Steps for Successfully Growing Sweetpotatoes in Washington

Plastic Mulch and In-row Spacing Effects on Sweetpotato Yield in

Northwest Washington

by Carol Miles and Srijana Shrestha

Mount Vernon, WA – Sweetpotato (Ipomoea batatas) production in the northern United States is limited due to the perceived barriers of a short growing season and relatively cool summer temperatures, yet recent studies have shown yield in northern regions can be greater than the national average when sweet potatoes are grown with plastic mulch. A study was conducted in northwest Washington to evaluate the productivity of ‘Covington’ sweet potato with polyethylene (PE) and soil-biodegradable (BDM) mulches and different in-row spacings (20, 30, and 38 cm) in 2019.

The objective of the 2019 experiment was to ascertain if sweet potatoes could be suitable for production in northwest Washington. Specific objectives were to 1) compare the impacts of PE mulch and BDM and different in-row spacings on productivity in northwest Washington, and 2) identify potential pest threats of sweetpotato production in this region and evaluate resistant accessions to manage the pest problem. Although many of the sweetpotato slips arrived for the experiment in poor health due to travel issues, sufficient slips were successful to determine at the conclusion that wireworm proved to be the only pest that severely affected sweetpotato production, damaging 98% of storage roots.

The objective of the 2020 experiment was then to evaluate wireworm-resistant accessions. Eight wireworm-resistant sweetpotato accessions were received from the USDA, Agricultural Research Service. Plant material arrived as tissue culture plantlets and on arrival were washed and transplanted into sterilized 4-inch pots filled with autoclaved potting mix (same product as in 2019) that was lightly moistened and all placed into a humidity box. Two accessions did not survive, and the remaining six accessions were propagated by vine cuttings in the greenhouse. The six accessions were transplanted in the same field where sweet potatoes were grown in 2019. On average only 16% of the crop was damaged by wireworm.

Both PE and BDM mulch increased soil temperature and led plants to become established and grow more quickly. Mulch type did not significantly affect total yield but the yield was higher with PE and there were seven times more jumbo sweet potatoes with PE mulch. One disadvantage of PE mulch is that it must be removed prior to harvest, while BDM mulch does not need to be removed or disposed of at the end of the season.

The study demonstrates there is potential to grow sweet potatoes successfully in northwest Washington for local fresh and processing markets. However, there are several limitations to production that must be addressed: sourcing a regional, reliable supply of healthy sweet potato slips; developing techniques to reduce wireworm damage; including genetic resistance; and the ability to increase field soil temperature.

The full article is available on the HortTechnology electronic journal website at:

‘Covington’ sweet potato grown with polyethylene and soil-biodegradable plastic mulch at three plant spacings (20, 30, and 38 cm) (left) and without mulch as a non-replicated reference plot at 38-cm plant spacing (right) at Washington State University Northwestern Washington Research and Extension Center, Mount Vernon, WA on 19 Aug. 2019; 1 cm = 0.3937 inch. (Photos by Carol Miles)

Carol Miles is a Professor in the Department of Horticulture at Washington State University, and is the Director of the Washington State University Mount Vernon Northwestern Washington Research and Extension Center. Carol is the Horticulture Specialist and specializes in vegetable crop production as well as cider apple production, and has a strong interest in alternative crops and organic production.

Srijana Shrestha is a PhD graduate research assistant with Carol Miles at Washington State University, Northwestern Washington Research and Extension Center.

Established in 1903, the American Society for Horticultural Science is recognized around the world as one of the most respected and influential professional societies for horticultural scientists. ASHS is committed to promoting and encouraging national and international interest in scientific research and education in all branches of horticulture.

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