Biochar is a carbon-rich material derived from plant residues and obtained by thermochemical techniques in an oxygen-limited environment or in the absence of it. The aim was to evaluate the physical properties of mixtures of rice husk biochar as a substrate component and their effect on the growth of cucumber seedlings. The treatments were: rice husk biochar (BC) and commercial peat moss (T; Sunshine mix 3), as well as mixtures of both in different proportions (20:80, 40:60, 60:40 and 80:20 %, respectively). For physical characterization, particle size, bulk density, total porosity, aeration porosity and water-holding porosity were evaluated. Additionally, nutrient concentration was determined. To evaluate the effect of biochar on cucumber seedling growth, stem length, fresh weight of aerial and root biomass, leaf area, dry weight of aerial and root biomass, and relative chlorophyll content (SPAD readings) were considered. It was found that the addition of up to 40 % BC to the universal growth medium (peat moss) does not alter the physical properties of total porosity, aeration porosity, and water-holding porosity; furthermore, the growth of cucumber seedlings in a 20 % BC mixture is similar in stem length, fresh biomass, dry biomass and leaf area to seedlings grown with 100 % peat moss.