The world of wood fibers—part one

With the ongoing drive to reduce the usage of peat in horticulture, interest in and use of other materials has increased. “Classic” non-peat inorganic materials such as perlite, loam, sand, and grit are only used in low percentages due to price and potentially undesirable physical properties. In addition there are three other products that are organic materials—widely used as professional growing media—coir, bark, and wood fiber. 

The wood component, most often referred to as “wood fiber,” was first used in the 1980s–1990s before becoming popular in the last decade. At the outset, very little was known in terms of handling requirements for such growing media components and mixes. Increased demand for wood fiber in horticultural container production has acted as a catalyst for thorough scientific studies and trials culminating in more in-depth knowledge of how to manage these mixes. 

As you would expect, wood fiber means fibers of wood but is a general term that does not specify:

• The species or mix of wood species used to create the fibers 
• The age or the parts of the trees (cuttings, branches, timber by-products, whole trees) 
• The mechanical manufacturing process using machines such as extruders, hammer mills, disc refiners, and knife ring flakers 
• The chemical manufacturing process—altering the C:N ratio by N fortification (impregnation) 
• The physical structure and inherent chemical nature of the product 

Significant variation

As a result, commercial wood fiber products from across the globe can vary significantly in physical and chemical properties—pH, porosity, water and nutrient holding capacity, and potential toxicity depending on the age and manufacturing process of the material.  

For instance in the US, yellow/loblolly pine (Pinus taeda) is mainly used, while in Europe European species of pine (P. sylvestris, P. maritima, P. pinaster, P. nigra) are the main sources. There are reports in the literature of Larix (larch) and Picea (spruce) species also being used for wood fiber production, as well as some anecdotal evidence of waste material from commercial wood products and crops being used to produce wood fiber for the horticultural market. 

To date, there have been over two dozen different hardwood and softwood tree species evaluated for their potential use as growing media components. Most have been found to be unsuitable due to poor plant growth, availability, cost, processing issues, or other reasons. 

Peat-based mixes containing 20–30% wood fiber can constitute high-quality growing media which can reliably produce good plant/crop growth in a variety of plant species. Peat works as a “blank canvas” and can form exceptional growing substrates on its own, as well as in combination with coir, bark, and wood fiber. The major challenge facing the growing media industry is the creation of peat-free mixes.