Many growers and gardeners regularly use crop residues including straw, hay, grass clippings, or leaves as mulches or cover and green manure crops which can be tilled into the soil. These types of mulches biodegrade well and can add valuable nutrients to the soil, assuming they have not been treated with pesticides, which are harmful to plants and animals. Sometimes these sources provide habitat for rodents or provide weed seed sources, so all materials should be used with caution.
In contrast, so called biodegradable mulches (BDM) have been in use since the 1980s to address the environmental deficiencies posed by the use of plastics. Many biodegradable mulch replacement products are bio-based. BDMs should not have any synthetic materials and are generally derived from natural sources, as opposed to plastics.
BDMs are not petroleum based or have non-bio-based additives and have not been chemically modified. They all use some form of plant starch bonded with polymers or plasticizers derived from renewable crops that binds them together.
To meet organic standards and biodegradability tests, BDMs are supposed to break down into non-harmful constituents, meet the ASTM standard for compatibility with at least 90 percent degradation within 24 months. Currently, paper mulch which meets the National Organic Program standard and deteriorates very quickly, is the only approved BDM that meet the USDA organic standard. However, a number of other products are becoming available which provide improved benefits.
A USDA SCRI (Specialty Crop Research Initiative) BDM multi-year 2015-2019 grant led by a tri-university group of researchers from University of Tennessee, Washington State University and Montana State University has been studying available mulches through field trials at the experiment stations in Mt. Vernon, Wash. and Knoxville, Tenn., testing products and grower perceptions on farms in each state (even near here at an Ephrata farm), determining and assessing BDM environmental and soil impacts and break down components, and evaluating costs and adoptability. Many of the tested products are available commercially. To learn more about the studies, costs and benefits for gardeners and farmers or to try a product go to ag.tennessee.edu/biodegradablemulch/Pages/usdascriproject.aspx
For answers to gardening questions, contact the Master Gardeners at the WSU Grant-Adams Extension office at 754-2011, ext. 4313 or email your gardening questions to firstname.lastname@example.org. Visit our web page at grant-adams.wsu.edu.