OSU Master Gardener goes sweet on potatoes

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Clear plastic provides the heat and humidity sweet potatoes need to thrive in the Willamette Valley. Photo by Gary Jordan.
Last Updated: 
May 1, 2015

EUGENE, Ore. – When he was 9 years old and riding with his brother on the back of a two-seater tractor on 30 acres in southern California, Gary Jordan planted sweet potatoes one at a time.

“It took us weeks to do,” said Jordan, a volunteer Master Gardener with Oregon State University’s Extension Service. “I’ve probably planted more sweet potatoes than anyone you’ll meet. I have great respect for sweet potatoes.”

The Eugene resident graduated from Extension’s Master Gardener program in Lane County a year ago and immediately put in a crop of his favorite vegetable. He entered a local contest and pulled in first and second place awards and soon was giving talks to packed rooms.

“People really want to know how to grow sweet potatoes,” said Jordan.

While few people may think to grow sweet potatoes in Oregon because the climate differs so much from the commercial sweet potato hub of North Carolina, Jordan wants to change that.

“Everyone says it’s too cold here to grow them,” he said, “but I want to show that’s not true. Sweet potatoes are a super food, one of the most nutritious vegetables. People want that, and what’s better than growing them, knowing what you’re eating and exactly where they come from? That’s a pretty good deal.”

Sweet potatoes need a lot of heat and humidity to grow, according to Jordan, and a little plastic makes all the difference in the Northwest. He uses clear plastic sheeting to cover the soil around the plants, a technique that keeps roots warm and sweet potatoes growing. The other essential is rich, well-draining soil. Mixing in compost or rotted manure before planting is a must.

Other than that, sweet potatoes need little else while they’re in the field. If the soil is fertile, Jordan said, there’s no need to fertilize. Especially don’t use a fertilizer high in nitrogen, which will cause foliage growth to the detriment of root growth. And the roots – or tubers – are what you’re after.

The trickiest part of growing sweet potatoes, Jordan said, is the curing process. And you have to cure them if you want sweet potatoes to taste sweet.

“The key is curing,” he said. “If you don’t, you don’t get the transition of the starches to sugar. Once cured, the flavor increases greatly.”

After they’re cured, Jordan likes to eat his sweet potatoes baked. Not only do they taste great, he said, but they also have the most nutrition when cooked whole in the oven. Not that he’s adverse to a sweet potato fry now and then.

What most people don’t know is that sweet potatoes and yams are not interchangeable. In fact, they’re from completely different plant families, Jordan pointed out, and yams are rarely grown in the U.S. The lighter-colored tubers you see in the grocery store are just a different variety of sweet potato.

“Most of us will never see a yam in our lives,” he said.

But everyone sees sweet potatoes, he said, and should eat plenty of them.

Gary Jordan Offers the Following Tips for Growing Sweet Potatoes

  • Choose an area in full sun.
  • Mix about 1 part compost or composted manure into 2 parts soil until good and fluffy. No need to add commercial fertilizer. Mound soil into a slight ridge 12 to 24 inches wide.
  • Cover soil with clear plastic pulled smooth and tight. Secure edges of plastic well with bricks, wood, U-shaped soil staples or rebar placed flat. It’s important to keep the edges sealed to get as much heat as possible.
  • Let soil warm until it gets to about 80 degrees. Use a soil or compost thermometer to check on the progress. Don’t worry about getting the soil too hot; sweet potatoes thrive in conditions above 100 degrees.
  • Make slits in plastic about 15 inches apart (more if you want very large sweet potatoes). Pull back each slit to create a 12-by-12-inch hole and make a depression. Plant sweet potatoes, which are called slips, into depressions.
  • Fill up planting holes with construction sand for drainage and to draw in heat.
  • Water late in the day when soil is at its warmest. Ideally, use a soaker hose under the plastic. To get the fastest growth, use water warmed by the sun in black plastic tubs.
  • Harvest carefully in fall when ambient temperatures drop into the 60s. Bruises, gouges or scratches can quickly lead to rot.
  • Cure immediately. Place in a warm, humid spot – 85 to 90 degrees – for five to seven days. Suggestions include a greenhouse or a small, heated room with a humidifier or a bucket of water.
Author: Kym Pokorny
Source: Gary Jordan