Sweetpotato

Introduction

Despite its name, the sweetpotato is not related to Irish or white potato. Sweetpotato belongs to the morning-glory family (Convolvulaceae). Sweetpotato originated in South America and is one of the most important food crops in the developing world. Sweetpotatoes are not the same as yam, although they are often marketed as such. True yams are in the family Dioscoreaceae, and are grown in tropical regions such as Africa and the Caribbean. Sweetpotato is a frost-sensitive crop that needs a frost-free growing season ranging from 90 to 150 days (depending on variety) to produce harvestable roots.

Types and varieties

The skin of sweetpotatoes 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, some markets 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

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

Sweetpotato does not need high levels of nitrogen (N), and yields may be reduced if nitrogen exceeds 75 pounds/A. If manure or compost is added, be careful not to add excessive fertilizer-N. Nutrients should be applied according to soil tests. See fertilizer table below. Sweetpotato needs high levels of phosphorous (P, up to 200 lbs/A) and potassium (K, up to 300 lbs/A). Both P and K can be applied at planting and N 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 N can be applied at planting and covered with plastic mulch to prevent leaching.

Sweetpotato requires 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

Planting

Sweetpotatoes are grown from rooted sweetpotato sprouts (slips) or vine cuttings. Slips can be produced by placing roots in warm (75º to 80º F) 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 65º F at a 4" depth for 4 consecutive days before transplanting.

Rows are 32"-42" apart with in-row spacing 8"-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 ensure success.

Field Culture

Because of their vulnerability to wireworms, sweetpotatoes 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. 

Harvest and Storage

Sweetpotato roots continue to grow until the leaves are killed by frost or until soil temperatures fall consistently below 65º F, 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.

Sweetpotato roots are very susceptible to damage at harvest. The 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. This minimizes damage and loss during storage by healing harvest wounds. To cure, maintain roots in temperatures between 80º F and 86º F and a high relative humidity (85%-95%) for 4-7 days. This forms a corky periderm layer below the damaged areas which limits microbial invasion and water loss. A freshly harvested sweetpotato is more starchy than sweet. During curing and storage, starches in the sweetpotato are converted to sugars, improving flavor. Wait at least 3 weeks after harvest before consuming the roots to permit the starches to convert to sugars for maximum eating quality.

Sweetpotatoes can maintain excellent quality for more than a year in proper storage conditions. The ideal storage conditions for sweetpotato are the same as for winter squash; moderately warm (55º-60º F) at 60%-75% relative humidity.