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 is one of the most important food crops in the developing world, where over 90% of the production is found. North Carolina and Louisiana are leading producers of sweet potato in the US.

The skin of sweet potatoes can be yellow, orange, copper, red, or purple; the flesh can be white, yellow, dark orange, red, and purple. Copper-colored skin with moist orange-colored flesh are the most common types sold in New England; however, many ethnic populations prefer the starchier white-fleshed varieties.

Sweet potato is a frost-sensitive crop that needs between 90 and 150 days of frost-free period to produce harvestable roots. Sweet potatoes are grown from slips of vine cuttings. Slips can be produced by placing sweet potato roots in warm (75-80F) moist sand or soilless media until sprouts are produced (about one month). 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.

Sweet potatoes grow best in well-drained, loam soils which result in larger and well-shaped roots. Sweet potatoes grown in heavy clay soils, or in soils with high soil OM may produce rough, irregular roots.

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.

Research at UMass showed that yields were increased by using raised beds covered with black plastic mulch. In New England, it is difficult to obtain good yields on bare soil.

Varieties

Beauregard. Copper/rose-colored skin with orange flesh. High yielding variety for New England.

Carolina Ruby. Deep red skin with dark orange flesh. Rough skin and irregular shapes.
Covington. An improved Beauregard-type, a 2005 NC State University release. High yielding, excellent uniformity and flavor.

Georgia Jet. Orange/red skin, deep orange flesh. Very early, susceptible to cracking.

Vardaman –Small roots with copper skin, deep orange flesh, outstanding flavor.

White-Fleshed:

Japanese – White flesh with pink/purple skin.

O’Henry–Large white-skinned roots with creamy moist flesh.

White Yam –Slender tapered tan-skinned roots with dry white flesh.

Plant Spacing

Rows are 32” to 42” apart with in-row spacing 8” to 12”, depending on cultivars. The sweet potato plants (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.

Lime

Sweet potatoes will grow at a soil pH of 4.5 to 7.5, but 5.8 to 6.2 is optimal.

Fertilizer

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

Sweet potatoes also require more boron than many vegetables. On boron-deficient soils, 0.5 pounds B per acre (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.

Harvest

Sweet potato roots continue to grow until the leaves are killed by frost or until soil temperatures fall consistently below 65F, 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 and Storage

Curing is recommended when selling sweet potato wholesale. This minimizes further damage by healing harvest wounds. Maintain roots in temperatures between 80°F to 86°F and a high relative humidity (85-95) for 4 to 7 days. This forms a corky periderm layer below the damaged areas which limits microbial invasion and water loss. Also during curing there is a loss of dry matter (respiration) and an increase in sugars, which increases flavor. A freshly harvested sweet potato is more starchy than sweet. It is recommended to wait at least three 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 up to a year in proper storage conditions. The ideal storage conditions for sweet potato are the same as for winter squash; moderately warm (55-60F) at 60-75% relative humidity. 

Weed Control

NOTE:  For the herbicides listed below, one product trade name and formulation is provided for each active ingredient (common name) as an example of rates, days to harvest (dh), REI, and special instructions. In many cases there are other products available with the same active ingredient. Please see Table 17 and Alphabetical Listing by Trade Name for more information on products with the same active ingredients. 

clomazone (Command 3ME): 20 to 64 oz/A (REI 12h, Group 13). Apply to the soil surface prior to transplanting. Some temporary crop injury (partial whitening of leaf or stem tissue) may be visible after crop emergence or after transplanting. Complete recovery will occur from minor early injury without affecting yield or earliness. See label for replanting restrictions.

napropamide (Devrinol 50DF): 2 to 4 lb/A (REI 12h, Group 15). Apply to the soil surface after transplanting and wet the soil to a depth of 2” to 4” within 24 hours to activate.

Insect Control

NOTE:  For the insecticides listed below, one product trade name and formulation is provided for each active ingredient (common name) as an example of rates, days to harvest (dh), REI, and special instructions. In many cases there are other products available with the same active ingredient. Please see Table 20 and Alphabetical Listing by Trade Name for more information on products with the same active ingredients.

Aphids

For more information see aphids in the pepper section.

acetamiprid (Assail 30SG): 2.5 - 4 oz/A (7 dh, REI 12h, Group 4).

azadirachtin (Neemix 4.5): 5 to 7 oz/A (0 dh, REI 12h, Group 18). Suppression and adult feeding deterrent. OMRI listed.

