Barking Up The Right Tree

Aged pine bark is the one of the most common organic substrate components in the US, with bark from loblolly and longleaf pines being the most prominent in the southern U.S.  Aging is a modified composting process (no nitrogen source added) in which the bark is piled on the ground in windrows and allowed to age for a period of time, usually six months to one year. Aging time can vary between suppliers, or even for the same supplier, based on factors such as space shortages, product demand, or preference. The resulting end-products from these various procedures are all sold to the consumer as the same product, but are completely different in terms of percent fines, water holding capacity, air space, and nutrient immobilization, which will cause them to act differently when used in a growing mix. Fresh bark (bark that is sold shortly after removal from a tree, then ground, and screened to an appropriate particle size), may also be sold as a growing mix, and is preferred by some growers.

Little research has been conducted concerning the differences between fresh and aged bark. Bark suppliers in the Southeast have indicated demand for fresh pine bark has increased because of its lighter weight and cheaper transportation costs. However, there are discrepancies in the literature as to whether fresh bark can be toxic to seedlings, and whether or not water holding capacity can be an issue due to the higher amount of naturally occurring tannins and waxes, as well as the larger particle size distribution. Because of these issues, growers using fresh pine bark may need to adjust their fertility and irrigation regimes, but the adjustments that need to be made are currently unknown.

The Horticultural Substrates Lab is working to gain a greater understanding of how pine bark substrates are influenced by aging, and how the chemical, physical, and hydrological changes over time may impact the growth of horticultural crops. Working with T.H. Blue Inc., a local pine bark supplier, we are trying to quantify the changes in longleaf pine bark over the course of the year during the aging process. Bark is sampled each month from the same windrows, and tests are run to determine changes in important characteristics such as pH and pH buffering capacity, particle size, bulk density, wettability, moisture retention characteristics, seedling toxicity, nutrient availability, and more. The results from this study should help to answer some of the questions about optimal utilization of pine bark substrates during the aging process.

Written and photo by Laura Kaderabek