If people had told me seventeen years ago that you could grow sweet potatoes in Canada, I’d have thought they were pulling my leg. At the time, my gardening mindset placed sweet spuds just this side of pineapples and bananas—way out of range for Maritime gardeners. After all, didn’t supermarket sweet potatoes hail from places like Louisiana and Tennessee?
But in 1985 an article by a New England gardener caught my eye. The author explained how she’d been easily raising sweet potatoes for years and that trials at Cornell University in upstate New York showed some promising new short-season varieties. I couldn’t resist trying them out for myself and have been happily growing sweet potatoes ever since.
There are many reasons why. Sweet potatoes are fairly undemanding. They tolerate drought, love heat, are light feeders, store like a dream, and when home-grown, taste sensational. Sweet potatoes pack loads of nutrition, containing high levels of carotenoids such as beta-carotene. For northern organic growers, they’re a great choice because we’re far away from the natural range of pests and diseases that attack sweet potatoes. I suggest that they’re worth a try in areas with at least a 90-day frost free season, essentially any garden where mid-season tomatoes can ripen before frost hits.
What’s in a name?
Despite these fine qualities, sweet potatoes suffer a lot of abuse in the nomenclature world. You’ll notice that I refer to “sweet potatoes” all in one word. As if the confusion over yams isn’t enough of a trial. Some commercial sweet potato producers name the moist, orange-fleshed cultivars they grow as “yams” to distinguish them from the drier, white-fleshed types. Botanically speaking, yams (Dioscorea genus), a tropical crop, aren’t even related to sweet potatoes (Ipomoea genus, the morning glory family) and neither are potatoes (Solanum genus). So, the thinking goes, the recent move to “sweet potatoes” instead of “sweet potatoes” helps to differentiate our favourites from the common spud.
Growing sweet potatoes
Sweet potatoes are not started from seed or cut pieces of tubers. They are propagated by small plants called slips. Usually, you’ll want your slips to arrive in the mail after the weather has settled to the stage when peppers and eggplants are normally set in the garden. At Mapple Farm, we’ve always tried to ship slips at the proper time; that is, when we’ve thought our customers needed them. This can be tricky especially when planting zones can vary so dramatically within relatively short distances (e.g. in B.C.). Also, a gardener’s requirements may differ greatly depending on whether a greenhouse or other protective techniques are used. Customers who know when they want slips, fill in the “When To Ship Slips” line on our order form. Otherwise, we’ll decide when it’s best to send them. Once you have grown your own sweet potatoes, you can consider saving your own slips.
The slips roughly resemble young tomato plants and are transplanted in the same way to the field or garden.
The planting site should be the warmest, most sunny, and weed-free place you can provide. To warm the soil even more, cover the beds with plastic mulch 2–3 weeks before planting. Best results come from placing the slips every 45–60 cm (18–24”) in raised rows on a cloudy day or late in the afternoon. The loosened earth of raised rows allows root systems to establish quickly and, later on, it allows tubers to expand more easily. Ideally the soil should be slightly acidic (pH 5.0–6.5) and light. Sweet potatoes are light feeders; they like a little phosphorus and a little more potassium. A sprinkling of greensand or wood ash is helpful. Avoid excessive nitrogen; this will promote vine growth at the expense of the tubers.
Like tomatoes, I put most of the plant underground and allow just 2 to 4 leaves to remain above the surface. For awhile, the plants might look like they aren’t doing anything, especially if it’s still cold outside. The frost-tender sweet potatoes should be planted when the weather is well settled. I usually plant mine from early June to Fathers Day. In the first few weeks (stage 1), all the action happens below the surface while the plants extend their roots.
Stage 2 takes place in July. After growing a healthy root system, the vines take off. As soon as you see them start to run, it’s a good time to thoroughly weed. Not long after, the vines will fill the paths. A mass of sweet potato foliage and its shade will prevent the next wave of weeds from germinating beneath the vines.
Towards the second half of August, the third and final stage takes place. Roots swell. As the tubers develop you face your trickiest decision—when to harvest. Sweet potatoes are said to double in weight every two weeks. If good weather holds, you want to keep them growing for as long as possible for the heaviest crop. The flip side, however, is that tubers won’t grow much at soil temperatures below 18OC (60OF) and soil temperatures below 10OC (50OF) can actually harm the tubers. “Chilling injury” leads to rot spots on sweet potatoes that can turn them into brown mush in storage. Once frost blackens the vines, the tubers should be harvested and cured as soon as possible by simply leaving the tubers in a warm (27OC/80OF) humid place for two weeks.
What other vegetable can you think of that keeps well at room temperature for a year or more? Once properly cured, sweet pota-toes are remarkably long-lasting with no effort. Save space in your cold room, fridge or freezer for other foods. Sweet potatoes prefer it relatively warm.
Finally, a well-grown sweet potato is a delicious treasure that actually improves in storage. We can’t help but include baked sweet potatoes in Thanksgiving dinner. And while they’re good then, it’s from Christmas to Easter and beyond when they’re at their best. The starches gradually convert to sugar to make a really sweet, as well as nourishing, treat.
Sweet Potatoes For The Home Garden. Ken Allan. Green Spade Books. Kingston ON. 1998. 204pp. Available from Mapple Farm for $20 (including shipping, tax and all other costs) and available for loan from the COG Library.