Understory plants in temperate deciduous forests are dependent on a spring window of high light availability prior to canopy leafout. Aesculus glabra Willd. (Ohio buckeye, Hippocastanaceae) produces leaves several weeks before other trees in the same habitat. Aesculus glabra canopy is hypothesized to reduce the spring high-light period, and thus to have a negative effect on understory plants, especially annual and spring-ephemeral herbs. Understory community composition (herbs plus woody plants <0.5-m height) and environment were compared under A. glabra– and non–A. glabra-dominated canopy in May and July in a mesic, secondary forest in northeastern Indiana. The understory under A. glabra canopy had lower cover and observed species richness in both seasons, and ordination indicated differences in species composition in both seasons. Annual and summer-green herbs and woody plants had lower cover under A. glabra in May, but spring ephemerals did not differ. Summer-green herbs and woody plants had lower cover under A. glabra in July. Aesculus glabra canopy reduced spring light availability, decreased spring soil temperature, slightly increased soil phosphorus, increased July (but not May) soil moisture, and had no effect on soil nitrate. Aesculus glabra litter extracts reduced germination and seedling growth of Raphanus sativus L. but not Lactuca sativa L., while soils from under A. glabra did not affect germination of Phlox divaricata L. The primary effect of A. glabra on the understory appears to be via lower light availability and soil temperature in spring.