Last revised February 15, 2010
Sweetpotatoes. Photo credit: Bill Mansour, Oregon State University
Sweetpotato is one word because the crop is distinctly different from potato (Solanum tuberosum) and yam (Dioscorea sp.), which are also grown and marketed in the U.S.A. Production of true yams (sweetpotato is also marketed as yam), however, is negligible. Almost all aspects of sweetpotato production, harvest, handling and storage, are different from potato, so these crops must not be confused nor treated similarly.
The most commonly grown sweetpotatoes are the orange-fleshed types. The market prefers those that have deep orange, moist flesh and smooth, thin, copper-colored skin.
White-fleshed types with red or magenta skin, commonly known as "Cuban Sweetpotato" or Boniato are also grown in the southeastern U.S. for ethnic markets.
Sweetpotato is marginally adapted only to the warmest areas of the Pacific Northwest, and only to those areas with a long growing season of over 150 days and warm nights.
Yam is a tropical crop important in north China and Japan and West Africa but of little consequence on the U.S. mainland outside of Florida. World-wide, ten species are important for food. Of these, Dioscorea batatas, D. japonica, and D. opposita are subtropical and marginally adapted to temperate regions, and might possibly be grown successfully in the Pacific Northwest. Yams are dry, starchy, generally white-fleshed, much larger than sweetpotato (usually 3-8 lb each), and are rarely found in Pacific Northwest produce markets. When found, they are usually imported from the Caribbean or are sweetpotatoes mislabeled as yams!
Sweetpotatoes are only adapted to the warmer parts of the state. They have been grown successfully in the Hermiston area for local market. They may be grown in the Ontario, Nyssa and Medford areas. Hot days and warm nights are important for successful commercial production. Long days promote vine growth while shortening days induce root development.
VARIETIES (130-150 days to maturity)
Many types of sweetpotatoes exist. Skin color may be various shades of tan, yellow-orange, red or reddish purple. Flesh color may be various shades of white, yellow-orange, orange, salmon-orange or red! The most common types are described:
Orange-fleshed: Beauregard (early, with some tolerance to soil rot, stores well, light rose skin, moderately deep orange flesh); Jewel (high yielding, deep copper skin, and deep orange flesh, resistant to root-knot nematode but intolerant to cool, wet soil). These two varieties accounted for about 90% of U.S. production in 1993. Others: Nemagold, Centennial (resistant to root-knot nematode and wireworm). For trial: insect resistant, USDA developed, varieties: Excel, Regal, Southern Delite (resistant to several wireworms and beetles, as well as root knot nematode). Others: Hernandez, Vardaman and Bush (bush types); Travis (earliest, for home garden).
Cream or White-fleshed: White Delight (purple skin, high yield, root-knot nematode resistant); others: Sumor, Nancy Hall, Picadita, Campeon, Star Leaf (generically known as Boniato).
Sandy and sandy loam soils that are well drained are generally best for sweetpotato production. Heavy soils and soils very high in organic matter are poorly suited. Soil pH should be 5.5-6.0. Long-time rotations are recommended because of scurf and Fusarium wilt. Scurf is also aggravated by high soil organic matter. Do not use soil that has been planted in sweetpotatoes for at least 3 years and do not use soil that receives the surface water drainage from sweetpotato fields. Soil preparation should be timed to minimize loss of soil moisture from the field at the time of seeding.
Sweetpotatoes are propagated from sprouts or from slips (vine cuttings); sprouts are preferred. Sprouts are grown from plant stock selected for its appearance, freedom from disease and off-types. Approximately 6-8 bushels of planting stock sweetpotatoes are needed to produce enough sprouts to plant one acre. For Pacific Northwest production it is recommended that sprouts for field production be purchased from reputable sweetpotato nurseries in California or elsewhere.
Pre-sprouting (root conditioning) procedure:
Buy only certified roots for sprout (planting stock ) production. Pre-sprouting roots is important for producing more sprouts per root and reducing the time needed to about 4-5 weeks. This is accomplished by holding the roots in a high temperature and high humidity environment until sprouts begin to develop. Temperature should be near 85 F and humidity about 95%. Do not allow root surfaces to become wet or white, hairy roots will begin to grow and root decay will increase.
