Interactive Effects of Root-Promoting Treatments and Media on Clonal Propagation of Two Western Pine Species

Root-promoting treatments and media are critical for the clonal propagation of Western pine species, yet long-term data for sugar pine (Pinus lambertiana Dougl.) and ponderosa pine (Pinus ponderosa Dougl. ex C. Lawson) remain limited. This multi-year greenhouse study (2024-2025) used a split-plot greenhouse design to evaluate interactions among four rooting media and various root-promoting treatments, including commercial mixed microbial/biostimulant products and indole-3-butyric acid (IBA). Coconut coir consistently outperformed other media, achieving rooting rates of up to 80% when combined with specific commercial products like Mykos, Dynomyco, or Wildroot. Seasonal timing also proved crucial; winter propagation (January 2025) yielded poor results (<30%) regardless of treatment, consistent with the influence of physiological dormancy. While commercial mixed microbial/biostimulant products were associated with improved rooting, their effectiveness depended on the use of an appropriate substrate. The superior performance of coconut coir is consistent with its physical characteristics (e.g., aeration, water retention), although these properties were not directly measured.