Sweet Potato

Introduction

Despite its name, the sweet potato is not related to Irish or white potato. Potatoes are members of the Solanaceae family whereas sweet potatoes belong to the morning-glory family (Convolvulaceae). Sweet potato originated in South America, and is one of the most important food crops in the developing world. Sweet potatoes are not yams although they are often marketed as such. Sweet potatoes resemble yams physically, but are not even closely related botanically. True yams are in the family Dioscoreaceae, which is more closely related to lilies and grasses.   Yams are grown in tropical regions such as Africa and the Caribbean. North Carolina and Mississippi are leading producers of sweet potato in the U.S.  Sweet potato is a frost-sensitive crop that needs between 90 and 150 days of frost-free period to produce harvestable roots.

Types and varieties

The skin of sweet potatoes can be yellow, orange, copper, red, or purple; the flesh can be white, yellow, orange, or purple. Varieties with copper-colored skin with moist orange-colored flesh (e.g. Beauregard, Covington) are the most common in New England; however, many ethnic populations prefer the starchier white-fleshed varieties.

Sweet Potato Varieties
Orange-fleshed White-fleshed
Beauregard Japanese
Covington O'Henry
Carolina Ruby - deep red, thick skin Japanese - purple skin, white flesh

 

Soil Fertility

Sweet potatoes will grow at a soil pH of 4.5 to 7.5, but 5.8 to 6.2 is optimal. Well-drained, loam soils result in large and well-shaped roots. Sweet potatoes grown in heavy clay soils, or in soils with high soil organic matter may produce rough, irregular roots. 

Sweet potatoes do not need high levels of nitrogen, and yields may be reduced if nitrogen exceeds 75 pounds/A. If manure or compost is added, be careful not add excessive fertilizer-N. Nutrients should be applied according to soil tests. See fertilizer table. Sweet potatoes need high levels of phosphorous (up to 200 lbs/A) and potassium (up to 300 lbs/A). Phosphorous and potassium can be applied at planting and nitrogen can be split between application at planting and before plants begin to run. Drip irrigation can be used to apply supplemental N under plastic mulch. Alternatively, all nitrogen can be applied at planting and covered with plastic mulch to prevent leaching.

Sweet potatoes require more boron than many vegetables. On boron-deficient soils, 0.5 lb B/A (5 pounds Borax or 2.6 pounds Solubor) should be added to prevent a disorder called blister. This disorder is characterized by small, raised bumps on the root surfaces and plant stunting

PLANT NUTRIENT RECOMMENDATION ACCORDING TO SOIL TEST RESULTS FOR SWEET POTATO
SWEET POTATO NITROGEN (N)* LBS PER ACRE PHOSPHORUS (P) LBS P2O5  PER ACRE    POTASSIUM (K) LBS K2O      PER ACRE
SOIL TEST RESULTS   VERY LOW LOW OPTIMUM ABOVE OPTIMUM VERY LOW LOW OPTIMUM ABOVE OPTIMUM
Broadcast and Incorporate 25 200 120 30-60 0-30 300 200 50-100 0-50
Sidedress When Vines Start to Run 25-50 0 0 0 0 0 0 0 0
Sidedress 6-8 Weeks after Planting 0 0 0 0 0 0 0 0 0
TOTAL RECOMMENDED 50-75 200 120 30-60 0-30 300 200 50-100 0-50
*SEE PLANT NUTRIENTS  FOR INFORMATION ON NUTRIENT MANAGEMENT AND APPLICATION.

Planting

Sweet potatoes are grown from rooted sweet potato sprouts (slips) or vine cuttings. Slips can be produced by placing sweet potato roots in warm (75º to 80ºF) moist sand or soilless media until sprouts are produced (about 1 month or longer). Slips are then pulled from the bedded roots and planted. In southern areas of the US, larger growers produce their own slips. It is recommended for New England that slips be purchased from companies that provide certified disease-free slips.

Field planting begins when all chance of frost has passed. Soil temperature in the production field should reach at least 65°F at a 4" depth for 4 consecutive days before transplanting.

Rows are 32" to 42" apart with in-row spacing 8" to 12", depending on cultivar. Slips are transplanted into the rows at a depth of 3" with no less than 2 plant nodes in the ground and leaving 2 leaves or more above the ground. If slips do not have good root development, transplanting during cloudy weather and maintaining adequate field moisture just after transplanting will help insure success.

Field Culture

Because of their vulnerability to wireworms, sweet potatoes should not be grown the first year after incorporating sod. 

Research in New England has shown that yields are increased, particularly during cooler summers, by using raised beds covered with black plastic mulch. In New England, it is difficult to obtain good yields on bare soil.

Harvest and Storage

Sweet potato roots continue to grow until the leaves are killed by frost or until soil temperatures fall consistently below 65ºF, whichever comes first. Time of harvest is often determined by digging up a few representative plants and determining the percentage of roots in the size classes. When tops of the plants turn black after the first frost, it is imperative to harvest as quickly as possible regardless of root size.

Sweet potatoes are very susceptible to damage at harvest. Sweet potato roots do not have a thick protective outer layer of cells such as that on white potato tubers. Abrasions and wounds can lead to rots in storage.

Curing immediately after harvesting is recommended when selling sweet potatoes wholesale. This minimizes damage and loss during storage by healing harvest wounds. To cure, maintain roots in temperatures between 80°F to 86°F and a high relative humidity (85% to 95%) for 4 to 7 days. This forms a corky periderm layer below the damaged areas which limits microbial invasion and water loss. A freshly harvested sweet potato is more starchy than sweet. During curing and storage, starches in the sweet potato are converted to sugars, improving flavor. It is recommended to wait at least 3 weeks after harvest before consuming sweet potatoes to permit the starches to convert to sugars for maximum eating quality.

Sweet potatoes can maintain excellent quality for more than a year in proper storage conditions. The ideal storage conditions for sweet potato are the same as for winter squash; moderately warm (55º-60º F) at 60% to75% relative humidity.