Sprouts are produced from the conditioned roots in cold frames, heated beds, or field beds of clean sand or fumigated sandy soil. Conditioned roots are covered by 1-2 inches of sand. Four to 5 weeks are needed to develop strong plants if the soil in the plant beds has been kept at 75-80 F. Six to 8 weeks may be needed if roots have not been "preconditioned". Adequate moisture is especially critical to germination of the sprouts and proper root formation on the sprouts.
In California's San Joaquin Valley, sprout production begins about March 1 in covered beds constructed as follows:
An eight-foot-wide, flat-bottom trench is prepared in sandy soil.
Three to 4 inches of cotton gin waste is placed uniformly in the trench. Although manure has also been used, it is thought to be too variable and to generate too much heat during decomposition. Cotton gin waste is considered to be more uniform and slower to decompose, generating comparatively less heat.
One to 3 inches of soil are used to cover the cotton gin waste.
Conditioned roots are placed one finger apart and end-to-end uniformly across the width and length of the bed.
Roots are covered with 1 to 2 inches of soil.
Steel rods of 3/8-inch diameter are arched over the bed and inserted into the soil at 6 to 8-foot spacing.
Clear 4-mil plastic sheets are used to form tunnels. One edge is covered with soil and the other held down with bags of sand. Tunnels can then be opened for irrigation or other maintenance.
Planting the sprouts
Sprouts should be taken form the plant beds when 6-10 leaves and a strong root system have developed on each one. They are set out into the field as early as possible when the soil has warmed and the risk of frost or a cold weather period is passed.
Plants should be spaced 12-15 inches apart in rows 3.5 to 4 feet apart. This requires approximately 14,520 plants per acre. Manage water carefully to avoid transplant shock.
For the most current advice, see Nutrient Management for Sustainable Vegetable Cropping Systems in Western Oregon, available as a free download from the OSU Extension Catalog
The following are general recommendations only. It is advisable to use a soil test for each field to be planted. Band all fertilizer at planting.
Nitrogen: 50-100 (N) lb/acre.
Phosphorus: 100-150 (P2O5) lb/acre.
Potassium: 100-150 (K2O) lb/acre.
Sandy soils need frequent light irrigations. Apply water uniformly and reduce applications as tubers begin to mature. Approximately 20-25 inches of water may be necessary in the Hermiston area.
Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
PLASTIC GROUND MULCHES AND ROW COVERS
Use of plastic mulch and trickle irrigation has been shown to be very effective. Early and total yields are increased and more than compensate for the increased cost. For black plastic mulch to properly increase soil temperature, it is imperative that the soil surface be smooth and that the plastic adhere to the soil surface. This can only be accomplished with a plastic-laying machine designed and properly adjusted for this purpose. Clear plastic mulch is very effective for increasing soil temperature but does not control weeds. A new generation of plastic mulch films allows for good weed control together with soil warming that is intermediate between black plastic and clear film. These films are called IRT (infrared-transmitting) or wavelength-selective films. They are more expensive than black or clear films, but appear to be cost effective where soil warming is important.
Non-woven or spunbonded polyester and polypropylene, and perforated polyethylene field covers, may be used immediately after transplanting. Row covers increase heat unit accumulation by 2 to 3 times over ambient. Two to four degrees of frost protection may also be obtained at night. Soil temperatures and root growth are also increased under row covers as are early yields, and in some cases total yields.
HARVESTING, HANDLING, AND STORAGE
Sweetpotato yields vary widely and are approximately 125 to 250 cwt/acre. The storage roots will continue to grow until frost kills the vines but yields will not increase after that. The danger of losing all or part of the crop still in the field increases after frost has killed the vines. Such losses may result from a hard freeze that damages roots near the surface of the soil or from chilling injury to all roots as the soil temperature drops to 50 F or lower. Excessive moisture in the soil may also greatly damage root quality by aiding decay-producing organisms to enter the roots.
Sweetpotatoes should be handled as little as possible after harvesting to prevent cutting, skinning, and bruising. The roots must also not be exposed to the sun for more than an hour or so after digging because of sunscald damage. To prevent infection by disease-producing organisms, the roots should be brought to storage immediately after harvesting and cured.
