This time of year, some of the prettiest produce on the shelves at my local organic market are sweet potatoes (Ipomoea batatas). Unlike many other crops, the quality of sweet potatoes improves in storage, so they taste better now than they did in November. But as their dormancy period ends and sweet potato tubers begin to sprout, growing sweet potatoes from slips (rooted cuttings) becomes a fun springtime project.
But wait. Aren't sweet potatoes a tropical crop, difficult to grow in cool climates? Yes and no. In recent field trials in chilly New Hampshire, for example, fast-maturing varieties like ‘Beauregard' and ‘Covington' produced three to four nice tubers per plant when planted into black plastic mulch, which helps warm the soil. In the UK, these and other vigorous varieties often are successful grown under plastic-covered tunnels. Still, you will get a bigger harvest when growing sweet potatoes in a warm climate than a cool one. In warmer climates, gardeners sometimes harvest eight or more tubers per sweet potato plant.
My own climate is a bit cool for sweet potatoes, but I still grow them because they are delicious, nutritious, easy to store, and grow like crazy in unusually warm summers. They also require little care, because vigorous sweet potato vines knit themselves together into a tangle of foliage that shades the soil and suppresses weeds. Besides, I wouldn't want to miss out on the drama of having sweet potato vines growing in my windowsills in late spring.
From Tuber to Sweet Potato Vine
Sweet potatoes are grown from rootable cuttings, often called slips. If you've never grown sweet potatoes before, it can be great fun to grow your own slips from small or medium-size sweet potatoes purchased at the market. One sweet potato will produce between three and five slips. This process takes about six weeks, so there is no need to hurry. At the Sand Hill Preservation Center in Iowa, Glenn Drowns emphasizes the importance of planting sweet potatoes after the weather turns warm and stable, in early summer. In my garden, the soil is not warm enough for sweet potatoes until the middle of June.
As a kid, I had fun sticking toothpicks around the middle of promising-looking sweet potatoes so I could suspend the other end in a jar of water and thereby grow a robust vining plant. This method works when conditions are right, or the result can be a scummy rotting sweet potato. A better idea is to nestle a sweet potato tuber diagonally in a bed of moist potting compost, with the sprouting end pointing up. Kept warm and lightly moist, the tuber will quickly sprout shoots that leaf out into vines. As the shoots, or slips, grow to 6 inches (15 cm) long, they can be broken off and transplanted to the garden, or to containers if outdoor conditions are too cold.
You can buy bundles of sweet potato slips if you are growing sweet potatoes in a big way, or you can use the bucket of sand method. Fill a bucket halfway with damp sand, and then lay in several sweet potatoes, half buried in the damp sand. Keep the bucket in a warm place, and it will be overflowing with slips in four to six weeks.
How do you tell the sprouting end from the root end of a sweet potato? This is not always easy, but the end of the tuber that was closest to the mother plant is fertile ground for foliar buds. If you cannot see little reddish buds on either end of a sweet potato, simply place the tuber in a warm place for two weeks. Buds will emerge, and you will know which end should go up.
Handling Sweet Potato Plants
Sweet potato slips that are grown indoors should be gradually exposed to strong summer sun over a period of one to two weeks – a task most easily accomplished by placing the mother plants in warm filtered shade. Prior to planting, the slips are snapped off, along with the lowest leaves. The slips are then positioned diagonally in well-moistened trenches, which are refilled so that only the top two leaves are showing at the surface. As long as the soil is kept lightly moist, the slips will develop roots and start growing within two weeks. Six weeks after that, the sweet potato vines will explode with growth and cover the ground with dense foliage.
Even the fastest-maturing sweet potato varieties should be allowed to grow for 90 days before you start looking for harvestable roots. It is also important to dig sweet potatoes before soil temperatures cool too much, because various skin diseases often develop in cold, moist soil. Some sweet potato varieties set tubers quite close to the primary growing crowns, while others wander a bit before initiating tubers. If you are not sure what to expect, begin digging from the outside of the row to minimise broken tubers.
Fresh sweet potatoes need to be cured for two to three weeks in a warm place. During this time, wounds to the skin heal over, and the flesh becomes sweeter and more nutritious. This process continues after curing, during the first months of storage at cool room temperatures, so stored sweet potatoes that are eaten in winter are often the best ones of the year.
The fastest-growing sweet potato varieties have orange flesh, but varieties with white, yellow, or even purple flesh are excellent options if you have a long, warm growing season. Sweet potato varieties vary in texture and flavour, though it can generally be said that orange-fleshed varieties cook up moist, white sweet potatoes become remarkably creamy, and purple sweet potatoes are dry and starchy, like regular potatoes. If you see these and other unusual sweet potatoes at local farmers markets, they are worth trying in your garden, too.
By Barbara Pleasant