The addition of Sphagnum to peat-based growing media ('Sphagnum replacement') influences plant performance. The primary physical effect of Sphagnum addition appears to be enhanced water retention. Good performance of plants cultivated in Sphagnum seems partly explainable in terms of its water retention properties. The large body of nutrient solution retained in Sphagnum can delay disadvantageous changes in its concentration during cultivation. The physical quantity of Sphagnum per unit volume, i.e. its bulk density, governs the volume of retained water and thus determines the strength of effects contributing to plant performance. When subjected to severe drought, plants cultivated in Sphagnum did not show clear signs of water deficit up to at least 1,572 hPa of matric suction, which is the estimated wilting point for plants grown in light peat. Using Sphagnum to replace peat in the growing medium appears advantageous to plants not only during drought but also during ordinary greenhouse cultivation.