Comparison of Pine Bark Substrates from Multiple Suppliers: White Wood Percentage and Volume

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 differences in white wood content among different pine bark suppliers and materials. Fresh and aged pine barks from four different sources were tested. White wood is a term used to describe the actual pine wood, or xylem that accompanies the bark during the debarking process. White wood percent and volume were determined in all pine bark sources/samples. Three replications of each bark material had white wood removed my hand (sorting and picking). After removing the white wood from the pine bark the volume and mass were calculated and recorded. There were differences between sources as well as among sources when comparing fresh versus aged pine barks. Percent volume ranged from 23 – 4.5% and percent mass from 19 – 4%. The four fresh bark materials from the different suppliers had white wood percent’s that ranged from 5% to 22% and in the aged bark it ranged from 5% to 14%. The size of the white wood particles was also extremely variable between bark suppliers/sources. There is no official standard or guideline for how much white wood is allowed in pine bark, for the bark to be considered “good quality” or “acceptable”. However, a general rule is that 10% white wood is acceptable in bark mixes. The variability in pine bark supplies can be likely influenced by many factors including species of pine tree, thickness of bark at time of removal, tree harvest season, method of bark removal from logs, variations in hammer milling, variations in bark handling and screening, and variations in bark pile management and aging. These variability’s can exist between/among different suppliers, but also it can (and very often does) vary from season to season at the same supplier.