Your Growing Guide to Sweet Potatoes: Plant Care Tips, Varieties, and More

In Tokyo, yaki-imo vendors sing out on fall and winter evenings, beckoning customers to buy hot sweet potatoes. Baked in a stone oven right on the seller’s little truck or cart, the pink-skinned, yellow-fleshed sweet potatoes are creamy comfort food. In Mexican markets, suppliers offer their smaller, sticky-sweet orange or dark purple camotes from a covered pot they carry through the plaza. In the Andes, Quechua and Aymara people freeze-dry sweet potatoes as chuño, while in the Pacific Islands, purple sweet potatoes are commonly mashed with coconut milk.

Not to be confused with Africa’s yams or with potatoes, these delicious tubers are widely varied and adapt well in many locations. Sweet potatoes are warm-weather crops that need plenty of space to spread their vines and dark, heart-shaped leaves horizontally. While they take time to mature, these plants are unfussy and easy to grow.

How to Plant Sweet Potatoes

While you can buy starts, or “slips,” from a seed company, it’s just as easy to start your own sweet potatoes from the ones you buy in the market or saved from last year. 

Growing Slips

There are several ways to get a sweet potato to sprout “slips”, the mini-plants that sprout from the nodes on the tuber. Many folks swear by suspending the sweet potato in glass above water; that will give you roughly two to six starter plants. But small farmers who are planting several rows of sweet potatoes have other methods—the most common among them is planting a healthy sweet potato from the previous year's crop in soil and collecting the sprouts.

The Kerr Center for Sustainable Agriculture recommends planting the sweet potatoes in 20-gallon bins with drainage holes, filled half full with moist potting soil. In a frost-free location, the tubers are placed on the surface, close together but not touching. As the new shoots reach 4-5 inches, they can be moved outdoors during the day to harden off. The Kerr Center report states that hardware cloth or chicken wire can be spread across the covered roots, and a couple more inches of potting soil can be put down. The hardware cloth keeps gardeners from pulling the roots out of the soil while plucking slips.

If you simply plant in a bin or bucket of potting soil, you can loosen the potato and then peel the slips off, carefully turning them sideways to keep the roots intact, and then gently separating the roots from each other. Growing slips takes about six weeks.

Some growers prefer to cut the slip off the potato without the roots, but it may take longer for the plant to get established.

Transplanting

Slips favor a warm environment, but once they are separated from the root, they should be maintained at a cooler temperature (50-60 degrees F) and moisture loss should be prevented in order to produce good yield. Slips can be planted immediately and are the healthiest if planted within six days. Larger slips, planted more deeply, may have an advantage in terms of growth above and below ground and in terms of yield.

A study from the University of Szeged Faculty of Agriculture in Hungary found that planting in ridges or hills resulted in the best yields. Once the soil has been prepared and hilled, slips can be planted a few inches deep, leaving at least 3-4 leaves above ground. Some growers who have a lot of slips to plant make a furrow in the ridge, lay the slips in place 10-14 inches apart, and then poke the root end into the soil with a forked wooden stick. Mechanized planting tools were tried by the Kerr Center and found to be of limited benefit.

Sweet Potato Plant Care

Sweet potatoes are hardy and attract few pests, but these plants do benefit from weed suppression and even a little bit of dressing with compost around the plant once it is growing steadily.

Light, Soil, and Nutrients

Sweet potatoes need full sun and warm weather to thrive. They also require a balanced level of nitrogen, according to a study carried out at Shangdong University in China. Too little will cause the plant to form thin roots and struggle to form storage roots; too much will inhibit the activity of the adventitious root cambium, also inhibiting the formation of storage roots. Their study showed that optimal nitrogen fostered more root-forming activity and larger sweet potatoes.

Since the roots of the plant will need to spread out and swell up, the soil should be well loosened ahead of planting. Compost can help the soil structure. Sweet potatoes prefer slightly acidic soil and may be more disease prone in neutral to alkaline soils, according to the National Gardening Association.

Create ridges between 6-10 inches high and 2-3 feet across. If it’s not too big a project, a long-handled hoe will do the job, but for larger planting areas, a wheel-hoe with a hiller attachment can help. Creating loose, friable texture is especially important in clay soils. 

Water

Water the plants deeply for most of the growing cycle. While drought stress will reduce the number and size of sweet potato tubers, tapering off watering at the end of the growing cycle, so that soil isn’t too heavy, will make digging them up easier.

Temperature and Humidity

When planting the slips, wait until a few weeks after your last frost. They are a hot weather crop with a long growing season, and so need the soil to be at least 50 degrees F, even at night. 

How to Harvest Sweet Potatoes

One of the benefits of soil ridges for sweet potatoes is that they let you know where to dig to harvest your crop. Removing the vines before digging also reveals the main stems at the center of the plant’s spread. Dig straight down at the edge of the ridge, then angle toward the center and lift; be careful not to damage the tubers. (It's almost impossible not to ding a few.) The Kerr Center tested machine-harvesting but found that a quality, long-handled digging fork was more effective and more cost-effective for the home gardener or small farmer. 

Don't forget to harvest sweet potato greens too; these greens are like spinach but taste a little sweeter. Clip some tender segments with newer leaves from the end of the vines, just not so much that it stunts the growth underground.

Sweet Potato Varieties

There are probably as many types of sweet potatoes (and related tubers) as there are cuisines in the world, and they are a colorful assortment. Though you may be drawn to the dark orange sweet potato for its carotenes or the dark purple for its anthocyanins—both are powerful antioxidants—your selection may depend on what you plan to cook. These are the most common sweet potato varieties to grow and cook with:

• Japanese sweet potatoes have pink flesh, a light yellow inside, and a creamy consistency when baked.
• Garnet sweet potatoes have red skin, deeper orange insides, and a dense, smooth texture when cooked.
• Jewel sweet potatoes have light, pink-orange skin and bold orange inside, featuring that slightly carrot-like, traditional “holiday yams” flavor.
• Hannah sweet potatoes have pale, pink-tan skin and cream-colored insides. This variety is drier and flakier than others but has a honey-sweet taste.
• Purple sweet potatoes, such as the Okinawan, are purple all the way through, dense, dry, sweet, and considered a super food.

How to Store and Preserve Sweet Potatoes

After harvesting, sweet potatoes have to “cure” for seven to 10 days at 80-90 degrees F and with adequate humidity. Uncured sweet potatoes are likely to dry out and not store well. The Kerr Center recommends a low-tech, passive energy method: Gardeners should choose a grassy area, water it, cover their harvest containers with a plastic tarp, and weigh down the edges to prevent it from blowing away, leaving some gaps for ventilation. In other places, growers cover the tubers with the leaves and vines they remove when digging.

Purdue University Extension recommends stacking storage crates and covering them with paper or heavy cloth to maintain humidity. Then, move the cured sweet potatoes to a dark location where a temperature of about 55-60 degrees F can be maintained.