Influence of Biochar Amendments on Green Roof Media Physical Characteristics and on the Growth of Basil and Peppermint

There are few research papers relating to the incorporation of biochar into green roof media and the subsequent effects of these additions on herb growth.  Nonetheless, there is an interest in biochar incorporations into green roof media as well as an interest in growing agronomic crops on green roofs.  The objectives of this research were: (1) to determine differences in media physical characteristics of biochar (Biochar Now, Loveland, CO) at 5%, 10%, or 15% (by volume) incorporations into heat-expanded clay (HEC; Garick, Cleveland, OH) and (2) to assess the growth effect of biochar incorporation on the growth of ‘Genovese Compact, Improved’ basil (Ocimum basilicum L.) and peppermint (Mentha x piperita L.).  A commercially available green roof substrate, Rooflite® Intensive Ag Media (RIA; Laurel Valley Soils, Landenberg, PA), was included for comparison.  All media were supplemented with 14.0:6.1:11.6 slow-release fertilizer.  Cylinders (17-cm X 17-cm; depth X diameter) were used for physical characteristic determinations.  The RIA media had the highest total porosity (TP), water retention or container capacity (CC), aeration porosity (AP) and AP after an applied suction of 6.3 kPa.  In the HEC media, there was a linear increase in TP and CC with increasing amounts of biochar.  The HEC containing 15% biochar increased CC to 24% compared to HEC alone (18%).  The RIA media had the highest water retention (CC = 28%).  Particle size distribution was similar for media except that RIA media had a greater proportion of particles larger than 4.0 mm and a smaller proportion of particles between 1.0 to 2.0 mm and of particles < 0.053 mm size.  There was significant shoot dry weight (g/shoot) response to biochar additions (P ≤ 0.01) and significant shoot dry weight and plant area coverage (cm²) difference between the two species (P ≤ 0.001).  Peppermint benefited more than basil from biochar incorporations.  Following the plant growth study, media containing biochar additions had similar final potential of hydrogen (pH) and electrical conductivity (EC) values as the 100% HEC control.  Stomatal conductance was significantly different among the two species following irrigation and successive drying of media over several days; peppermint typically having much greater stomatal conductance following initial irrigation and during the drying period of several days. In conclusion, biochar additions to HEC media increased total porosity and container capacity which contributed to increased shoot dry weight of peppermint.