Growing Sweet Potatoes in a Home Garden

The sweet potato (Ipomoea batatas) belongs to the family Convolvulaceae. The vegetable probably originated in the tropical parts of South America, and was domesticated there at least 5,000 years ago. The large, starchy, sweet-tasting roots are an important vegetable crop worldwide. The true yam (Dioscorea species) is an unrelated plant and grown primarily in tropical and semitropical areas for its tubers.

Planting sweet potato

• Hardiness: Very tender warm-season annual. Frost will injure top growth; needs warm weather to grow. 
• Planting: Plant sprouted new shoots (called "slips") after the soil has warmed to 65°F. Planting in ridges allows the soil to warm faster in the spring, improves drainage, and gives the roots room to expand. Ridging also makes harvesting easier. Sweet potatoes grow best in light, sandy soils but grow well on heavier soils, high in clay and amended with organic matter. Full sun requires direct light at least 6 hours/day; prefers 8 to 10 hours per day.
• Days to maturity: 85 to 120
• Spacing: 12 inches between plants within a row, and 40 inches between rows
• Fertilizer needs: Medium requirement for nutrients, either from soil organic matter or fertilizers. Incorporate compost prior to planting. On light, sandy soils, side-dress with a nitrogen source once or twice during the growing season, if needed. Refer to Fertilizing Vegetables for details.
• Approximate yield: 15 to 30 pounds per 10-foot row

Common sweet potato problems

• Scurf
• Voles

Growing and care of sweet potatoes

• Sweet potatoes are storage roots, not tubers. New shoots will emerge along the entire root, and young leaves and shoots can be eaten as greens.
• Established sweet potato plants will tolerate relatively dry weather.
• Cultivars with moist flesh (Georgia Jet, Centennial, Vardaman) predominate in the United States. They are derived from the cultivar 'Porto Rico', and are sometimes called yams. Japanese and Korean sweet potatoes are drier, store better, and are becoming more popular in Maryland.
• Sweet potato slips are often difficult to find, though there are online sources. You can also grow your own, and the process from bedding to planting takes six weeks.
  1. Buy certified seed stock at a garden center or your favorite varieties from a market. Wash any purchased roots to remove anti-sprouting chemicals.
  2. Cover the bottom of a plastic nursery flat or container with 1 to 2 inches of coarse sand or soilless growing mix (potting soil).
  3. Slice each root in half lengthwise and place it cut-side-down in the container. Cover with 2 inches of coarse sand or growing mix.
  4. Keep the roots moist, warm (75° to 85°F), and covered with plastic until new shoots emerge.
  5. Remove the plastic and grow the plants directly under fluorescent or LED grow lights. (Use a timer to have the lights on for 14 to 16 hours per day.)
  6. Pull slips from bedded roots and plant outside after the danger of frost has passed. Slips are rootless when pulled off of the original root, so keep them well-watered. You can also take cuttings from the slips and pot them up into containers.

Watering

• Keep plants uniformly supplied with moisture for best performance.
• Sweet potato is sensitive to drought for the first 50 to 60 days after planting.
• Water deeply and regularly during dry periods, but don't keep the soil saturated since excessive water can cause root rot and splitting of the storage roots.

Weeding

• Remove weed seedlings by hand or with a hoe until the plant begins to grow.
• As they establish, the rapidly-growing vines will shade out weeds, so little additional weeding will be needed.
• Covering the soil between plants with an organic (biodegradable) mulch is beneficial where weed pressure is high.

Harvesting sweet potatoes

• Sweet potato foliage is edible and very nutritious. Eat it fresh in a green salad or sauté with other vegetables.
• Sweet potatoes generally mature 85 to 120 days after planting. Check the root size after 80 to 85 days because they don't stop growing and can start to split when overgrown.
• Harvest roots as soon as they reach eating size and before a frost. Digging out the roots is easier if you cut the vines off first.
• Use a garden fork or spade to loosen the soil and gently lift up and expose the sweet potatoes. Handle them with care, gently removing attached soil clumps. Don't rub the skin or wash roots prior to storing indoors.
• If the vines are exposed to frost, dig the roots immediately because decay in dead vines moves into the roots. If immediate digging isn’t possible, cut away vines and throw loose soil over the rows to protect the sweet potatoes from the cold. Soil temperatures below 50°F can cause chilling injury. 

Curing, storing, and preserving a sweet potato harvest

Commercial growers cure sweet potatoes in special rooms at 85°F and 80% to 90% relative humidity (RH) for 5 to 10 days. Curing causes the conversion of starch to sugar and helps to heal cuts in the skin of the roots that could lead to rot. The roots are then stored for 6 to 8 weeks at 55 to 60°F to further increase the sugar content.

Duplicating this process would be very difficult for home gardeners! You can at least partially cure the roots by leaving them on a porch or other covered outdoor location (or in a garage) for a week or so. This works best if you harvest in September when temperature and humidity are still relatively high.

• Spread the roots out on a mesh screen in a single layer so they get good airflow.
• If curing outdoors, be sure to protect the roots from rain and animals.
• Move the cured roots to a cool, dry area (such as a basement) and store them through the winter in boxes, bins, or baskets, lined with newspaper that allow for good air movement. The sugar content will slowly increase in storage. Roots store best at 55° to 60°F at 85% to 90% RH.
• The roots will shrivel and sprout if temperatures are too high.
• Sweet potato roots usually store successfully even if the one-week curing process is omitted.