Evaluation of Different Biochar Sources as Potential Media Amendments for Container Plant Production of Native Plants

Biochar, an organic material produced by pyrolysis, is being explored as a sustainable alternative to horticultural-grade bark and peatmoss in growing media. In this study, biochars derived from the following four feedstocks were evaluated as partial replacements for peats: (1) dried dairy manure; (2) Pinus ponderosa sawdust; (3) ground wood waste; and (4) commercial nonfood biomass products. Media formulations containing concentrations of biochar of 0%, 5%, 10%, 15%, or 20% were combined with 70% aged bark and the remaining was filled with sphagnum peatmoss. Plants of three native shrub species, Douglas spirea ( Spirea douglasii ), mockorange ( Philadelphus lewisii ), and firechalice ( Epilobium canum var. garrettii ) were grown in these substrates for 3 months. To determine the effects of the media, at harvest, the shoot dry weights for all species were determined. Similarly, data regarding the heights of spirea and mockorange and the diameter of the canopy of firechalice plants were also measured. All substrate mixes exhibited acceptable physical properties, including air capacity, water-holding capacity, and total porosity. However, chemical analyses revealed that nutrient contents varied by biochar source, which could have influenced plant growth responses. Dairy manure biochar applied at more than 5% suppressed spirea and mockorange growth, likely because of excessive salt content. Ponderosa pine biochar generated shoot dry weights comparable to those of the control, but mockorange height declined by at least 17%. In contrast, biochar from ground wood waste and commercial biochar maintained plant size across most treatments, with the greatest shoot growth observed for spirea and firechalice in the 10% commercial treatment. Firechalice shoot dry weight was reduced in all ground wood waste treatments, whereas in both spirea and firechalice, an increase in shoot dry weight was observed only with the 10% commercial treatment amendment. Overall, commercial biochar demonstrated potential as a peatmoss substitute, while dried dairy manure, Pinus ponderosa sawdust, and ground wood waste biochars negatively affected at least one of the species assessed.