STORAGE (Quoted or modified from USDA Ag. Handbook 66 and other sources)
Store at 55 to 60 F and 85 to 90% relative humidity. Sweetpotatoes are usually stored in non-refrigerated commercial or farm warehouses. The primary purpose of storing is to permit orderly marketing during several months after harvest. Sweetpotatoes should first be cured by holding at 85 F and 90 to 95% relative humidity for 4 to 7 days. Curing helps prevent the entrance of decay organisms by healing cuts and other injuries received in harvesting and handling. Such injuries should be kept to a minimum by careful handling. If the curing temperature and relative humidity are lower than recommended, healing is slower and less effective in preventing subsequent decay in storage or marketing.
Usually, sweetpotatoes will not keep satisfactorily if they have been exposed to excessively wet soil conditions just before harvest; chilled before or after harvest by exposure to temperatures of 50 F or below for about a week, or subjected, upon harvest, to a delay of 2 or more days before being provided with optimum conditions for healing.
Prompt curing after harvest is stressed, but it is especially important for sweetpotatoes that are harvested during or after a period of cold weather. Enough ventilation should be provided during curing to prevent accumulation of carbon dioxide, depletion of oxygen, or condensation of moisture.
Holding: After curing, the temperature should be reduced to 55 to 60 F, usually by ventilating the storage with outside air. The relative humidity should remain at 85 to 90% during storage. Most cured cultivars will keep satisfactorily for 4 to 7 months under these conditions. Storage at relative humidity above 90% is not recommended because of the possible development of surface discoloration and surface mold on the roots. Weight loss can be expected to be 2 to 6% during curing and about 2% during subsequent storage. Transformation of starch to sugar in sweetpotatoes takes place during curing and continues in storage for approximately 5 months.
The sweetpotato is of tropical origin and is chilled if held at temperatures below about 55 F. Uncured roots are more susceptible to chilling that cured ones. Cultivars differ slightly in their ability to withstand chilling injury, but all cultivars are sufficiently susceptible to make it desirable to avoid chilling conditions. Short periods at temperatures as low as 50 F need not cause alarm; but after a few days at 50 F, or shorter periods at lower temperatures, sweetpotatoes may develop discoloration of the flesh, internal breakdown, off-flavors and hard core when cooked, and increased susceptibility to decay.
Temperatures above 60 F stimulate development of sprouts (especially at high humidity), pithiness, and internal cork (a symptom of a virus disease) when it is present.
Refrigeration is now used in some large sweetpotato storages to extend the marketing season into warm weather, when ventilation will not maintain low enough temperatures.
Sweetpotatoes are usually stored in bulk bins or slatted crates. Palletization of crates and use of pallet boxes facilitate handling. Some of the newer storages equipped for palletized handling have separate curing and storage rooms. Sweetpotatoes can be cured in palletized field boxes in a room designed to provide recommended conditions for curing, and after curing, the sweetpotatoes can be carefully moved with a fork-lift to a room in which storage conditions are maintained continuously.
Curing and storage rooms must be kept clean. If storage containers are re-used, they should be steam heated to 122 F for 6 hours before use to reduce black rot and scurf contamination.
Sweetpotatoes are usually washed and graded, and sometimes waxed, before being shipped to market. They should be treated with a fungicide to reduce decay during marketing. An effective fungicide is 2,6-dicloro-4-nitroaniline (Botran).
Consumer packaging of sweetpotatoes in film bags or over-wrapped trays is done mainly to aid marketing and should not be done prior to storage. The shelf life of washed and fungicide-treated roots in consumer packs is only 2 to 3 weeks. Weight loss of roots during marketing is much less in perforated film bags than in mesh bags. Perforation of 3 to 5-lb polyethylene bags with about thirty-two 1/4-inch holes is essential to lower the internal relative humidity and avoid excessive sprouting, root growth, and dampness.
Controlled-atmosphere storage of sweetpotatoes is not a commercial practice, and the feasibility of long-term storage in controlled atmosphere remains unclear. Research showed that roots stored in 2 to 3% carbon dioxide and 7% oxygen were better than check roots held in air, as indicated by lower total losses due to decay and weight loss. However, sweetpotatoes stored at above 50% carbon dioxide or below 7% oxygen often developed an alcoholic flavor or other off-flavor.
Rhizopus soft rot is the most important and rapidly developing storage and transit rot.
Sweetpotatoes are commonly packaged in 50-lb cartons, crates and bushel baskets, or 40-lb cartons.