Sweet potatoes (Ipomoea batatas, Zones 10–12) are fun and very easy to grow as long as you have enough space for their vigorous vines. At season’s end, the plants will produce delicious and nutritious roots that can store well for up to a year or longer. Though many gardeners think of this as a crop that needs warm weather and a long growing season, sweet potatoes can be easily grown wherever there are at least 90 to 100 frost-free days.
Tiny slips (above) will produce an abundance of vigorous vines. As long as the vines have plenty of room to ramble, this undemanding crop will not need much else from you. Many growers use black plastic mulch, but even a bed without plastic (below) will soon be covered in an attractive living mulch of leaves and vines.
Sweet potatoes need warm soil that is not too fertile
Sweet potatoes are grown from cuttings called “slips.” These can be purchased from a reputable grower, though gardeners may want to try growing their own. There are many excellent sweet potato varieties to experiment with; these differ greatly in flesh and skin color, as well as their adaptation to different growing regions.
Sweet potato plants are sensitive to chilling and should be planted only after the soil temperature is above 65°F. Here in Zone 5, I aim to transplant around June 10 and harvest in early October.
This is a crop that grows best in well-drained soils that are not too fertile. Avoid compacted or heavy clay soils, which can prevent roots from getting the air and water they need. Excess nitrogen will cause roots to become long and skinny rather than plump, so don’t apply compost or manure prior to planting. Although a good yield of sweet potatoes will remove the equivalent of 1 pound of 10-10-10 fertilizer per 100 square feet, for the best results only apply nutrients based on the results of a recent soil test.
The right mulch makes a big difference
To prepare the bed for planting, loosen the soil at least 8 inches deep—deeper if you can. While it is possible to grow sweet potatoes successfully without plastic mulch, we have found that we get better production if we use black plastic applied tight against the soil. This can raise soil temperatures around the young plants, allowing for an earlier start, which is particularly useful in cooler climates. However, black plastic mulch also conserves water and provides weed control in all locations.
Space your slips 9 to 12 inches apart in rows 3 to 6 feet apart. Usually a slip will have several nodes that will produce roots or shoots. Try to bury at least two or three nodes in the soil, leaving the growing point above ground. It is not important for the slip to have healthy-looking leaves; even the scraggliest looking slips can produce large, vigorous plants.
Transplanting conditions have a big impact on success. To avoid dessicating young slips that don’t have many roots, plant them out on a day when it is cloudy or raining, and water the slips immediately after planting. If your slips arrive when the weather is not conducive to transplanting, you can hold them for a week or more by placing the bundled slips in a deep pot, loosely placing potting mix around them, and watering regularly. When you are ready to plant, simply detangle the slips and plant individually.
Keep pests away from your sweet harvest
While sweet potatoes are not attacked by Colorado potato beetles and other potato pests, they do have enemies. Deer love to eat sweet potato foliage, so you may need to use fencing or row covers to keep them at bay. Voles enjoy feeding on sweet potato roots, and their damage is often not evident until harvest time. Maintaining a weed-free area around the planting, or mowing the grass very short, makes the habitat less favorable for voles. Wireworms and grubs feed upon young sweet potato roots, leaving unsightly tunnels. Both are larvae of insects that lay their eggs in grass or sod, so avoid planting root crops into newly prepared beds where grass was growing recently.
Scurf is a fungal disease that causes harmless but unattractive discoloration on the surface of sweet potato roots. Purchasing disease-free slips from a reputable supplier is the best defense against scurf and several other viruses.
Harvest as late as you can
Sweet potatoes should be harvested as late as possible—before the soil falls below 60°F. Before digging, use clippers or weed trimmers to remove the vines. Be gentle as you dig, since sweet potato skins are very soft and can rub off easily until they have been toughened up by curing.
After digging, cure your roots by keeping them in a warm place (80°F to 85°F) for four to seven days. A garage, a tool shed, the second floor of a barn, or a mudroom can be a good place to do this. Once the roots are cured, move them to their final storage place: somewhere that is moderately warm (55°F to 60°F) and humid. Take special care to avoid chilling the roots, which will be damaged by temperatures below 45°F. Under the right conditions, sweet potato roots can be stored for more than a year.
It is critical to wait a few weeks after harvest before eating the roots. While they are edible, the roots of recently dug sweet potatoes are very starchy and have poor eating quality. Throughout the first three weeks after harvest, these starches are converted into sugars, and the roots develop their excellent eating quality.
How to grow your own slips
After growing a successful sweet potato crop, it is easy to make more plants the following season. Here’s how.
Select and save the very best roots from your fall crop for use the following year. Choose nicely shaped roots that are completely free of any disease or insect pests. Keep in mind that some varieties are less inclined to sprout than others, and roots that have experienced chilling injury may exhibit poor sprout production.
Six to eight weeks before you want to transplant slips outside, place the sweet potato roots on their sides in trays of potting mix. Cover them completely with moist sand (or more potting mix), and keep the trays between 75°F and 80°F.
Twist and plant
When the sprouts are 4 to 6 inches long, remove them from the sweet potato root by twisting. The root will continue to produce new sprouts. The sprouts can be planted right away into prepared ground, or you can place them in a jar of water for a few days to start roots prior to transplanting.
Rebecca Sideman is a sustainable horticulture specialist and professor of sustainable agriculture and food systems at the University of New Hampshire in Durham.
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