One of the largest and rarest Bebb willow (Salix bebbiana) communities in the United States occurs at Hart Prairie, Arizona. Low recruitment of the willow over the past several decades has been linked to inadequate soil water content for seed germination and seedling establishment. We tested a hypothesis that a prescribed burn would reduce biomass of and evapotranspiration by herbaceous plants, thereby increasing soil water content. Three treatments (unburned control, early-growing season burned, late-growing season burned) were applied in year 2001 to replicated plots in fern- and grass-dominated herbaceous communities. Soil water content (0–30 cm) was measured weekly in plots during the 2001, 2002, and 2003 growing seasons. Both early- and late-season burning reduced herbaceous biomass in the fern-dominated community in 2002 and 2003 and reduced biomass in the grass-dominated community in 2002 but not in 2003. Soil water content increased for approximately four weeks in 2001 following the early-season burn, but the early-season and late-season burns reduced soil water content in both communities over much of the 2002 and 2003 growing seasons. Thus, early-season burning may benefit willow seed germination by increasing soil water content immediately following burning but be detrimental to germination in the second and third growing seasons after burning because of drier soil. Large temporal variation in the effect of prescribed burning on soil water content will complicate the use of fire as a restoration tool to manage soil water available for threatened plants such as Bebb willow and for recharge of groundwater.