2017 ASHS Annual Conference

Biochars, organic matter heated through pyrolysis, are one potential amendment nursery stock growers could use to reduce reliance on horticultural grade bark and peat moss, but the impact of biochars on landscape plants grown in soilless potting mixes is unknown. Biochars from four parent materials, dried dairy manure (DM), ponderosa pine sawdust (PP), ground wood waste (GWW) and a commercial biochar made from non-food biomass (CB), were tested as alternate amendments in potting mixes in two trials (DM and PP tested 2015; GWW and CB tested 2016) in 1-gallon containers. Amended biochar media contained 0, 5, 10, 15 or 20% biochar by volume, 70% aged bark and sphagnum peat moss to bring each mix’s volume to 100%. Douglas spirea (2015, 2016), mockorange (2015) and firechalice (2016) were grown in the mixes for three months. At harvest, increases in shoot heights were recorded for spirea and mockorange, whereas plant diameters were measured for firechalice. Shoot dry weights were determined for all plants. Physical properties (air capacity, water holding capacity and total porosity) were evaluated for all mixes and were found suitable for container production. Chemical analyses of each raw biochar were completed and nutrient levels varied depending on parent material, especially the CEC which ranged from ~2 cmole(+)/kg (DM and PP) to greater than 30 cmole(+)/kg (CB and GWW). Plant growth in response to biochar type also varied. Spirea and mockorange shoot dry weights and heights were negatively impacted by >5% DM biochar. High salts (electrical conductivity = 58 mS·cm^-1, extractable chloride = 11,993 mg·kg^-1) in the DM biochar were likely responsible. Shoot dry weight of spirea and mockorange and the height of spirea grown in PP mixes were equivalent to control plants, but mockorange height decreased at least 17% with incorporation of PP biochar. Mixes amended with GWW or CB biochars produced plants with equivalent height (spirea, p = 0.1132) and shoot diameter (firechalice, p = 0.431) regardless of treatment. In contrast, spirea and firechalice plants produced the most shoot dry weight in the 10% CB mix, whereas firechalice shoot dry weight decreased at least 11.2 % with any concentration of GWW biochar. One of the four biochars evaluated increased plant growth (10% CB), whereas three of the biochars (DM, PP and GWW) were detrimental to the growth of at least one plant species when grown in soiless media in 1 gallon containers.