Pine Bark Substrate Source and Age Affect Seedling Toxicities

Bark has been an important component in horticultural substrates for years (and is rapidly increasing in greenhouse professional mixes and in retail potting soil mixes), and in the nursery industry bark is the most common substrate component used in the United States. Bark is obtained as a byproduct of the timber industry (pulp mills, saw mills, pole peeling operations, etc.) when it is striped off logs after harvest. The objective of this work was to evaluate the potential and/or severity of pine bark toxicities on seedling germination and growth. A grow test method was used to observe any toxicities expressed in plants grown in pine bark substrates. Six pine bark sources and two different ages (fresh and aged) were tested.  The method and duration of aging varied among suppliers.  The aged materials were “ready for sale” from each supplier. For each substrate, 12, four-inch pots were filled and then seeded with one of three species: marigold, radish, and tomato. Four replications were used for each species with five seeds per pot. A control mix (60:20:20 peat/vermiculite/perlite) was used as well for comparison. Seeds germinated and grew for two weeks and then were thinned to one plant and grown for an additional two weeks. Seedling germination percentage was recorded. In addition, toxicities were observed and recorded. Toxicities were exhibited through foliar symptoms such as chlorosis, reddening, and limited growth (did not get beyond cotyledon stage). Seedling germination varied among pine bark sources and ages of pine bark when compared to the control mix for both marigold and tomato species. However, all sources and ages of pine barks showed an increase in germination percentage of radishes as compared to the control mix (95-100% vs. 90% for control mix). With few exceptions, all pine barks showed an equal or greater germination percentage than the control mix for marigolds and tomatoes. Those that did have lower germination percentage cannot be attributed to one particular source or age. Therefore, germination percentage is not predictable since two of three species varied greatly amongst different sources and ages. The variation in germination between and among bark ages and sources can be likely due to the particle size of the barks (which was extremely varied), with coarser materials not being supportive of seed germination even if there were no toxicities preventing germination.