RGR) increased with increasing light levels between 90–540 μmol m−2 s−1, and it was reduced at 900 μmol m−2 s−1. However, differences in RGR were not significant among treatments. Specific leaf area did not vary consistently as a function of light level, whereas leaf area ratio and leaf mass ratio tended to increase with increasing PPFR, reaching maximum values at 360–540 μmol m−2 s−1. However, none of these growth variables differed significantly across the range of PPFR levels. The transfer of plants to lower PPFR had no significant effect on any of the growth components. Biomass production for the species appeared to be optimized at PPFR of 360–540 μmol m−2 s−1. Growth might be restricted by an insufficient supply of photosynthates at low PPFR and by photoinhibitory processes at higher PPFRs.