Growing Your Seedstock
Improved planting stock is being produced in North Carolina by carefully selecting superior-yielding, high quality hills, eliminating disease using meristem tip culture, reducing mutations and providing better, true-to-type clones by maintaining planting stock in a vegetative state. Approximately 75 to 90 percent of the North Carolina sweet potato acreage is produced using this improved planting stock. Sweet potato roots for bedding and plants (slips) for transplanting into the field can be purchased from NC Certified Seed Producers or you can buy plants at a garden center if storing roots and pre-sprouting is not an option for you.
Planting sweet potato root pieces directly in the field, as done with white potato tubers, does not result in sufficiently uniform storage roots. True seed is not a viable option because Sweet Potatoes are genetically complex and plants growing from true seed are extremely variable. At all stages there is potential for disease transmission. Good sanitation practices and use of disinfectants will reduce the need for fungicides, but it is difficult to grow sweet potatoes without any crop protection.
In early March, the stored roots are pre-sprouted, a process by which sweet potato seed stock is conditioned to produce sprouts. Research has shown that pre-sprouted seed will yield 2 to 3 times as many plants and will produce them earlier than seed that has not been pre-sprouted.
To pre-sprout, warm the seed roots for 2 to 4 weeks at temperatures from 75-85 degrees, with a relative humidity of 90 percent. Ventilation is necessary during pre-sprouting because roots are taking up oxygen and giving off carbon dioxide. Roots with scurf, black rot or other diseases, obvious off-color flesh or skin mutations should be discarded before bedding. In conventional production, seedstock is treated with fungicides before bedding. Pre-sprout until most of the seed stock has sprouts approximately 1/4-inch long. Sprout development can be delayed if necessary, by lowering the temperature in the pre-sprouting room.
Once pre-sprouting is finished, the seed roots are bedded to produce sprouts or slips. Several types of beds may be used for growing sprouts – hot beds with or without plastic covers. The most practical type for the majority of North Carolina growers is the field bed covered with clear plastic. Seed stock is usually bedded by the end of March and plants are ready for transplanting into the field by early May in eastern North Carolina. Plastic greenhouses are recommended for early plant production. Plants can be produced in 4 to 5 weeks in greenhouses if artificial heat is provided and pre-sprouted seed stock is used.
Bed Site Selection and Preparation
To avoid any carry-over of disease and other rot pathogens from a previous crop, select a well-drained, sandy, loamy soil that has not been used to produce sweet potatoes for at least 3 years. Unshaded field sites that have been out of sweet potato production for 3 to 4 years are best. Good drainage is necessary to prevent rotting of the bedded roots. Locate the bed where water is available, as it will be necessary to irrigate periodically. However, do not use water sources that are fed by drainage from old sweet potato fields. Allow 12 square feet for each bushel of seed stock 1-1/8 to 2 inches in diameter. Allow 20 to 30 square feet for seed stock 1 to 2 inches in diameter. Additional space will need to be provided for driveways and alleys between beds.
The width of the bed is a matter of individual preference. Narrow beds, 24 inches to 28 inches wide, are popular because they can be prepared (opened) easily and the seed stock can be covered with soil mechanically. Equipment is also available to place the plastic covers over the beds mechanically. Cutting transplants from narrow beds is preferred because workers damage fewer developing plants and mother roots.
Plant bed sites are usually not fumigated. If serious weed and disease problems are anticipated, the site may be treated in the fall when the soil is warm. Do not treat when the soil temperature at the 4-inch depth is less than 55 degrees F.
Each bedded seed root will produce up to 15 plants. Often, as many as 6 sprouts will be growing on each root at one time. To prevent the production of small, weak, spindly plants, give the seed roots sufficient space: at least 1 inch between roots that are less than 2 inches in diameter and just enough space that the roots do not touch for those more than 2 inches in diameter. Be sure that the depth of the soil covering the seed stock is at least 1 inch. Seed stock covered with more than 3 inches of soil may rot as a result of oxygen starvation.
Fertilizing Plant Beds
Proper fertilization will increase the size and vigor of the plants as well as the number of transplants produced. Fertilizer may be applied to the top of the bed, 75 pounds of 8-8-8 fertilizer per 100 square yards of bed space, after the seed roots have been covered with soil. After application, rake the top of the bed lightly, without injuring the roots, to incorporate the fertilizer into the top inch of soil. For those bedding larger quantities of seed stock, it is best to broadcast the fertilizer and mix it into the top 6 inches of soil before making the beds. Use 3,600 pounds of 8-8-8 fertilizer per acre. While this method greatly reduces the labor and time required to apply the fertilizer, the alleys and walkways are also fertilized.
Covering Plant Beds
After bedding and fertilization is completed, cover each bed with 1.5-to 2.0-mil clear plastic film to trap and hold the heat in the bed. Do not attempt to keep the plastic off the soil. Cover the edges of the plastic with soil to keep the wind from blowing it off. Punch a few small holes in the plastic cover (a 1/2 inch diameter hole every 4 inches) for ventilation. These holes allow carbon dioxide to escape and oxygen to enter, minimizing oxygen starvation, carbon dioxide buildup, overheating, and decay. If oxygen is limited or the carbon dioxide cannot escape, the roots will decay. This usually occurs when the soil in the bed is extremely wet. It is undesirable to cover the sweet potatoes with soil and wait until after a rain to put on the plastic covers. If the cover is airtight, the soil surface under the plastic may reach 120 degrees on a clear day when the outside air temperature is only 70 degrees. If such conditions persist for an extended period, sprout production may be reduced.
Managing Plant Beds
Remove the plastic covers when the plants begin to emerge throughout the bed or until chance of a killing frost has passed. After each cutting or pulling of plants, and after each leaching raintop dress the bed with 1 to 3 pounds of nitrogen per 100 square yards of bed. This amount is enough to keep the plants growing but not enough to make them tender and brittle. The beds should be irrigated as needed to maintain adequate soil moisture for plant growth. Irrigation after each cutting helps rejuvenate the beds and activate applied fertilizers. Disease and weed treatments are usually neither warranted nor practical after plants have emerged.
Producing and Handling Quality Plants
The quality and vigor of plants are best determined by the size of the stems and the number of leaves. The best plants are 8 to 12 inches long and have 8 or more leaves. Plants that have less than 8 leaves and are slender-stemmed and weak, do not survive in the field, especially if the soil is dry immediately after transplanting. Plant cuttings longer than 12 inches create a problem if a precision-type transplanter is used, and they will be difficult to cultivate even if they go through the transplanter. Transplant as soon as possible after removal from the bed, supply 1.5 to 2 ounces of water to each plant when planted, and press the transplanting hole closed. Do not dip the plants in water because this will spread the pathogens that cause bacterial soft rot, pox, fusarium root, stem rot and other diseases.
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 top-dressed after each pulling or cutting.
Plant Bed Destruction
After the plants have been harvested, destroy the bedded seed stock. Even though “mother roots” may look good enough to market, they should not be eaten. This used seed stock has been exposed to materials and conditions that render it unsuitable or even dangerous for human consumption.