The objectives of this research were: 1) to assess the effect of biochar incorporation on the growth of basil (Ocimum basilicum ‘Genovese Compact, Improved’) and peppermint (Mentha × piperita) and, 2) to determine the physical characteristic differences in heat-expanded clay (HEC) substrate following incorporations of biochar at 5%, 10%, or 15% (by volume). A commercially-available green roof substrate, Rooflite Intensive Ag (IA) substrate, was included for comparison. The IA substrate had the highest total porosity (TP), container capacity (CC), and air-filled porosity (AP). The HEC substrate showed a linear increase in TP and CC and a linear decrease in dry bulk density with increasing amounts of biochar. The commercially available IA substrate had the highest water retention (CC = 25.0%). Overall, there was a maximum increase of peppermint shoot dry weight (g/shoot) response in the HEC substrate using 15% biochar. Coverage area measurements indicated that peppermint benefited more than basil from the incorporation of biochar. Biochar alone did not influence stomatal conductance, although basil or peppermint grown in the IA substrate had higher stomatal conductance than plants grown on HEC with all three biochar incorporation rates at 3 and 4 d after irrigation, probably due to the lower aboveground biomass of the IA-grown plants. In conclusion, the addition of biochar amendment to HEC substrate had a minor influence on peppermint growth and no influence on stomatal conductance of either basil or peppermint.