Biochar, a carbon-rich material resulting from pyrolysation, is one of the proposed alternatives as a substrate component to peat. The aim of this study was to investigate the effect of different ratios of peat substitution with a locally available hardwood biochar on the chemical characteristics of growing media, seed germination, plant morphological and nutritional parameters of Tagetes patula. Experimental results show that biochar can be used as a partial peat replacement, showing a complementary liming effect, rising the initial pH of peat (2.8) to values of 4.5 and 7.3 in mixtures with 50% and 75% substitution of peat by biochar, respectively. The mixture with a 50% biochar rate exhibited the strongest change between initial and final pH, rising from 4.5 to 5.7. Increasing content of biochar in the mixtures corresponded to increasing content of total K, Ca, Mg, available K and decreasing level of available Mg. The highest germination percentage was observed in the mixtures with biochar rates from 50% to 100%. Among all substrates, the greatest plant diameter, true leaf area and total root length were measured in the 50% peat substitution mixture, where the highest contents of P, K and Ca in plant shoots were found.