Beauveria bassiana (Mycotrol O): 8 to 32 oz/A (0 dh, REI 4h, Group 22). Treat when populations are low and thoroughly cover foliage. Takes 7 to 10 days after the first spray to see control. Repeat applications may be needed. OMRI listed.

imidacloprid (Admire Pro): 0.3 to 0.7 oz/1,000 row ft, 4.4 to 10.5 oz/A (3 dh leaves, 125 dh corms, REI 12h, Group 4A). See plant-back restrictions. To avoid resistance, do not use another nicotinoid (Group 4) insecticide if imidacloprid was used at planting.

imidacloprid (Provado 1.6F): 3.5 oz/A (7 dh, REI 12h, Group 4A). See plant-back restrictions. For resistance management purposes, do not use a Provado foliar application following another nicotinoid (Group 4) insecticide application on the same crop. To protect pollinators, avoid use during bloom and for at least 10 days prior to bloom.

lambda-cyhalothrin (Warrior*): 2.5 to 3.8 oz/A (14 dh, REI 24h, Group 3A).

pymetrozine (Fulfill): 2.75 to 5.5 oz/A (14 dh, REI 12h, Group 9A). Selective control of aphids. Translaminar. Apply before populations build up. 

pyrethrin (PyGanic EC5.0): 4.5 to 18 oz/A (0 dh, REI 12h, Group 3A). OMRI listed.

pyrethrins + piperonyl butoxide (Pyrenone): 1 tsp/gal, or 1 to 12 oz/A (0 dh, REI 12h, Group 3A).

spirotetramat (Movento): 4-5 oz/A (7 dh, REI 24 h, Group 23). Immature stages; may also reduce adult fertility.

thiamethoxam (Actara): 3 oz/A (14 dh, REI 12h, Group 4). Foliar spray.

thiamethoxam (Platinum): 5 to 8 oz/A (30 dh, REI 12h, Group 4). Systemic insecticide applied to seed pieces in-furrow during planting, impregnated on dry granular fertilizer, or as directed spray at plant emergence or during last hilling operation. Must incorporate into root zone with sufficient irrigation within 24 hours. DO NOT apply as a foliar spray.

Cutworms

Caterpillars hide under the soil surface adjacent to the plant stem during the day and feed on stems after dark. For best results, make application between midnight and dawn while cutworms are feeding aboveground. See cutworms in the Pepper and Tomato (Outdoor) sections for more information on the black and variegated cutworms.

Bacillus thuringiensis aizawai (XenTari): 0.5 to 1.5 lb/A (0 dh, REI 4h, Group 11). Must be ingested; apply in evening before larvae are actively feeding and direct sprays to base of stem. Use high rate at cool temperatures. OMRI listed.

Bacillus thuringiensis kurstaki (Dipel DF): 0.5 to 1 lb/A (0 dh, REI 4h, Group 11). Must be ingested; apply in evening before larvae are actively feeding and direct sprays to base of stem. Use high rate at cool temperatures. OMRI listed.

beta-cyfluthrin (Baythroid* XL): 0.8 to 1.6 oz/A (0 dh, REI 12h, Group 3A).

carbaryl (10% Sevin Granules): 20 lb/A (7 dh, REI 12h, Group 1A). Apply evenly over soil surface. Less toxic to aphid and mite natural enemies than XLR Plus formulation.

deltamethrin (Decis* 1.5EC): 1 to 2.4 oz/A (3 dh, REI 12h, Group 3A).

lambda-cyhalothrin (Warrior*): 1.9 to 3.2 oz/A (14 dh, REI 24h, Group 3A).

methoxyfenozide (Intrepid 2F): 6 to 10 oz/A (1 dh, REI 4h, Group 18). Insect growth regulator. Works on all instars. Feeding stops within hours but death takes several days. Suppression only.

pyrethrins + piperonyl butoxide (Pyrenone): 1 tsp/gal, or 1 to 12 oz/A (0 dh, REI 12h, Group 3A).

zeta-cypermethrin (Mustang*): 1.4 to 4.3 oz/A (1 dh, REI 12h, Group 3A).

