All About Sweet Potatoes
Sweet potatoes (Ipomoea batatas) are often misidentified, at times making shopping for them a bit of a guessing game. Many shopkeepers have started labelling the sweet, moist orange varieties as yams. True yams (Dioscorea rotundata) are dry, white-fleshed roots that are neither botanically nor even culinarily related to sweet potatoes. These bland, starchy roots are usually found in stores catering to people who hail from tropical regions. To further complicate matters, in order to differentiate white-fleshed sweet potatoes from regular white potatoes (Solanum tuberosum), some retailers have started labelling them as "sweetpotatoes."
There are hundreds of named varieties of sweet potatoes in numerous skin and flesh colours. The skin might be beige, pink, yellow or purple and the flesh, white, orange, yellow or stunning vivid purple. Textures range from rich, dry and mealy to sweet, smooth and moist.
Sweet potatoes are incredibly nutrient-dense. They are so blessed with complex carbohydrates that diabetics can comfortably savour their sweetness. The high amount of soluble fibre can benefit those with circulation problems. Their high antioxidant levels might even reduce cancers. They are an exceptionally good source of vitamins A, C, K and B complex. And they are an excellent source of potassium and manganese. You need not feel guilty enjoying something so delicious.
Records indicate that sweet potatoes were used in Peru over five thousand years ago. Genetic analysis suggests they originated in the Caribbean well before then. From South America, Polynesian navigators apparently took them to many Pacific islands. Then they made their way to China, Japan and India. Europeans did not become aware of this fine vegetable until Christopher Columbus returned from his first voyage with samples.
Sweet potatoes did not become common in North America until the late eighteenth century, when the proper method of curing the freshly harvested roots became known. Prior to this, the roots would only remain in usable condition for a couple of months after harvest. This was no problem in the tropics, where they could be grown every month of the year, but the curing technique made growing them in cooler climates a more practical enterprise.
Curing is accomplished by placing the freshly harvested sweet potatoes into a hot (32° C) and humid (90%) space for about ten days. This initiates a process in which minor harvest damage to the root's skin is repaired and the flesh becomes more rich, sweet and moist. Following this the sweet potato can remain in good shape for over one year as long as the temperature is always kept above 14° C.
Sweet potato seeds are used only by breeders; gardeners start their crop from "slips." You can order slips (which are sprouts from a saved sweet potato root) from some seed catalogues. Ask for them to be delivered the first week of June. Mapple Farm in New Brunswick offers a good selection of varieties suitable for northern gardens. They typically arrive looking wilted, but sweet potatoes are amazingly resilient. Promptly transplant into 1-inch pots filled with a ratio of two-thirds potting soil to one-third topsoil. Water well and keep warm but out of direct sunlight until they become established.
You can produce your own slips by laying your chosen variety of sweet potato into a tray and then half covering it with soil. Water well. As sprouts form, cut them off when about two inches long. Root these in the above-mentioned soil blend. Start this process some time between mid-March and mid-April. Hold back the rooted plants by letting them wilt and become root-bound. Each root gradually produces innumerable slips.
For such a nutritious vegetable, sweet potatoes require surprisingly little fertility. A generous dose of low-fertility compost, such as horse manure or leaf mulch, helps maintain moisture in sandy soils and makes digging out the mature roots easier in clay soils. Deeply loosen the soil to about 10 inches with a digging fork. Forming 8-inch-high by 15-inch-wide beds in heavier soils will ease harvesting even more. Every 20 inches form a 10-inch circular depression.
In lieu of fertility requirements, sweet potatoes demand a lot of soil heat (30° C is the target), so for success in our climate, preheat the soil with clear plastic. (Black plastic merely warms the soil; clear plastic heats the soil.) Though I do not like using plastics in my garden, this is the crop to break the rule with. In mid May, lay 4-mil construction-grade clear plastic over the bed. Firmly anchor and seal all the edges by digging a 3-inch-deep trench, lay in the plastic edge, and then backfill the soil. Moisture will be retained and weeds will germinate but then quickly succumb to the heat.
After summer has seriously arrived (about June 15), cut an 8-inch slit in the plastic over each depression. Deeply transplant a slip into each slit, ensuring that only the leaves are above ground. Water each plant well. Cover each slit with a couple of handfuls of sand. Rain or overhead irrigation will then drain towards each plant's roots. The plants should receive about one inch of water every week. Stop watering around the middle of August (and pray that the rain also stops).
After the leaves have been blackened by the first frost or the soil temperature has dropped below 14° C, gently dig up the roots. Cure them by putting them into a closet with a small heater and some damp towels to maintain them at 32° C and 90% humidity for about ten days. Then store them in a cool dry space that never goes below 14° C. You can enjoy them right away but they reach peak flavour after the new year. If grown well and properly stored, they should remain in good shape for many months.
Uses for Sweet Potatoes
If you're impatient while waiting for your roots to mature, sweet potato leaves are a delicious summer green. Use them in place of spinach or chard, which tend to become bitterly astringent during hot summer weather. Or use them in place of kale, which can become sulphurous in very hot weather. Pick the newest, tenderest leaves.
When considering substituting sweet potatoes in your favourite regular potato recipes, use caution. The starches behave differently and the richer sweet potato flavour needs to be compensated for. But go ahead and experiment.
Sweet potatoes are so delicious on their own that I prefer cook them unadorned, in one of the following ways:
• Poke a couple of holes in the skin to prevent them from exploding. Bake at 400° F for about 40 minutes. This method enhances the dry, mealy texture.
• Simmer whole for 30 minutes. This method adds moistness.
• Peel and slice the sweet potatoes and then steam them with 1/4-inch water for 12 minutes. (Add a lemon slice to maintain the bright orange colour.)