PRODUCTION PRACTICES

Sweetpotato production is more complicated than that of many other vegetables because of the need for vegetative propagation. Growers must:

1. acquire storage roots and bed them to produce transplants,
2. plant the transplants,
3. grow plants for storage root production,
4. harvest the storage roots,
5. cure the storage roots and
6. store the storage roots.

At all stages, there is potential for disease transmission. Good sanitation practices and use of disinfectants will reduce the need for fungicides. In conventional production, fungicides are used to reduce disease transmission. Sweetpotatoes can be grown organically in the South but production is more difficult and the results less satisfactory than with organic production of other vegetables, because of this high potential for disease transmission.

Soils and Fertilization

The best soil types are well-drained, fine sandy loams. Poor aeration caused by poor drainage decreases yields and in sensitive cultivars can cause either 'souring' (tissue breakdown of the storage roots) with severely impeded drainage or water blisters (enlargement of lenticels on the periderm) if the drainage problem is less severe.

Heavy clay soils result in rough, irregular roots, but high (more than 2 percent) organic soils also reduce production. Sweetpotatoes will grow at a soil pH of 4.5 to 7.5, but 5.8 to 6.2 is optimal.

Harvesting one ton of sweetpotatoes removes 4 to 5 pounds N, 1.4 to 3 pounds P2O5, and 7 to 11 pounds K2O from the soil. Sweetpotatoes only need moderate amounts of nitrogen and phosphorus, but need significant amounts of potassium. Beauregard, a new introduction, requires even less N than Jewel. Low potassium reduces yields and increases the number of long, slender, malformed roots.

Sweetpotatoes also require more boron than many vegetables (see Degree of sensitivity of various plants to deficiencies of micronutrients. On boron deficient soils, .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. Except for very susceptible cultivars, blister does not show up until sweetpotatoes have been stored for several months. The table below lists plant nutrient requirements based on soil tests. See Soil Management for organic sources of these nutrients.

Planting

Acquiring and pre-sprouting seed roots. For growers in temperate zones, the first step in production is to acquire storage roots which are as true-to-type and free of disease as possible. Such high quality propagation material is available from certified seed growers but buying enough certified seed to produce the entire commercial crop would be very expensive. A good compromise is to buy enough certified seed to produce next yearâs 'seed' roots and to store these roots separately from the commercial crop. The certified material should be planted in a field that has not been used for sweetpotato production for several years. Small (0.75-to-1.5-inch diameter) disease-free and true-to-type roots are selected from roots produced on the certified material. Small and large roots will produce similar numbers of sprouts. This is because sprouts on the proximal or stem end of the root inhibit other sprouts until pulled off. Larger roots produce sprouts over a longer period of time, however. One bushel (50 pounds) of small roots will produce 500 to 1000 transplants. Approximately .3 to .6 tons of roots are required per acre.

Sweetpotato

Presprouting stored 'seed' at 75 to 85 degrees F and 90 percent relative humidity for 2 to 4 weeks doubles or triples the number of sprouts produced and also speeds up sprout production. Ventilation is necessary during presprouting, because roots are taking up oxygen and giving off CO2. Roots with scurf, black rot or other diseases or with obvious off-color flesh or skin mutations should be discarded before bedding. In conventional production, seedstock is treated with fungicides before bedding.

Bed selection and preparation. Unshaded sites on sandy, well-drained soils, out of sweetpotato production for 3 to 4 years are best. Water should be available for irrigation. Long beds, 24 to 30 inches wide are cultivated, and in conventional production, fumigated and fertilized with 75 pounds 8-8-8 per 100 square yards.

Small seed roots are placed 1 inch apart in the bed and large roots placed so they almost touch. Plastic netting is sometimes laid over the roots to hold them in place when the sprouts are pulled. Roots are then uniformly covered with 1 to 2 inches of soil.

A clear plastic mulch is laid over the bed to warm the soil. IRT (infrared transmitting) mulches are sometimes used to reduce weeds while still warming the soil. Holes ¸ inch in diameter are punched into the plastic at 4-inch spacings for aeration, but if the soil was too wet at the time the mulch was laid, anaerobic soil conditions may develop under the mulch and the bedded roots will rot. When the sprouts appear, the danger of frost is past, or temperatures under the plastic get above 90 degrees F, the cover must be removed. Sprouts, also called 'slips', are ready for transplanting when they have 8 or more leaves and are 8 to 12 inches long, which usually takes 4 to 6 weeks.

Cultivars vary in the number of sprouts produced by each root, as previously discussed. 'Jewel' roots average 10 to 15 sprouts, usually producing 6 at one time. Roots continue to produce sprouts for several weeks, depending on cultivar, root size, and the vigor of the bedded seedstock. An additional 20 pounds of NaNO3 per 100 square yards is topdressed after each pulling or cutting.

When transplants are pulled out of the ground, underground portions of the stem, roots formed near the mother root and some mother root tissue remains attached. To reduce the risk of disease, sprouts are sometimes allowed to grow longer and sprout cuttings, usually called vine cuttings, are made above the soil line. In a recent study in Georgia, cutting the shoots of 'Georgia Jet', 'Red Jewel' and 'Jewel' above ground resulted in more marketable U.S. No. 1 roots than pulling the shoots.

Field planting. Soil temperature in the production field should reach at least 65 degrees F at 4 inches for four consecutive days before transplanting. Feeder roots of the transplant will not grow at soil temperatures below 65 degrees F, and, if planted too early, vines develop a purple color, root yield is low, and roots are round or chunky rather than oblong. Silt or clay loams and sandy soils with hardpans are usually formed into wide, 8-to-10-inch high ridges to provide an appropriate drainage rate.

Rows are usually placed 32 to 42 inches apart with in-row spacings 6 to 12 inches apart, depending on the cultivar (see cultivar descriptions). Closer spacings delay harvest in most cultivars.

If water is provided at transplanting and as necessary for the next 40 days, the plants will probably survive later water stress. Thus, sweetpotatoes are sometimes thought of as a drought-tolerant crop. In reality, however, sweetpotatoes are vulnerable to reductions in yields and root quality if they receive irregular watering, too little water or too much water. Uneven water availability will cause growth cracks and drought may reduce yields. Too much water also injures sweetpotatoes. Storage roots of most cultivars cannot tolerate extended periods of very wet soil due to the lack of oxygen. In saturated soils, lenticels expand, becoming more noticeable. These expanded lenticels are sometimes called water blisters. If rainy conditions persist, roots 'sour' and rot. Thus late season rains often reduce yield and quality on poorly drained soils (see below).

Physiological Disorders

Growth cracks are caused by uneven growing conditions, usually uneven watering, and are sometimes associated with secondary disease problems. Cracks are most common on large roots and on nematode-infested roots. Certain viruses also increase cracking. Souring occurs in waterlogged soils and in airtight storage containers. Roots are very active metabolically and become oxygen-starved at low soil oxygen levels. Ethanol and CO2 accumulate as byproducts of anaerobic respiration. This accumulation breaks down root tissues of sensitive cultivars in the soil or later in storage.

Mutations. Sweetpotatoes have an unusually high rate of mutation, as high as 20 percent in the cultivar Jewel. Sometimes the entire plant is affected and sometimes only part. Multi-colored roots, called chimeras have areas of differently colored skin or flesh. An important function of certified seed producers and the foundation seed programs that support them is to rogue out or remove such mutations.

Sweetpotato cultivars with insect and disease resistance

U.S. Grade Size Requirements for Sweetpotato

* each root or usable piece