For two consecutive harvest seasons, Ajay Nair and his team of researchers have unearthed an unusual vegetable from Iowa soil—the warm weather-loving sweet potato—confirming that the southern crop can be grown successfully in the Midwest.
In fact, the productive yields of these trials at the Iowa State University Horticulture Research Station north of Ames lead Nair to recommend that growers first identify a market before expanding their acreages to include sweet potatoes. He also suggests curing methods to preserve the vegetable for sale through the winter months.
Nair is a vegetable production Extension specialist and assistant professor at the ISU Department of Horticulture. He has received financial support from the Leopold Center for Sustainable Agriculture to establish his research program, which focuses on vegetables suited to Iowa's soil and climate types.
Before you start growing this crop, make sure you first find a market for it
Sweet potatoes are typically grown in the South, where the hotter climate and long growing season suits the plant's needs. But Nair has found that by using black plastic mulch and drip irrigation, three cultivars of sweet potato—the Beauregard, Covington and Evangeline—produce yields very close to the national average of about 20,000 pounds per acre and, in one case, exceeds this.
"If you're growing half an acre, or let's say one-fourth of an acre, you have 5,000 pounds! So, before growing, find a market," says Nair, adding that either the grower or buyer should have the ability to cure and store the produce so it doesn't go to waste.
Is that a hard sell? Not at all—sweet potatoes are rich in beta-carotene and other valuable nutrients, and are also high in sugar, and thus delicious. Additionally, like potatoes and onions, the sweet potato is "not a perishable crop." If the yield is cured for 10 to 14 days at 85 degrees F and about 90% relative humidity—and then stored at about 55 degrees F—Nair has found that the crop will last six to seven months. "The farmer can harvest in the fall and then slowly sell it throughout the winter."
You can harvest this crop in the fall and then slowly sell it throughout the winter
Nair points out that growing sweet potatoes in Iowa has the added appeal of being a niche product, since the big sweet potato producers are all out of state—North Carolina, Louisiana and California. He recommends that farmers appeal to buyers interested in promoting local foods throughout the winter, when fresh produce is scarce. With the right consumer base—through farmers markets, CSAs, restaurants, schools or hospitals—farmers can earn anywhere from the USDA wholesale price of 20 cents a pound to as much as $1 a pound at the farmers market.
Nair has identified several growing and harvesting practices that suit the sweet potato plant's needs, particularly on the commercial scale. The key to success is controlling temperature and moisture—"Black plastic mulch is a definitely a must. That gives you an insurance against erratic weather."
The black plastic absorbs heat, warming the soil while keeping in moisture. It also keeps out weeds, so that apart from an initial weeding between the mulch rows, the sweet potatoes don't need much hoeing or herbicide treatment. Their trailing vines soon cover the ground, making it hard for weeds to germinate.
With some planning, Iowa-grown sweet potatoes could easily become a regular feature of the holiday season
Drip irrigation is also important for growing sweet potatoes in Iowa particularly this year, with unseasonal rains and a very dry summer. "You don't need a lot," says Nair, "just uniform irrigation."
Nair will be comparing his two years of data against each other to learn more about the sweet potato's performance with drip irrigation and plastic mulch to control against climate variables. He is also looking at spacing requirements that maximize sale-able sweet potatoes not too large or too small. And for commercial growers, Nair has found that sweet potatoes can be harvested using a U-shaped undercutter implement attached to a tractor. It pulls the potatoes up out of the soil, where they can then be easily picked up by hand.
So, with some planning, an Iowa-grown sweet potato could easily become a regular feature of the holiday season, says Nair. He shares a recipe for sweet potatoes that can be found at the Leopold Center website. Here's a teaser: Baking sweet potatoes at 350 degrees F activates enzymes within the vegetable that break down its starches to sugars. That's what makes them so sweet.