(© Bonnie Schiedel. Originally published in Canadian Gardening, October/November 2005) Everything you need to know to grow your own tasty, nutritionally packed super tubers

You don't usually equate the sweet potato with terms of endearment, but
singer-songwriter James Taylor does just that when he refers to his girlfriend as Sweet Potato Pie--who's responsible for the divine happiness he feels one particular day--in his appropriately titled song "Sweet Potato Pie." I fully understand: sweet potatoes (Ipomoea batatas) are sublime, not only with respect to taste and cooking versatility, but in nutritional value as well. These healthful superstars are an excellent source of vitamin A (in the form of beta carotene, which also offers antioxidant properties) and a very good source of vitamin C, not to mention the dietary fibre and vitamin B6 they provide. And great news: if your region has at least a 100-day frost-free season, you can likely grow them, as an increasing number of Canadian gardeners have discovered. Short-season cultivars (90 to 100 days) such as 'Georgia Jet' and Tainung 65 thrive in home gardens across the country, especially during dry, hot summers. (One gardener in Northampton, New Brunswick, raised a 3.4-kilogram Tainung 65 monster last year.) But while they like it hot, sweet potatoes are no prima donnas. They require little watering, weeding or feeding, and they store well.

Planting and growing
Although members of the morning glory family (Convolvulaceae), sweet potatoes are not sown from seed, but rather from slips, small shoots that grow from the tuber that are then transplanted. While slips are not readily available at garden centres, there are a few Canadian suppliers you can order from--or you can grow your own (see 'Let It Slip').
Sweets can't tolerate frost, so you should plant your slips about the same time you normally transplant warm-temperature veggies such as peppers and eggplants. Harden off as you would other tender plants.
Sweet potatoes prefer light, sandy, somewhat acidic soil (pH 5 to 6.5), but New Brunswick grower Greg Wingate of Mapple Farm notes they do just fine in his clay soil. Plan to plant them in the warmest, sunniest spot possible.
You can help speed soil warming by planting in wide raised beds or rows about 20 centimetres high, and by covering the soil with sheets of clear or black plastic mulch (highly recommended, given the short summers throughout most of Canada) several weeks before you plant; the soil needs to be at least 13°C.
On transplanting day, thoroughly weed the area. Wingate sometimes adds a side dressing of potassium-rich wood ash--which promotes stronger, more disease-resistant root growth (avoid excess nitrogen, which will produce lush, leafy growth at the expense of the tubers)--working it into the soil as he weeds. Cut holes in the plastic about 23 centimetres in diameter, and 45 to 60 centimetres apart; use the soil to anchor the slip, tucking it into the centre of the hole and leaving about two or three leaves above ground. Water well, and provide some shade for the first few days if there's a lot of sun.
Sweet potatoes can also be grown in containers, according to Ken Allan, a grower in Kingston, Ontario. At a minimum, you'll need a 30-centimetre pot--bushel baskets and halfbarrels work well, too. The advantages are that the soil in containers will heat up quickly and you can move them to a warm, sheltered spot as needed. The main drawback is that the soil will cool faster than a bed in the ground, so you need to be vigilant--a soil temperature of 10°C or lower will ruin your crop (below 10°C, harmful rot spots may form, turning your sweets into brown mush during storage). You'll also need to water more often, as the roots can't reach deep into the ground for moisture as they do in regular beds.
Regardless of your planting location, growth may seem slow at first as the slips take root. Then, toward midsummer, the vines will grow like crazy.
Weed thoroughly (but carefully, so as not to damage the young, shallow roots) as the vines begin to spread. The vines may try to root into the bare ground. If you wish, you can lift them to slightly dislodge any roots that may have formed, but it's not critical to do so. Come August, your sweet potato tubers will be growing and swelling.

Harvest and storage
Sweet potato growth slows when the soil temperature falls below 18°C. When the vines turn black after the first frost, harvest your potatoes immediately. Dig them out carefully because the skins are susceptible to bruises and cuts. Leave them outside on the ground to dry for several hours, brush off the dirt, then gently transfer to a box lined with newspaper. Leave them in a warm (27°C), humid place for two weeks. Proper curing is essential to create a layer of suberin (a waxy substance that keeps moisture in and helps to heal any nicks or cracks) in the skin. Then store at room temperature for up to a year. "[The flavour of] a wellgrown sweet potato actually improves in storage," says Wingate. "The starches gradually convert to sugar."


