How to grow Sweet Potato

How to grow Sweet Potato - Introduction

  • Sweet potato, Ipomoea batatas, is a tender, warm-weather vegetable that requires a long frost-free growing season to mature large, useful roots. 
  • "Bush" varieties with shorter vines are available for situations where space may be limited.
  • Though orange-fleshed varieties are most common today, white or very light yellow-fleshed types were once considered the finest types for sophisticated people. 
  • Some white-fleshed types are still available, though they may be hard to find outside the Deep South of USA.
  • They are mostly propagated by stem or root cuttings or by adventitious roots called "slips" that grow out from the tuberous roots during storage. True seeds are used for breeding only.
  • They grow well in many farming conditions and have few natural enemies; pesticides are rarely needed. 
  • Because they are sown by vine cuttings rather than seeds, sweet potatoes are relatively easy to plant. 
  • Because the rapidly growing vines shade out weeds, little weeding is needed, and growers can devote time to other crops. 

How to grow Sweet Potato - Site & Soil

  • Sweet potatoes are easy to grow in places with more than a 120-day growing season. 
  • They are usually grown in sandy soil, which makes them easier to dig, but they thrive in most soils, even heavy clays.
  • Sweet potatoes are grown on a variety of soils, but well-drained light and medium textured soils with a pH range of 4.5-7.0 are more favourable for the plant.
  • They can be grown in poor soils with little fertilizer. However, sweet potatoes are very sensitive to aluminium toxicity and will die about 6 weeks after planting if lime is not applied at planting in this type of soil. 
  • The plant does not tolerate frost. It grows best at an average temperature of 24 °C (75 °F), abundant sunshine and warm nights. 
  • Annual rainfalls of 750–1000 mm are considered most suitable, with a minimum of 500 mm in the growing season. 
  • The crop is sensitive to drought at the tuber initiation stage 50–60 days after planting and is not tolerant to water-logging, as it may cause tuber rots and reduce growth of storage roots if aeration is poor.
  • Depending on the cultivar and conditions, tuberous roots mature in two to nine months. 
  • With care, early-maturing cultivars can be grown as an annual summer crop in temperate areas, such as the northern United States. 
  • Sweet potatoes rarely flower when the daylight is longer than 11 hours, as is normal outside of the tropics. 

How to grow Sweet Potato - Soil Preparation

  • Till or hand-dig the potato bed to a depth of 8” to 10”. 
  • Sweet potatoes do well in low fertility soil, even clay, but the addition of compost will loosen the soil and make the roots easier to dig in the autumn. 

How to grow Sweet Potato - Starting Slips

  • Sweet potatoes are started from plants called "slips." 
  • Always buy plants grown from certified disease-free roots. 
  • Upon receipt of your slips, placing them in a jar of water until you are ready to plant will perk them up, allow you to wait until weather conditions are perfect for your area, and give you time to prepare your soil.
  • To grow your own plants, there are several proven methods:

Sweet potato - Hotbed technique

  • Place several sweet potato roots about one inch apart in a hotbed and cover with two inches of sand or light soil. 
  • Add another one inch of sand when the shoots begin to appear. 
  • Keep the soil in the bed moist throughout the sprouting period, but never allow it to become waterlogged. 
  • Keep soil temperature between 70° and 80°F. 
  • Plants are ready to pull in about 6 weeks (when they are rooted and 6 to 8 inches tall). 
  • You can allow roots to continue possibly producing additional flushes of plants if more are desired. 

Sweet potato - Pantry technique

  • To start potatoes, purchase your favourites at the grocers in mid-winter. 
  • Buying organic sweet potatoes improves the chances they will sprout. 
  • Let them sit in the pantry and when they begin to sprout, immerse the opposite end in a jar of water and bring into the kitchen. 
  • You may need to use tooth-picks to keep the potato upright in the jar, see right.
  • The sprouts, called slips, are what will be planted.
  • The slip will be detached later from the potato, and when grown-on will produce its own roots to support a full growing plant and several tubers.

