Sweet Potato

Introduction

Despite its name, the sweet potato (Ipomoea batatas) is not related to Irish or white potatoes. Sweet potato belongs to the morning glory family (Convolvulaceae). Sweet potato originated in tropical South America and is one of the most important food crops in the world. Sweet potatoes 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. Sweet potato is a frost-sensitive crop that needs a frost-free growing season ranging from 90-150 days (depending on variety) to produce harvestable roots.

Types and Varieties

New varieties come on the market and older varieties fall out of favor or become available over time. The information below is updated every 2 years, but with any questions, please contact your local Extension staff.

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, some markets prefer the starchier white-fleshed varieties and purple-skinned/purple-fleshed varieties are also available.

Soil Fertility

Sweet potato 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, sweet potato may produce rough, irregular roots. 

Sweet potato does not need high levels of nitrogen (N), and yields may be reduced if N exceeds 75 lb/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. Sweet potato 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.

Sweet potato requires more boron (B) than many vegetables. On B-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 Recommendations According to Soil Test Results for Sweet Potato

Planting

Sweet potatoes are grown from rooted sweet potato sprouts (slips) or vine cuttings. Slips can be produced by placing roots in warm (75-80ºF) 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 65ºF at a 4" depth for 4 consecutive days before transplanting. If not planting immediately, keep slips out of direct sunlight and wind, with roots wrapped in a moist cloth, keeping leaves and stems dry. Do not set them in water.

Rows are 32-42" apart with in-row spacing 8-12", depending on cultivar. Tighter spacing can result in competition and delayed root sizing. 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. Keep irrigated until plants are established.

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. 

Sweet potato quality can be heavily influenced by field conditions before harvest. Managing fields to avoid excessively dry or overly wet conditions at harvest can significantly reduce storage-related disease problems. In a wet year you may experience more cracking, especially on more sensitive varieties. 

Sweet potatoes are a favorite of deer and voles. If these pests are a concern on your farm, you should take preventative measures to protect the crop from damage. 

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 potato 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. Studies have shown that mowing vines 10-14 days before harvest can help increase skin toughness and minimize harvest damage. Roots should be loaded immediately after digging as sunscald damage can occur as soon as 30 minutes after digging in sunny weather.

Curing immediately after harvesting is recommended. This minimizes damage and loss during storage by healing harvest wounds. To cure, maintain roots in temperatures between 80-86ºF 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 sweet potato is more starchy than sweet. During curing and storage, starches in the sweet potato 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.

Sweet potatoes can maintain excellent quality for up to a year in proper storage conditions. Use vented crates, maintain airflow, and do not store with onions or apples to prevent sprouting. The ideal storage conditions for sweet potato are the same as for winter squash; moderately warm (55-60ºF) and 58-90% relative humidity. Storage temperatures that drop below 55ºF can cause chilling injury which will render tubers unmarketable. Cold damage results in pithy flesh, internal darkening, hardcore, pitting, and shrunken legions.