Growing Sweet Potatoes in Cold Climates
Learning How to Grow Sweet Potatoes Can be Fun and Delicious
America’s favorite candied root vegetable has an ancient history of warding off famine. Sweet potatoes were domesticated in South America over 5,000 years ago but other civilizations began growing sweet potatoes long before European exploration.
Captain James Cook found a variety in Polynesia and brought samples back to London; researchers have since dated the genetic makeup to about 1,000 A.D. It is believed Polynesians sailed to Peru or Ecuador and brought the plant back, a theory supported by linguistics. “Kūmara,” the Maori word for sweet potato, closely resembles “kumar” in the Peruvian native language Quechua.
Sweet potatoes entered China in 1594 in response to crop failure. And though Europeans considered them an exotic delicacy, they took hold within cuisine before the standard “Irish” potato did. In Africa, where the fluffy white variety is most popular, health officials strive to convince locals to grow the orange strains to prevent childhood blindness. The efforts have been well received by locals and doctors, since growing a more colorful crop is easier than trekking to remote villages to distribute vitamin A capsules.
Now sweet potatoes are eaten in many forms around the world, such as a dried chips dipped in peanut sauce in Uganda, a soup in China, flour for baked goods in Kenya, as a pickle in India, noodles in Korea, served for breakfast with sambal and coconut in Sri Lanka, and as a sweet dessert in Malaysia and Singapore. Leaves and vines are eaten as a vegetable in West Africa and Taiwan. Because they grow well in the American South and were associated with survival during famine, they fell out of popularity with affluent society. After the Great Depression, per-capita consumption fell from 29 pounds per year down to 3-4 pounds. It’s most commonly eaten as French fries, in sweet potato pie, and candied for Thanksgiving dinners.
A Potato by Any Other Name
Sweet potatoes vs. potatoes. Sweet potatoes are only distantly related to commonplace starchy russets or Yukon golds, also known as “Irish” or “English” potatoes. In fact, sweet potatoes are a member of the morning glory family while standard potatoes are nightshades. True yams are rough-skinned tubers, related to lilies and sometimes toxic while young or in wild varieties. Western markets have tagged some sweet potatoes with the deceptive name “yams” to differentiate the southern crops from northern varieties. While “Irish” potatoes and true yams can be toxic, all parts of the sweet potato are edible. Orange varieties are high in beta carotene and vitamin A while purple tubers contain large amounts of cancer-fighting anthocyanins. The foliage contains more vitamin C and folate than the roots. A low-glycemic food, they are usually a good choice for diabetics who crave a little sweetness. Roots are high in fiber and contain no cholesterol until you add butter or cream.
It Grows Like a Weed…
Propagating from stem or root cuttings, or from slips (leaves sprouting from the tuber), sweet potatoes can be a continuous crop in tropical areas. A single plant flourishes and reproduces, allowing families to dig up only what they need. They grow so well that some gardeners take care not to let clippings fall on fertile soil lest they sprout and spread. Though they tolerate most soils, they prefer acidity between 4.5 and 7.0. Optimal conditions for growing sweet potatoes are sunny locations with warm nights and well-drained soil with a temperature of about 75 degrees. Poor soils should be amended with lime and natural fertilizer such as compost or manure, as sweet potatoes are extremely sensitive to aluminum toxicity. Vine cuttings easily root in water or moist soil. After the plant takes hold, leaves soon cover the ground and make most weeding unnecessary.
Except Where it Doesn’t.
Because sweet potatoes simply cannot withstand frost, northern gardeners must be vigilant of the weather. They grow shorter-season varieties which produce in four months instead of nine. A growing season shorter than 120 days is simply not long enough for any variety. Though they are rarely grown as a staple crop above zone 9, they can still provide a unique and satisfying addition to your table.
Acquiring Plants and Cuttings
Don’t attempt to smuggle cuttings back from Hawaii. If you’re lucky, the nice lady at the agricultural inspection station will simply chastise you while confiscating your sweet potatoes. Your trip could be delayed and you could pay fines up to $1,000. If the plant makes it safely back to your home state, you could unleash parasites into your local soil.
Option #1: Buy sweet potatoes from your local grocery store. Select organic tubers, since conventionally grown stock is sprayed with chemicals to prevent sprouting. Insert skewers around the circumference of the tuber, near the middle. Suspend in a mason jar of at least a quart size and fill the jar with water. Place in a sunny window or under plant lights. Within a few weeks root buds will form under the water and slips will sprout on top. When the slips are over two inches in length, gently pluck them off and either suspend them in a small container of water or insert directly in moist soil, leaving at least half of the slip above the medium. The entire tuber may also be planted, but this usually results in growth of the original root and not as many new tubers as if you plant each separate slip.
Option #2: Obtain vine cuttings from another local gardener. This can be done just before frost, or while the gardener harvests his own sweet potatoes, and kept alive through the winter in a warm room with strong plant lights. Insert the cut end directly into a large mason jar of water, using a barrier at the lip to keep the cutting suspended. A coffee filter works well because it doesn’t disintegrate with moisture and can easily be cut away from the vine. Place the jar in a warm, sunny location. Roots should form within a couple days. Once the vines are well rooted, plant in loose, moist soil. With care, the plant will flourish and you can take cuttings from it to propagate more plants.
