FARMER'S BOOKSHELF

An information system of tropical crops in Hawaii
Department of Tropical Plant & Soil Sciences
University of Hawaii at Manoa



Sweet Potato

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Ask the Experts--Sweet Potato
Sweet potato insects, pests, and plant disease pathogens, Knowledge Master, CTAHR
Sweet potato production guidelines for Hawaii, Knowledge Master, CTAHR



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Adapted Cultivars


Moist Types

Dry Types

Semi-Dry Type

Sources of Planting Materials


Culture


Soil and Soil Preparation

Good soil drainage is essential for the production of a successful crop. Sweet potato cuttings should be planted on ridges. Height of the ridge is usually 8 - 14 inches, depending on soil drainage.


Irrigation

Yield can be increased by 30% with timely irrigation. Generally, sweet potatoes require less water than most vegetable crops. Irrigate moderately to get new planting established. When tuber formation starts at about the 7 - 9 week stage, avoid prolong water stress up till about the 14th week. Irrigation after a prolong period of late season water stress may cause tubers to crack. Irrigation should be stopped about a month before harvest.


Hilling

Done at about five weeks after planting. Usually the remaining half of the recommended fertilizers are applied just before hilling (assuming half was applied preplant). Soil is pulled up from both sides of each ridge increasing the width and height of e ach ridge by 1 - 3 inches. Hilling may aid in reducing damage by weevils.


Fertilizer


Soil pH

Optimum range 5.1 to 6.8.


Soil Amendment and Fertilization

Rely on soil test for lime and fertilizer applications. If soil pH is below 5.0 and/or the available calcium content is below 1,000 lbs. per acre, apply agricultural lime at the rate of 2,000 lbs./acre (4.5 lbs. per 100 square feet) 4 - 8 weeks before pl anting with adequate moisture in the soil. For a soil test with moderate levels of available phosphorus and potassium, a total of 650 lbs./acre (1.5 lbs./100 square feet) of 10-10-10 plus 130 lbs./acre (0.3 lb./100 square feet) of 0-0-61 (murate of potas h) fertilizer should be applied per crop.

If 15-15-15 is used, apply 425 lbs./acre (1.0 lb./100 square feet) plus 130 lbs./acre (0.5 lb./100 square feet) 0-0-61 (murate of potash) fertilizer. Approximately half of the fertilizers should be applied at planting (with the treblesuperphosphate) and the remainder about five weeks later as a side-dress just before rehilling. For soils very low in available phosphate, 1,000 lbs./acre (2.25 lbs./100 square feet) of treblesuperphosphate (047-0) should be applied preplant broadcast in 12 - 15 inch bands in the plant rows and incorporated into the soil 8 - 12 inches.

Sweet potato is a crop that requires not only nitrogen and phosphorus but especially adequate potassium for optimum production of tubers. High nitrogen fertilizers will cause excessive vine growth and deeper ridged tubers. Manures should be avoided beca use it renders the tubers to become more susceptible to scurf infection.


Harvest


Estimated Yield= 11,000 lbs. per acre


Planting to Harvest= Four to six months.

Harvesting

Most varieties are harvested at 4-1/2 to 5-1/2 months. If plants are left to grow longer, there may be greater percentage of "jumbos" which are marketed for less than Hawaii #1. Also, leaving the crop in the field longer may increase weevil in festation and disease infection.

The tubers should be dug with care to prevent bruise and cuts when the soil is dry. The vines are usually cut at the base and either removed or rolled into adjacent rows before digging with a middlebuster or potato harvester. If the digging must be done in wet soil, do not wash the tubers. Permit the wet adhering soil to dry in a shaded area until the soil dries and could be flaked off by gently rubbing with the hands.


Storage

After curing, keep at 55 - 60 degrees F with 85 to 90% relative humidity.


Insects recorded in Hawaii


Agrius cingulatus (Fabricius) sweetpotato hornworm
Agrotis ipsilon (Hufnagel) black cutworm
Amorbia emigratella Busck Mexican leafroller
Bedellia orchilella Walsingham sweetpotato leafminer
Bemisia tabaci (Gennadius) sweetpotato whitefly
Chaetanaphothrips orchidii (Moulton) thrips
Chaetocnema confinis Crotch sweetpotato flea beetle
Chrysodeixis chalcites (Esper) green garden looper
Conoderes amplicollis (Gyllenhal) gulf wireworm
Cylas formicarius elegantulus (Summers) sweetpotato weevil
Euscepes postfasciatus (Fairmaire) West Indian sweetpotato weevil
Hyperomyza lactucae (Linnaeus) an aphid
Macrosiphum euphorbiae (Thomas) potato aphid
Melanaphis sacchari (Zehntner) sugarcane aphid
Myzus persicae (Sulzer) green peach aphid
Omphisa anastomosalis (Guenee) sweetpotato vine borer
Pulvinaria urbicola Cockerell cottony sweetpotato scale
Pycnoderes quadrimaculatus Guerin-Mene.bean caspid
Spodoptera exigua (Hubner) beet armyworm
Tetranychus tumidus Banks tumid spider mite
Trialeurodes vaporariorum (Westwood) greenhouse whitefly


Planting


Propagation Method

Vine terminal cuttings or sprouts from tubers.

Amount of Cuttings

Spacing of 10 inches by 4 feet requires 13,068 cuttings per acre.

Spacing of 12 inches by 4 feet requires 10,890 cuttings per acre.


Planting Space

Four feet between rows and 10 to 12 inches between plants.


Selection of Cuttings

Select 12 - 14 inch vigorous, large vine terminals with many short internodes. Cuttings which have more internodes along a given length are considered to be more productive. (8 nodes per foot is good). Tubers will develop from the buried nodes.


Best Planting Period

Generally March, April, May are best. Poorest are October, November, December.


Planting Method

If irrigation is unlimited, horizontal planting is recommended because this method produces a greater number of uniform tubers. If irrigation is limited, vertical or slant planting is recommended. This method will usually yield larger tubers on nodes closer to the soil surface and smaller ones from deeper nodes. Generally, horizontal planting gives better yields than vertical or slant plantings. When planting vine cuttings, bury at least 3 - 4 nodes.


Planting Depth

Four to nine inches below the soil surface. Generally, horizontal plantings are 4 - 5 inches deep and vertical planting goes down to 9 inches. Too deep planting may reduce yields.


Vine-Turning

Turn over vines only if necessary. The main purpose of vine-turning is to prevent roots from growing from the nodes of vines in contact with the soil surface and to discourage formation of small irregular roots, once formed, it will check the growth of the normal tubers. Usually, on moist fertile soil 2 - 3 vine-turnings may be necessary several days after irrigation or if there is vigorous vine growth.


Curing

The following are reasons for curing sweet potato tubers.

Best conditions for curing is at 85 degrees F with 85 to 90% relative humidity. In most lowland areas in Hawaii, curing can be done by storing the tubers in a ventilated rodent-proof shed for about ten days.