Making smart use of milling byproduct

Departments - Case Study

A half million dry ton of Douglas fir bark is an annual byproduct of the milling process in Washington state.


Smart Bark is produced by heat treating discarded Douglas fir bark, a byproduct of the milling industry. Photo: After Image Visual Services

A half million dry ton of Douglas fir bark is an annual byproduct of the milling process in Washington state. It is the most readily available bark in the Pacific Northwest, where it is commonly used for decorative purposes, burned as “hog fuel” or becomes a disposal issue for mills.

Recent research has shown that heat treating tree bark to produce a “Smart Bark” may serve as a suitable substitute for vermiculite and/or perlite. The bark is first screened to remove any debris and wood particles. It is then ground into uniform sized particles. The bark is then ready to be heat treated.

The bark represents an organic material devoid of toxic resins and low moisture content after the heat treatment. The treated bark could serve as a natural cellulosic sponge capable of absorbing and retaining water and plant nutrients.

Research conducted by Washington State University’s Wood Materials and Engineering Laboratory and Department of Horticulture and Landscape Architecture, has shown that heat-treated and nutrient infused Douglas fir tree bark typically performed similar to peat-perlite and peat-vermiculite growing media in the production of marigolds, impatiens and peppers.

Infused fertilizer, alone, in the experimental barks appeared not sufficient to produce a salable crop of marigolds, peppers or impatiens when evaluated over an extended growth period. However, with the addition of top-dressed macronutrients, the potting media made from the treated barks typically performed similarly to standard mixes used by commercial growers.

Typically the 50 percent experimental bark plus 50 percent peat moss generally outperformed plants in the other experimental bark mixes. Based solely on its water holding capacity, compared to that of vermiculite and perlite, the Smart Bark product appeared comparable.

For more: Plant Care Technologies Corp.; (208) 892-0113;

Authors: Ken Duft is professor emeritus, and Yihao Jiang is a student intern, Washington State University, School of Economic Sciences,