By Frank Giles
Plastic mulch provides growers numerous benefits in establishing beds to grow crops. Some of those benefits include improved pest and weed management, water conservation and better yield and crop quality.
But what do you do with the mulch after growing the crop? It creates an expense for growers to deploy labor to remove the plastic from the fields. Some of that plastic is recycled, but most data indicate that it is a relatively small amount (less than 10%).
That leaves a lot of plastic that must be removed and disposed of in landfills or burned. A team of researchers from various educational institutions have been studying the potential of biodegradable mulch to lessen the environmental impact of plastics and save growers the expense of removing and disposing of the material.
The U.S. Department of Agriculture funded this multi-state project. It officially started in September 2022. The objective is to evaluate the potential of soil-biodegradable plastic mulch as an alternative to traditional polyethylene mulch.
“The project involves interdisciplinary collaboration among several universities and industries and extensive outreach activities,” said Shinsuke Agehara, University of Florida Institute of Food and Agricultural Sciences associate professor and one of the research cooperators on the project. “Strawberry is the model crop in this project. In Florida, we conducted the first-year strawberry field trial during the 2023–24 season, and we are currently conducting the second year trial.”
PI of Mulch Study
Lisa DeVetter, an associate professor at Washington State University, is the principal investigator (PI) of the biodegradable mulch study. She said the team also is looking for recycling of mulches to achieve better outcomes.
“We understand soil-biodegradable mulches may not be a viable option for many operations, and we still have much to learn about their performance across production systems and regions,” DeVetter said. “We are also learning about these mulches’ residence time in soils and building on soil health research. There are field trials across the United States studying this in Florida, California and Nebraska.
“One key barrier for soil-biodegradable mulch adoption is organics. Certified organic growers cannot use currently available mulches as none of the materials meet the program’s requirements (e.g., biobased content requirements, use of prohibitive materials, etc.).”
On the recycling front, the research team is looking at more efficient mulch removal techniques to reduce contamination levels (which presents a barrier to recycling as soil and other contaminants must be removed) as well as advanced recycling techniques that can handle the high contamination load of mulches.
What is Biodegradable?
There are various standards set to be able to call a mulch biodegradable. The EN 17033 standard specifies that all plastics that claim to be biodegradable reach 90% biodegradation within two years in an aerobic incubation test at a controlled temperature between 68 and 82 degrees. However, in-field biodegradation of this mulch may take longer than in the laboratory test because seasonal variation observed in field soil temperature and moisture is not accounted for in a laboratory test.
“Some growers are already using soil-biodegradable mulch. But there are concerns about impacts on soil health, economics, fate and residence time for biodegradable mulch, and a recycling infrastructure that is still developing,” DeVetter added. “The project ends September 2026, but we will request a one year no-cost extension in all likelihood to finish the project, so it will officially end in September 2027.”
Florida Results
In trials conducted at the Gulf Coast Research and Education Center, biodegradable mulches showed similar or even better horticultural benefits compared to standard polyethylene mulches. The trial also showed early-season marketable strawberry yields were higher for white biodegradable mulch than black polyethylene and biodegradable mulches. The higher early-season yields would benefit growers economically due to typically higher strawberry prices during that period.
While early-season yields differed, the study showed that total marketable yields were not significantly different between the mulches over the course of a full season. Agehara said the field trial suggests that mulch colors and compositions may affect strawberry growth, but further research is needed.