Flea Beetles

Sweetpotato flea beetle adults consume narrow strips of leaves on young plants, rather than producing numerous round holes that are characteristic of most species. Larvae rarely feed on sweet potato, but may damage fibrous roots or etch channels into the surface of tubers. 

acetamiprid (Assail 30SG): 1.5 to 2.5 oz/A (7 dh, REI 12h, Group 4).

beta-cyfluthrin (Baythroid* XL): 1.6 to 2.8 oz/A (0 dh, REI 12h, Group 3A).

carbaryl (Sevin XLR Plus): 1 to 2 qt/A (7 dh, REI 12h, Group 1A).

chlorpyrifos (Lorsban 75WG): 2.67 lb/A (125 dh, REI 24h, Group 1B). Apply as pre-plant broadcast spray and incorporate into top 4” to 6” of soil.

deltamethrin (Decis* 1.5EC): 1.5 to 2.4 oz/A (3 dh, REI 12h, Group 3A).

endosulfan (Thionex* 50W): 1 lb/A (1 dh, REI 24h, Group 2A).

imidacloprid (Admire Pro): 0.3 to 0.7 oz/1,000 row ft, 4.4 to 10.5 oz/A (3 dh leaves, 125 dh corms, REI 12h, Group 4A). See plant-back restrictions. To avoid resistance, do not use another nicotinoid (Group 4A) insecticide if imidacloprid was used at planting.

imidacloprid (Provado 1.6F): 3.5 oz/A (7 dh, REI 12h, Group 4A). See plant-back restrictions. For resistance management purposes, do not use a Provado foliar application following another nicotinoid (Group 4A) insecticide application on the same crop.

kaolin (Surround WP): 25 to 50 lb/A or 0.25 to 0.5 lb/gal (0 dh, REI 4h). Suppression and repellence only.  Follow label instructions for mixing. Generally compatible as a tank mix with other insecticides. OMRI listed.

lambda-cyhalothrin (Warrior*): 2.5 to 3.8 oz/A (14 dh, REI 24h, Group 3A).

pyrethrin (PyGanic EC5.0): 4.5 to 18 oz/A (0 dh, REI 12h, Group 3A). OMRI listed.

pyrethrins + piperonyl butoxide (Pyrenone): 1 tsp/gal, or 1 to 12 oz/A (0 dh, REI 12h, Group 3A).

thiamethoxam (Actara): 1.5 to 3 oz/A (14 dh, REI 12h, Group 4). Foliar spray.

thiamethoxam (Platinum): 5 to 8 oz/A (30 dh, REI 12h, Group 4). Systemic insecticide applied to seed pieces in-furrow during planting, impregnated on dry granular fertilizer, or as directed spray at plant emergence or during last hilling operation. Must incorporate into root zone with sufficient irrigation within 24 hours. DO NOT apply as a foliar spray.

zeta-cypermethrin (Mustang*): 1.9 to 4.3 oz/A (1 dh, REI 12h, Group 3A).

Tortoise Beetle

Adults and larvae feed on underside of leaves producing numerous holes. 

carbaryl (Sevin XLR Plus): 1 to 2 qt/A (7 dh, REI 12h, Group 1A). May encourage buildup of aphids by killing natural enemies.

lambda-cyhalothrin (Warrior*): 2.5 to 3.8 oz/A (14 dh, REI 24h, Group 3A).

pyrethrins + piperonyl butoxide (Pyrenone): 1 tsp/gal, or 1 to 12 oz/A (0 dh, REI 12h, Group 3A).

Slugs

Damage appears as shredded foliage. Look for silvery slime trails on leaves or turn over soil clods or debris to find slugs during daylight hours. Grow plants away from moist, shaded habitats, use clean cultivation, control weeds, hand pick/crush slugs or scatter baits on the ground near infested plants. See the Cabbage Section for more information on slugs.

iron phosphate (Sluggo: Snail and Slug Bait): 20 to 44 lb/A (0 dh, REI 0h, Group 9B). Apply around perimeter, scatter around base of plants, or band down rows. Apply to moist soil in the evening. OMRI listed.

metaldehyde (Deadline Bullets): 20 to 40 lb/A (REI 12h). Soil surface treatment or band treatment between rows after formation of edible parts. Do not apply directly to or contaminate edible portions of plants.

Whiteflies

For more information see whiteflies in the tomato section.

