Hydrological properties, in particular water-retention characteristics (WRC) and hydraulic conductivity (K), were studied in eight different growth media based on low-humified sphagnum peat with coarse (perlite) and fine (sand) constituents. The temporal change of matric potential(ψ)in the media, which were connected to two contrasting mineral soils (fine and coarse sandy soil) with higherψ, was studied in the laboratory in order to emulate the hydraulic situation in field conditions after outplanting of container seedlings. It was shown that the hydrological properties of peat medium and its hydraulic interaction with sandy soil can be altered markedly by the additives used. Adding a coarse constituent (perlite) to peat increased air-filled porosity (AFP) and saturated K of the medium, but decreased water retention (WR) (especially in wet conditions) and unsaturated K. Addition of a fine constituent (sand) decreased AFP and saturated K, but increased WR and K in dryish conditions. In general, the effect of additives on these hydrological properties increased with increasing proportion of additive in the medium. By using ternary mixtures (peat+perlite+sand), extreme values of these properties were avoided. When in contact with sandy soils, fine-textured sand constituent added to peat was found to cause the greatest movement of water into the growth medium. The results suggest that, after outplanting, additives in peat growth medium could be important for the movement of water from the surrounding soil and for the availability of water to seedling roots.