Bio-based, plastic or fabric mulches for the garden

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Mark Amara/Courtesy photo - Biodegradable mulches being installed in a field near Ephrata this past in May.

Plastic mulches, made from non-renewable petroleum based sources, have been available since the 1960s. These mulches form an impermeable barrier that helps reduce weed competition, conserve water, minimize soil and water erosion, increase yield and crop quality, and helps crops mature earlier. However, these types of mulches do not transmit water, may create an adverse microclimate or very hot conditions especially in summer heat, and should be disposed of offsite since they do not break down and have questionable environmental effects. 

Determining how and when mulches are removed after the growing season affects labor costs. Since plastic mulches have limited recycling options, they are routinely taken to landfills or burned which can release harmful chemicals into the environment and air or left out in the field where they break into pieces, blow or fall apart but do not deteriorate. Residual plastic left in the field or elsewhere does not break down and negatively impacts wildlife and water quality.

Fortunately, there are other alternatives to plastic. Fabric mulch or other woven fabrics and naturally occurring mulches have application for gardeners and farmers alike. Though fabric mulches do not break down in the soil and should be removed at the end of the season, they do transmit air and water and can serve as excellent weed, insulation, and moisture control barriers. They can be cleaned and reused for multiple years which can cut down on disposal costs.

Many growers and gardeners regularly use so called crop residues including straw, hay, grass clippings, or leaves as mulches. These types of mulches biodegrade well and can add valuable nutrients to the ground, assuming they have not been treated with pesticides which are harmful to plants and animals. But these sources can provide habitat for rodents or provide weed seed sources, so all materials should be used with caution.

In contrast, so called biodegradable plastic mulches (BDM) have been in use since the 1980s to address the environmental deficiencies posed by the use of plastic mulches though few use them.

Many biodegradable plastic mulch replacement products are bio-based. True BDMs do not have any synthetic materials and are derived from natural sources. These products are not petroleum based or have non-biobased 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 holds them together. To meet organic standards and biodegradability tests, they must break down into non-harmful constituents; meet the ASTM standard for compostability with at least 90 percent degradation within 24 months. 

Currently, other than plastic mulch which has to be pulled up on organically certified fields every season, and paper mulch which meets the NOP standard and deteriorates very quickly, there are no approved BDMs that meet the USDA organic standard (OMRI). But there are products available in Canada and Europe that carry the biodegradable identity and/or other products that claim to be biodegradable. Gardeners can choose which products to use and may wish to consider what the environmental pluses and minuses are.   

For answers to gardening questions, contact the Master Gardeners at the WSU Grant-Adams Extension office at 754-2011, Ext. 4313 or email questions to ga.mgvolunteers@ad.wsu.edu. Visit our web page at http://grant-adams.wsu.edu.

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