How growth and morphology of wild plants are controlled by the water vapor saturation deficit of the air (vapor pressure deficit, VPD) is not sufficiently understood. We tested the hypothesis that VPD acts on temperate woodland herbs independently from soil moisture by exposing two species (Mercurialis perennis L. and Stachys sylvatica Torr.) to variable VPD levels in climate chambers and in open-top chambers on the forest floor. A decrease in air humidity from 85% to 40% in the climate chamber experiment, which simulates a microclimate change after canopy gap creation, resulted in a 40% decrease in biomass production of both species when grown in hydroponic culture. This result is supported by the more realistic open-top chamber experiment, which showed a productivity decrease by approximately 25% when plants were continuously exposed to an atmosphere with 15% lower air humidity than ambient. Elevated VPD levels reduced biomass production through either a reduced leaf expansion rate or a lower number of formed leaf buds. We conclude that many woodland herbs require sufficiently high air humidity for optimal growth; permanently decreased air humidity, as may occur in a future drier climate, after gap creation, or after clear-cutting of the forest, may threaten the vitality and survival of woodland herbs.