They come in all shapes and sizes, in hues of golden orange, pale white, and green. Painted, carved, or baked in a pie, pumpkins are cheerful symbols of autumn that are grown in every county of Washington.

Scientists at Washington State University are helping Northwest farmers grow this striking squash more sustainably. Research that wrapped up this fall at WSU’s Northwestern Washington Research and Extension Center in Mount Vernon explored the pros and cons of pumpkin production on biodegradable mulch, a practice that puts less plastic waste in the landfill.

U.S. farmers grow more than a billion pounds of pumpkins annually, and many growers use sheets of polyethylene plastic as mulch, blocking weeds and preventing water loss to ensure their plants thrive. Nationwide, farmers use about a billion pounds of plastic annually. Unfortunately, that plastic ends up in landfills, and in some regions, burned in the fields.

For several years, WSU horticulturist Carol Miles has studied a newer product, soil-biodegradable plastic mulch that can be tilled into the ground after harvest, then broken down by microbes in the soil.

“You don’t need to pull it out of the field and take it to the landfill every autumn,” saving farmers time and money, Miles said.

Over the last two crop years, pumpkins in her experimental plots thrived on the biodegradable mulch.

“Mulch warms the soil by two to three degrees,” Miles said. “We have cool soil in our region, so if we can warm the soil by just a couple of degrees, our plants can grow better and faster.”

Studying diseases on pumpkins

In the Pacific Northwest, pumpkin diseases can be weather-dependent, says WSU plant pathologist Lydia Tymon.

“If the growing season is hot and dry, we have fewer bacterial problems,” she said. “If the season is cold and wet, more problems can emerge.”

Cool and wet, 2019 saw nasty weather for pumpkin rots.

However, Tymon has found that biodegradable mulches don’t affect the prevalence of Verticillium wilt, a common soilborne fungal disease, compared to standard plastic and bare earth in past years.

Extra work when using mulches

Most pumpkins grow along lengthy vines that trail off the weed-blocking mulch, but Miles specifically wanted to see how pumpkin fruit performed atop the plastic. She planted Cinnamon Girl, a cultivar that sets its fruit near the crown, the heart of the plant. That revealed a challenge: the soil-biodegradable mulch stuck to the bottom of the fruit.

“We have a lot of dew in the morning, and we found that if we let the fruit dry after harvest, the mulch would stick more firmly to the pumpkins,” Miles said. “Nobody wants plastic stuck to their pumpkin, even if it’s biodegradable. But if you wipe the fruit before the dew dries, the mulch comes right off.”

Farmers who grow varieties that don’t set fruit on mulch won’t encounter this challenge. For those who do, it means an extra step that they’ll have to weigh against trade-offs for sustainability.

Challenges aside, working with this crop can bring on a happy outlook, Tymon said.

“Pumpkins are cheery,” she said. “They’re this bright color, and really fun to work with.”

Media contact:

  • Carol Miles, professor, Department of Horticulture, 360-848-6150, milesc@wsu.edu