Tuber troubleshooting
Canadian gardeners are far from the sweet potato pests and diseases that traditionally affect growers in the rest of the world. If you experience any problems, it's likely to come from critters: mice may munch on the sweet tubers below ground; deer and rabbits on the leaves above ground. One way to deter rabbits is to spray the plants with a solution made from three or four chopped garlic cloves soaked in two tablespoons (30 mL) of mineral oil, then combined with a pint (570 mL) of water and one tbsp. (15 mL) of fish emulsion. Dilute one part mixture with 10 parts water; refrigerate unused portion.
If deer are an issue, try hanging bars of strongly scented soap around the perimeter of your garden.

Grower Greg Wingate of Mapple Farm in New Brunswick, who ships slips throughout Canada and the U.S., notes that on arrival, slips may look a little sad--yellowed or browned--but they'll soon perk up if properly cared for. If you can't plant them immediately, place the slips in a clear jar and cover the rooted area with room-temperature water. You can also temporarily plant them in a seed flat and keep them in a warm place with light until you're ready to go.

Yam, not! Are too!
So are sweet potatoes and yams the same thing? No...and yes. True yams (Dioscorea), which are large, starchy tubers, grow only in tropical and subtropical countries. It's thought that African slaves in the southern U.S. began calling sweet potatoes nyami, which was the name for yams in their homeland. To add to the confusion, some American growers of moist, orange-fleshed sweet potatoes have recently begun referring to them as yams to distinguish them from sweet potato cultivars with a whiter, mealier flesh. The latest wrinkle: other sweet potato growers are starting to spell the name as "sweetpotatoes."

Let it slip
Here's how to grow your own slips:
Stick toothpicks around the middle of a sweet potato and suspend it in a jar (or sturdy glass) of water, submerging the bottom half of the tuber (the end that has remnants of previous stems--often the wider end, depending on the veggie's shape--should point up). After about a month, you should have several slips about 20 centimetres long. Remove them with a knife or simply by giving them a twist. Another way to grow slips is to place several sweet potatoes in bed of sand, covered with a moist layer five centimetres thick. Once the shoots start to grow, add an additional 2.5 centimetres of sand, keeping the it moist but not waterlogged and between 15 and 27°C. Slips should reach 20 centimetres in about six weeks. (Keep in mind, however, that supermarket-bought sweet potatoes may not produce slips if they have been kept in cold storage--below 10°C.)

Cultivars available in Canada
B-18 90 to 100 days; orange skin, yellow flesh; excellent flavour (especially immediately after harvesting); good yield, large number of small- to medium-size tubers; may have better resistance to chilling injury
'CARVER' 100 days; copper-coloured skin, orange flesh; excellent taste; very good yield; stores well
'FRAZIER WHITE' 105 days; both skin and flesh are white; excellent yield, small to medium; stores well
'GEORGIA JET' 90 to 100 days; pink skin, deep orange flesh; very good taste; excellent yields, small to medium size; stores well; goof for regions with cooler, shorter seasons
JAPANESE (a.k.a. Japanese yam) 120 days; burgundy skin, white flesh; dryish texture with a sweet taste; medium yield and size; stores well
KOREAN PURPLE 100 days; purple skin, white flesh; milder and less sweet than other varieties, subtle clove-like undertone; excellent yield; stores well
'REGAL' 110 days; red skin, orange flesh; rich, sweet taste, excellent for baking; very good yield, large tubers; average storage length
'SUPERIOR' 100 to 110 days; copper-coloured skin, orange flesh, striking ivy-like foliage; good, sweet taste, slightly mild; very good yield; stores well
TAINUNG 65 90 to 100 days; red skin, yellow, flesh, drier, firmer and milder than others; very good yield, large tubers; excellent storage; good for regions with cooler, shorter seasons
'TOKA TOKA GOLD' (a.k.a. 'Golden Kumara') 90 to 100 days; yellow skin, deep yellow flesh with orange streaks; sweet but slightly dry; medium yield, large size, good for baking