Option #3: Purchase from a seed company. This offers more varieties, such as the coveted Hawaiian strain Molokai Purple. Reputable companies obtain plants from tissue cultures to avoid parasites and diseases, selling a clean specimen to customers. Most sweet potato slips and plants may not be shipped until April to avoid cold damage. Be ready when the package arrives. Bring it inside and open immediately, inserting slips or roots in tepid water and setting them in a sunny location. Do not immediately plant them outside, even if you live in zone 9, because they were probably raised in a greenhouse and need to be hardened off.
Growing in Cold Climates
Articles on how to grow potatoes won’t help you here, because the crops are different from planting to harvest. But growing sweet potatoes can be done even up in Alaska. Plant in full sun. If it’s still cold outside, keep your babies in a greenhouse or a sunroom. Use strong ultraviolet lights if necessary. Introduce plants gradually to the outside to harden them off.
Maintain an ambient temperature above 60 degrees for the foliage and between 70 and 80 degrees for the roots. You can achieve this several ways. Install a greenhouse thermometer to keep the ambient temperature at 60-70, which your tomatoes will love, but use additional heating methods for small cups of soil. Use a warmer, such as a heating pad covered by a waterproof barrier, and set containers atop. Black plastic draws sunlight toward the soil. If you heat your greenhouse, give sweet potatoes the coziest spot. Transplant as necessary because the roots grow fast. If you move them outside during the day, carry them back in during cold nights. Do not set out permanently until all danger of frost has passed. In zones 5 to 8, cover soil with clear or black plastic a few weeks before planting outside. Test soil temperatures then cut x-shaped slits in the plastic and insert plants directly through the holes into the ground. Fold flaps back around the stems and tack down with rocks or landscape pins. This keeps the soil warm and moist while foliage flourishes in the cooler air. If the summer becomes a scorcher and plants wilt, spread a thin layer of light-colored straw over the plastic, beneath the foliage. Keep soil moist but not wet. Do not apply much fertilizer because this will encourage foliage growth instead of large tubers.
Cold and erratic climates may require that you use containers for growing sweet potatoes and save the ground space for growing beets. Twenty-five-gallon planters, painted dark colors and set atop concrete or blacktop, draw in heat. Cover the soil with plastic, as described above, and insert plants through slits. Is your location still too cold for growing sweet potatoes? Keep plants in a greenhouse through the entire growing season. Use large, dark containers such as plastic storage totes with drainage holes drilled in the bottom. Greenhouses can nurture your plants when snow falls outside. Whether in the ground or in containers, a stainless steel meat thermometer can gauge soil temperature. Choose the largest possible container size. Smaller containers result in small, cramped roots.
If you wish to continue a rare variety next year, take cuttings before the frost hits and propagate roots as described above. Ignore advice about leaves dying back, because these are not “Irish” potatoes. Leaves will not die unless frost hits or soil becomes inadequate. Judge tuber maturity by the variety you planted and time spent in the ground. Some gardeners claim a nip of frost sweetens the roots as it converts starch to sugar. Whether harvesting before frost or after, pull back plastic and gently loosen the soil with a spading fork or a shovel, digging at least eighteen inches away from the plant to avoid cutting the tubers. Pull the plant straight up. Sift through the soil and remove both large and small sweet potatoes, as both sizes cook up well. Lay roots on the ground for a few hours to dry then cure them in a warm location for up to two weeks. Similar to food preservation methods for onions and squash, store in a cool, dry location and enjoy!
Varieties to Try
Beauregard: A heavy yielder in a short time, this popular orange variety has short vines and tubers that grow close to the stem, making them a good choice for growing sweet potatoes in colder climates and containers. Slips are available through many online seed suppliers.
O’Henry: This one is lightly sweet and dense, cream-colored inside and out. Developed from a mutation of Beauregard, it bears the same benefits of heavy yields within short seasons.
Toka Toka Gold: Also known as Golden Kumara, this New Zealand variety has yellow skin and yellow flesh streaked with orange. Expect small yields of large, sweet, dry tubers. Locating slips may be difficult. Search for specialty companies online.
Okinawan: Tubers can be found in Asian markets, but it’s debatable whether those are organic. This variety is white-skinned, with lavender flesh streaked with stunning purple. Not the best choice for growing sweet potatoes in containers, Okinawan has long roots that intertwine and bind up. Delicious and impressive if you have the space.
Molokai Purple: Good luck finding this one in a grocery store. Plants must be ordered from rare seed companies or obtained from dedicated gardeners. Deep purple stems sprout leaves of dark green tinged with an aubergine hue. Royal purple tubers grow long and thin without becoming rootbound. Whether you live in zone 9 or below, or way up in zone 4, you can learn about growing sweet potatoes and enjoy this nutritious treat within your own little homestead.