Beauveria bassiana (Mycotrol O): 8 to 32 oz/A (0 dh, REI 4h, Group 22). Thoroughly cover foliage. Takes 7 to 10 days after application to see control. OMRI listed.

endosulfan (Thionex* 50W): 2 lb/A (1 dh, REI 24h, Group 2A).

imidacloprid (Admire Pro): 0.3 to 0.7 oz/1,000 row ft, 4.4 to 10.5 oz/A (3 dh leaves, 125 dh corms, REI 12h, Group 4A). To avoid resistance, do not use another nicotinoid (Group 4A) insecticide if Admire Pro was used at planting.

imidacloprid (Provado 1.6F): 3.5 oz/A (7 dh, REI 12h, Group 4A). For resistance management purposes, do not use a Provado foliar application following another nicotinoid (Group 4A) insecticide application on the same crop.

lambda-cyhalothrin (Warrior*): 3.8 oz/A (14 dh, REI 24h, Group 3A). Suppression only.

novaluron (Rimon 0.83EC): 9 to 12 oz/A (14 dh, REI 12h, Group 16B). No more than two applications per season. Insect growth regulator, active on immature stages only. Suppression only.

pyrethrin (PyGanic EC5.0): 4.5 to 18 oz/A (0 dh, REI 12h, Group 3A). OMRI listed.

pyrethrins + piperonyl butoxide (Pyrenone): 1 tsp/gal, or 1 to 12 oz/A (0 dh, EI 12h, Group 3A).

spiromesifen (Oberon 2SC): 8 to 16 oz/A (7 dh, REI 12h, Group 23). Immature whiteflies only.

spirotetramat (Movento): 4-5 oz/A (7 dh, REI 24 h, Group 23). Immature stages; may also reduce adult fertility.

Wireworms and White Grubs

These root pests are favored by low, heavy soils that have been is sod or pasture.  Delay planting susceptible crops, such as corn, potatoes or sweet potatoes, on such land for at least 2 years after the sod has been broken. 

chlorpyrifos (Lorsban 75WG): 2.67 lb/A (125 dh, REI 24h, Group 1B). Apply as pre-plant broadcast spray and incorporate into top 4” to 6” of soil. Wireworms only.

ethoprop (Mocap* 15G): 20 to 26 lb/A, or 1.6 to 2.1 lb/1,000 row feet (REI 48h, Group 1B). Apply in a 12-15” wide band, 2 to 3 weeks prior to planting. Mix in top 2” to 4” of soil right after application.

Disease Control

Prevention is essential for disease control in sweet potato. The most important diseases are root diseases which become systemic; once infected it is not possible to cure the plant. Vegetative propagation is an ideal vehicle for disease perpetuation. Start with healthy propagating stock and inspect frequently in the production cycle. Planting stock should be produced and stored separately from commercial sweet potatoes. Careful handling and proper curing at harvest are primary disease prevention measures. Application of fungicides and bactericides are not as effective as proper curing in controlling post-harvest diseases. Practice good sanitation of farm equipment, storage bins, and storage rooms.

NOTE:  For the disease control products listed below, one product trade name and formulation is provided for each active ingredient (common name) as an example of rates, days to harvest (dh), REI, and special instructions. In many cases there are other products available with the same active ingredient. Please see Table 22 and Alphabetical Listing by Trade Name for more information on products with the same active ingredients.

Rhizoctonia Root Rot and Stem Canker

azoxystrobin (Quadris): 0.4 to 0.8 fl oz/1000 row feet (0 dh, REI 4h, Group 11). Apply in-furrow or banded applications over row. Rotate with a fungicide with a different mode of action.

thiabendazole (Mertect 340F): 107 fl oz/100 gal water. (REI 12h, Group 1). Apply as a seed root treatment at bedding. Do not use treated seed roots as food or feed.

Black Rot (Ceratocystis fimbriata)

One of the most significant diseases of sweet potato, black rot is the common name for the decay of fleshy roots and symptoms on sprouts and vines. Successful control of black rot depends on selection of healthy seed roots, fungicide treatment, crop rotation, proper curing, and effective sanitation.

1, 3-dichloropropene plus chloropicrin (Telone C 17): 10.8 to 17.1 gallons/A (REI 5 days). Soil fumigant. High acute inhalation toxicity and carcinogenicity.

thiabendazole (Mertect 340 F): 107 fl oz/ 100 gallons (REI 12h, Group 1). Seed root treatment. Do not use the treated roots for food or feed.

 Soft Rot (Rhizopus)

Careful handling and proper curing at harvest are primary disease prevention measures. Application of fungicides and bactericides are not as effective as proper curing in controlling post-harvest diseases. Practice good sanitation of farm equipment, storage bins, and storage rooms.

dichloran (Botran 75 W): 1.0 lb/100 gal water. (REI 12h, Group 14). Apply at bedding or as post harvest dip.