For about ten years now we’ve been growing sweet potatoes on our farm. This has not always been easy—sweet potatoes are a tropical, heat-loving plant, and the growing conditions in most of Canada are the opposite of what they want.
But much to my surprise, our sweet potatoes thrived in this very rainy year, while our regular potatoes struggled. How is this possible?
The answer has to do with the evolutionary origin of sweet potatoes, and the reason that they are such a popular crop around the world.
Both sweet potatoes and “regular” potatoes were domesticated in Latin America some 8000 years ago. The potato was grown in the Andean mountains, in what is now Peru. Because of its mountain origins, the potato can tolerate colder weather and thrive in fields and gardens all over the world (from Ireland to Ontario to mountainous parts of China).
The heat-loving sweet potato, in contrast, was domesticated in what is now Mexico, in low river valleys, and quickly spread the Caribbean.
(The yam—a name sometimes erroneously used for the sweet potato in grocery stores—is actually a very different species native to Africa and Asia.)
The sweet potato later spread to the Polynesian islands, Japan, the Philippines, and coastal Asia. Why so popular? The sweet potato could happily tolerate heavy rainfall and even typhoons.
While some other crops failed in typhoon flooding, the sweet potato would survive.
All of which brings us back to the present day, and explains why our potatoes suffered as our sweet potatoes thrived. Andean potatoes, domesticated in soils that were high and dry, simply were not suited to the record rainfalls we received.
But sweet potatoes do have a real weakness: cold.
In autumn we watch carefully for frost. Most plants can survive a temperature as cold as 1 degree Celsius, or just under.
Sweet potatoes, however, are more vulnerable. If they get down to about 4 degrees Celsius—the temperature of a refrigerator—they’ll suffer a chilling injury that will damage the whole plant.
In fact, is is for that reason that you should never store sweet potatoes in the refrigerator. Refrigeration ruins both the flavour and texture of a sweet potato; it’s better to keep sweet potatoes in a dry, dark place, a little cooler than room temperature. Even putting them on the kitchen counter is better than putting them in the fridge.
The sweet potato’s intolerance for cold is the reason it is only now becoming more popular as a crop in Ontario. Our growing season is warm enough to grow sweet potatoes for just a few months each summer.
So a certain amount of babying is required to produce sweet potatoes. But understand that we don’t grow them from seed (like corn or carrots). Nor do we grow them by re-planting the tuber (as with regular potatoes).
Instead, we start them from tiny shoots called “slips”. In early February we take specially reserved sweet potatoes out of storage and let them sit in a warm spot; in March we put them in a tray of soil in our greenhouse. The tubers will send up little shoots to the surface, and each of these shoots has its own set of roots.
One tuber might produce a dozen or more shoots, which can be plucked off and planted outside once spring comes.
Even then, the sweet potato garden beds need careful preparation. The soil is too cold, so we lay old greenhouse plastic over raised beds to warm them. Soon the shoots grow into a dense layer of vines and leaves.
When the fall comes we dig up the plants; if the soil is loose enough, you can actually pull all the roots of one plant up as a bundle of sweet potatoes. To ensure that the tropical tubers will store through winter, we have to cure them—which means putting them in a hot, dark room for a few weeks, artificially creating the climate of their origin.
If you want to grow potatoes yourself, it’s easy to raise them as a houseplant indoors through the winter. Unlike regular potatoes, the leaves and vines of a sweet potato are edible. You can get ornamental varieties of sweet potato, but a small tuber, planted in a pot indoors, will soon produce a healthy bunch of leafy vines. They’re beautiful to look at, and you can put the chopped leaves in a stir fry or sauté them like spinach.
Aric McBay is an author and farmer at Root Radical CSA.