Growing on

  • At least 2 weeks after the date of the last average frost, clip the slips from the potatoes with a scissors or knife. 
  • Remember which end of the slip was attached to the potato; that end will go in the ground and become roots and produce more potatoes. 
  • Longer slips should be cut in 4” to 6” pieces.
  • The bare-root slips can be transplanted into their growing positions when they are 6-8 inches tall without roots, however, an alternative method is to use small bedding plant trays known as “six-packs” or small flower pots. Fill the pot or flat with potting soil and plant the slips, one to a cell or pot. Allow 2 weeks for roots to grow, then plant in the prepared beds. This method requires more materials and work, but is much less of a shock to the slip and will save growing time in temperate climates.

How to grow Sweet Potato - Planting Out

Transplanting potted plants into a raised bed with a plastic mulch
  • Transplant the slips as soon as the soil warms up after the last frost to allow the maximal warm-weather growing period. 
  • The sprouts (slips) are planted directly in the garden from the sprout bed.
  • A string from one end of the bed to the other will help to keep the row straight. 
  • Using a tape measure or a stick cut to length, poke the slips into the soil 12” apart. 
  • Water well after planting and keep watered until signs of growth are observed. 
  • The slips have leaves but no roots so don’t be alarmed if they lose a few, or all, of their leaves. They will survive.

How to grow Sweet Potato - Size & Spacing

  • Set the plants 12 to 18 inches apart, preferably on a wide, raised ridge about 8 inches high. 
  • A ridge not only dries better in the spring but also warms earlier than an unridged area. 
  • Black plastic or membrane mulch can be a good way to speed early season growth by capturing and storing more of the sun's heat in the soil under the plastic cover. 
  • Because the vines of spreading varieties need a great deal of space, allow at least 3 to 4 feet between rows.

How to grow Sweet Potato - Care & Cultivation

  • After early cultivation (which is not necessary with black plastic or membrane), sweet potatoes need minimal care to keep down weeds. 
  • Sweet potato slips grow slowly at first but much more quickly by mid-summer, forming a tangled mass of vines 2’ deep and 8’ wide. 
  • Keep well watered, feeding every other week with a high-potassium liquid feed.
  • Plants either side of the sweet potato bed may be engulfed so allow plenty of room.
  • Sweet potatoes have almost no pest problems, but they do require weeding until the canopy forms. After that, few weeds can survive. If rainfall is insufficient, make sure potatoes get 1” of water per week.
  • The tubers will be ready to dig from 100 to120 days after planting. 
  • The longer they are in the ground, the higher the vitamin content and the larger the potatoes.

How to grow Sweet Potato - Harvesting

  • Do not water during the last 3 to 4 weeks before harvest to protect the developing roots.
  • Early roots may be "robbed," starting in late summer, by digging into the side of the ridge and carefully removing some developing roots while leaving the plant in place. 
  • Dig when leaves yellow but don’t wait until plants are killed by frost as those tubers may not last in storage. 
  • Most of the potatoes are in a tight cone-shaped cluster directly below the main stem. 
  • However, some will be growing a foot or even 18” out from the stem, and there is no way to know where they are. 
  • Digging carefully, start about 18” from the stem, below the level of the ridge until the fat roots are exposed. Some sweet potatoes will get nicked but they will heal.
  • Use a potato fork or garden fork and be careful not to bruise, cut or otherwise damage the roots. 

How to grow Sweet Potato - Store & Preserve

  • Proper curing can be a problem in the cool autumn season. 
  • Ideally, the roots should be allowed to dry on the ground for 2 to 3 hours, then placed in a warm room for curing (85°F and 85 percent humidity (if possible) for 10 to 14 days and then stored in a cool (55°F) location. 
  • Sweet potatoes should be handled as little as possible to avoid scuffing and bruising. 
  • In case of frost, cut the vines from the roots immediately to prevent decay spreading from the vines to the roots and dig sweet potatoes as soon as possible. 
  • Cold soil temperatures quickly lessen the roots' ability to keep in storage. 
  • Do not allow roots drying in the garden to be frosted because they are quickly ruined. 
  • For best quality, use the potatoes as soon as possible after they have been stored.