Jobin, P., Caron, J. and Rochefort, L. 2014. Developing new potting mixes with Sphagnum fibers. Can. J. Soil Sci. 94: 585-593. Researchers are developing techniques to mass produce Sphagnum fibers (SF) on a sustainable basis since this material has properties that could benefit the growing-media industry. The objective of this study was to incorporate SF into peat-based substrates to enhance the value of brown Sphagnum peat and/or replace perlite in blond peat mixes. Nine substrates were prepared by mixing brown Sphagnum peat (BrSP) or sieved brown Sphagnum peat (sBrSP) with 0, 15 and 30% of SF and substituting 0, 50 and 100% of the perlite in a 70% blond Sphagnum peat (BSP) and 30% perlite mix with SF. The growth of Pelargonium was unaffected by the addition of SF, with the exception of above-ground biomass which was 15% lower in the BrSP substrate containing 15% SF. In the case of Petunia, above-ground biomass increased with a 30% SF addition to sBrSP, and below-ground biomass increased with a 30% SF addition to BrSP and sBrSP. Adding SF to peat increased water retention and hydraulic conductivity, but either reduced or had no impact on air-filled porosity. Removing fine particles from BrSP and adding 30% SF provided promising results, although the effect varied with plant species. Moreover, it is clear that SF can be used as a substitute for perlite in BSP mixes.