Exploring Biodegradable Plastic Mulches in Red Raspberry

Raspberry growers in Washington are increasingly using tissue culture (TC) plugs as their choice of planting material. TC plugs are generated by micropropagation through aseptic techniques, which produces planting materials more quickly and with less space than traditional bare root canes and root cuttings.

Tissue culture ‘Meeker’ raspberry planted with a biodegradable plastic mulch. (Photo: Lisa DeVetter)

Additionally, the use of proper aseptic techniques creates virus- and disease-free planting materials. Some cultivars of raspberry preferred by growers are exclusively produced by TC, which has been a large driver in the increased acreages of raspberry planted as TC plugs.

Considerations with TC
Despite some of the benefits of TC, resultant plugs are more expensive on a per-plant basis. They are also more delicate and difficult to establish compared to bare root canes and root cuttings. This is because TC plugs compete poorly with weeds and have foliage at the time of planting, which increases the risk of phytotoxicity and plant death due to herbicide misapplication.

Growers using TC are in need of affordable techniques to enhance TC establishment through improved weed management and promotion of crop growth. New research funded by the Washington Red Raspberry Commission and Washington Commission on Pesticide Registration will be evaluating biodegradable plastic mulch application in raspberry and may provide growers with the information they need to improve TC establishment.

Biodegradable plastic mulches (BDMs) are manufactured as alternatives to polyethylene (PE) plastic mulches. Ideally, BDMs should function similarly to PE mulch in their application and provision of horticultural benefits, such weed management, conservation of soil moisture, and soil temperature moderation. However, they are engineered to biodegrade in soil or composting conditions after their use.

Looking at Alternatives

Dr. Carol Miles, Professor of Vegetable Horticulture at the Washington State University Northwestern Washington Research and Extension Center (WSU NWREC), has been doing research on alternatives to plastic mulches in vegetable production systems for more than 10 years. She is involved in the new research project that will be testing plastic BDM application in raspberry.

“While agriculture produces only a very small portion of the plastic waste produced worldwide (about 1% of the total), if biodegradable plastic mulch can be tilled into the soil after use without causing environmental harm, it can serve as a model to help develop plastics that can safely biodegrade in soil,” Miles says. “Currently, a significant amount of plastic is thrown into the environment, and 80% of the plastic that is in the oceans comes from being thrown away on land; if those plastics were biodegradable in soil, they would not end up in the oceans.”

What are the research needs before these potential promising tools can be recommended? PhD student Shuresh Ghimire in Dr. Miles’ program has been researching BDM application in pumpkin and sweet corn.

“While we have evidence of equal vegetable yield and quality with biodegradable plastic and PE mulches, farmers are concerned about mulch biodegradation in the field post-soil incorporation and its impact

‘Meeker’ raspberry after 10 months of growth with a biodegradable plastic mulch. In this preliminary, non-replicated trial, raspberry grown with the biodegradable mulch was on average 9.6 inches taller and produced 7 more canes compared to the non-mulched control. (Photo: Lisa DeVetter)

on soil health and productivity of subsequent crops,” he says. “Since we currently lack a standard method to measure mulch biodegradation under field conditions, we are working to fill this information gap.”

“There are more specific research needs for plastic BDM application in perennial systems like raspberry, as much of the previous research on BDMs has focused on annual vegetable cropping systems and these products have not been widely tested during overwintering conditions,” adds Huan Zhang, M.S. student in Horticulture at WSU.

Future Research
For one, the persistence of BDMs in the raspberry system needs to be tested. Because they are biodegradable, they should ideally be gone within six months in spring planted systems and 12 to 18 months in fall-planted systems. If the material biodegrades too rapidly, then the horticultural and weed management benefits may be lost.

Plastic BDMs should also be compatible with soilborne disease management. In a previous study, British Columbia researcher Eric Gerbrandt found in TC raspberry planted in 2013 that growth was improved under non-degradable PE mulch, but populations of root lesion nematode increased.

Dr. Inga Zasada, USDA-ARS plant pathologist and nematologist in Corvallis, Oregon, who is also involved in BDM research in raspberry, explains, “We know plant-parasitic nematodes are a major production constraint for raspberry growers.  Not managing nematodes is risky, therefore growers rely on soil fumigation. Further research is needed to help raspberry growers transition to new ways of applying fumigants and also how to manage nematodes in an evolving system that integrate BDMs.”

Applying biodegradable mulch on a raspberry farm. (Photo: Lisa DeVetter)

BDMs are also relatively expensive. While BDMs use could reduce growers’ cost of production due to reduced herbicide applications and hand weeding during establishment, as well as the cost of labor to remove and dispose of mulch, it is important for growers to have a comprehensive understanding of the forecasted economic ramifications of BDM use for their operations.

This is where WSU economist Suzette Galinato comes in on another proposed project that seeks to better understand the economic impacts of BDM use in raspberry.

“Growers look for ways to improve profitability.  On-farm adoption of BDM will depend on whether or not benefits outweigh the costs of adoption, and by how much adoption affects the grower’s bottom line.  In our proposed project, we will estimate the costs and benefits of mulching practices in a TC raspberry system, which we expect to aid in the grower’s decision-making.”

Despite the information gaps, which are currently being addressed by this research team, some raspberry growers in Washington are already adopting BDMs.

“Biodegradable plastic mulch film has been a great tool for me in the last two seasons of using it in the establishment year of my raspberries,” Randy Honcoop, a raspberry grower in Lynden, WA, says. “I needed help controlling weeds, especially as I’m now using tissue culture plugs for planting material. Using the BDM film allows me to get by with no herbicide application on the rows, as well as providing outstanding control of the moisture levels in the soil around the new plants. It does appear that I’m also experiencing increased growth in the establishment year, probably due to the soil warming that the film provides. I would hate to be without it.”

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