How to grow Sweet Potato

Where to plant

Sweet potatoes need full sun, at least 8 hours a day, and loose, well-drained, sandy loam. Heavy clay or rocky soil gives misshapen roots, so grow them in raised ridges about 8 to 10 inches high where soil is heavy. They tolerate a range of pH but do best at about 5.5 to 6.5. The vines trail along the ground, so keep them away from tall crops that would shade them.

Plant only after the soil has warmed to at least 65°F and all danger of frost has passed, usually 3 to 4 weeks after the last frost. The crop needs 3 to 4 months of warm weather, between about 65°F and 95°F.

• Florida: plant slips from March to July in North and Central Florida; South Florida can plant from February to October.
• Gulf South, Texas and the Mid-South: plant from April or May through June. In Mississippi, slips go in from early May through June.
• Desert Southwest and Southern California: plant from April into July; water deeply in the heat.
• Mid-Atlantic, Southern Plains and Northern California: plant in May and June; July plantings do not size up before frost.
• Northeast, Midwest, Northern Plains and Mountain West: plant in late May or June into warm soil. Black plastic mulch warms the soil and helps, and quick varieties such as Beauregard are the best bet.
• Pacific Northwest west of the Cascades and Alaska: summers are usually too cool for a reliable crop.
• Hawaii: plant year round.

Choosing a variety

• Beauregard is the leading variety in the South: high yielding, light rose skin, deep orange flesh, about 90 to 100 days, and a good choice for shorter northern seasons.
• Covington and Jewel are orange-fleshed, about 110 to 120 days. Evangeline is high in sugar, about 90 to 100 days. O'Henry is a white-fleshed form of Beauregard.
• Okinawan (purple flesh) and Murasaki-29 (cream flesh, purple skin) are drier, nuttier specialty types.
• Buy certified, weevil-free slips from a reputable supplier. Several states, including California, North Carolina and South Carolina, restrict moving sweet potato plants and roots from areas where the sweetpotato weevil is found, which includes all or parts of many Southern states and Hawaii, so check before ordering slips across state lines.

How to plant

1. Work compost and a low-nitrogen fertilizer into the bed, following a soil test if possible, and form ridges about 12 inches wide and 8 to 10 inches high.
2. Choose firm, green slips 8 to 12 inches long with a few leaves.
3. Plant slips with the cut end down, 4 to 5 inches deep and 9 to 18 inches apart, leaving the top few leaves above the soil. Wider spacing gives larger roots.
4. Water in immediately and keep the soil moist for the first week or two while the slips root.
5. Mulch between the ridges to suppress weeds until the vines cover the ground.

Watering and feeding

Sweet potatoes are drought tolerant once established, but the best quality and yield come from about 1 inch of water a week.

Go easy on nitrogen. Too much gives lush vines and few, thin roots. A low-nitrogen fertilizer worked in before planting is usually enough.

Pruning and care through the year

• Let the vines run; there is no need to prune them, though you can lift them occasionally to stop them rooting at every node.
• Young leaves and shoot tips are edible, cooked like spinach; take a few from each plant.
• Keep weeds down early, before the vines cover the ground.

Harvesting

Sweet potatoes do not have a set ripe stage; most varieties are ready 90 to 120 days after planting. Dig a test plant to check the size. Harvest before the soil cools below about 55°F to 60°F and well before frost, as cold soil and frost damage the roots.

Dig carefully with a fork, starting well out from the plant, as the skins scrape and bruise easily and damaged roots store poorly. Avoid digging in very dry soil, which increases skinning. Do not wash the roots.

Cure them right after harvest at about 80°F to 85°F with 85 to 90 percent humidity for 7 to 10 days, for example in a warm room in a box covered with damp cloth. Curing heals wounds and converts starch to sugar. Then store them in a dark place at 55°F to 60°F, where they keep for months. Never refrigerate raw sweet potatoes; below 55°F they suffer chilling injury and hard cores. They taste sweeter after a few weeks of storage.

Growing in pots

Sweet potatoes can be grown in a large grow bag or tub at least 20 inches wide and deep with a loose, sandy potting mix. Expect a smaller harvest than in the ground, and water regularly, as containers dry out fast.

Using it

Bake, roast, mash or fry sweet potatoes, or use them in pies and breads. The young leaves and tender shoot tips are a nutritious cooked green, common in Asian and African cooking.

Growing your own slips

About eight weeks before planting time, lay small sweet potatoes from last year's crop in a warm bed or a tray of moist sand or potting mix, cover with 1 to 2 inches of mix and keep them at about 75°F to 85°F. When the shoots are about 9 to 13 inches long, cut them about an inch above the soil and plant them; they root quickly. Use your own healthy, disease-free roots rather than store-bought ones.