Allelopathy is often treated as an innate characteristic of a species rather than a phenotypically plastic trait that can vary with environmental conditions. Lianas are a highly competitive phenotypically plastic life form that typically occur in both shaded and unshaded environments. As such we hypothesized that temperate lianas may conditionally change allocation to allelopathic chemicals in response to light availability though the expected direction of change is unclear. Shading may reduce resource availability and therefore reduce allocation to allelochemicals induce allelopathy as a competitive mechanism or may not be related to allelopathy. To test the conditionality of allelopathy sun and shade leaves of five common liana species (Toxicodendron radicans Parthenocissus quinquefolia Celastrus orbiculatus Lonicera japonica and Vitis vulpina) were collected from a young deciduous forest in New Jersey USA and tested with laboratory bioassays to detect allelopathic potential. All liana species showed allelopathic potential and three species exhibited induction of increased allelopathic potential in shaded environments. The two species that were not shade induced are late successional lianas that persist for long periods in forest canopies. In contrast the inducible lianas were early successional species that typically decline with canopy closure. This research indicates that lianas have the potential to be allelopathic and allelopathic potential conditionally responds to shading only for species that would normally be excluded from the forest canopy. As early successional lianas are present throughout forest regeneration in a range of light environments allelopathic plasticity may increase their success by differentially allocating resources based on environmental conditions.