Background and aims Soils are composed of both fine and coarse materials. Coarse material (> 2 mm) is considered to be inert and is usually discarded in models of plant water balance, even though it affects soil properties. No studies have yet attempted to assess whether rock fragments may act as a water reservoir for plants. Methods Cuttings of Populus euramericana were planted in 5-L pots containing reconstituted soil made up of fine earth (silty clay loam texture) and either limestone or inert (quartz) pebbles (rock fragments 2–5 cm) at 0, 20, and 40% volume in a cross-factorial experiment. Two drought periods were applied and the growth, evapotranspiration, water stress status by stomatal conductance, and water content of the two soil phases (fine earth and pebbles) were monitored. Results First, pebbles can contain water, and ignoring this water induced and underestimations of the soil available water content by respectively 11% and 30% for the treatment with 20% and 40% limestone pebbles. Second, the plants grown on limestone pebbles were up to 70% less stressed than the plants grown on inert pebbles during drought. Third, stomatal conductance, a water stress indicator, was correlated with the water content of both the fine earth and the limestone pebbles.