Sweet potato is grown throughout the U.S. for its enlarged storage roots. More than 147,500 acres were planted in 2019. In Missouri, it is produced mainly by small produce growers, but the market is larger than what is grown locally and is expected to increase further. Therefore, Missouri growers have an opportunity to increase local production to supply local markets. The initial steps in sweet potato production are cultivar selection and propagation for field planting. See the "Midwest Vegetable Production Guide for Commercial Growers" for cultivars and pesticides recommended in Missouri and the Midwest. This article discusses on-farm sweet potato slip (sprouts or cuttings for field planting) production to optimize quality and timing for field planting.
Sweet potato is a perennial tropical crop but cultivated as annual in the U.S. It is sensitive to chilling temperatures (cold temperatures between 32°F and 60°F) and will die with freezing temperatures. However, production of sweet potato slips begins early in the spring under protected systems to harvest slips for field planting as soon as the soil temperature in the field reaches 65°F and the risk of freezing has passed.
Sweet potato is propagated by vegetative slips from sprouted seed-roots (storage root used for vegetative propagation). Select good quality, certified G1 or G2 seed-roots that are uniform and free from insects and diseases to plant in slip production beds. 'G' is referred to the generation or annual cycles after certified virus tested, true to type mother plant from tissue culture (G0). Plan to start propagation from seed-roots 10 to 12 weeks before the optimal date for field planting. Start by pre-sprouting seed-roots in a room at 85°F (29°C) and 90% relative humidity for 2 to 3 weeks or until sprouting is visible. Make sure room is ventilated because the process requires oxygen. Bedding can be done in the field with floating row covers as well as under low and high tunnels. Avoid sites that had sweet potato in the past 3 years to reduce the risk of soil-borne diseases. Fertilize with 4-5lb/100sqft bed area of 8-8-8 or equivalent fertilizer. Prior to bedding, treat seed-roots with appropriate fungicides to reduce decay. Plant seed-roots in beds 8 to 9 weeks before expected planting date. Spread seeds-roots (one layer) in beds 2-3 ft wide (Fig. 1) and cover with 2-3 inches of moist soil or sand. Large growers use an offset rotary ditcher to throw soil on top to cover the seed-roots. Smooth the bed top for good contact and cover with black plastic mulch to warm the soil (Fig. 2). Lay two lines of drip-tape before laying plastic mulch and irrigate as needed. Clear plastic mulch can be used but weed problems may arise and herbicides will be necessary. Punch holes every 3-4ft on each side of the bed for ventilation to prevent accumulation of carbon dioxide. Moist soil and temperatures between 75°F and 85°F (24-29°C) are optimal for growth.
Remove plastic mulch when sprouts begin to emerge (2 to 4 weeks after bedding) and cover with floating spun-bonded row cover or low tunnel to promote growth and protect against cold temperatures (Fig. 3). Irrigate either by drip or overhead sprinkler as necessary to promote growth especially during drought periods. Remove row cover when sprouts reach 12 to 14 inch or 5-7 day prior to planting to top trim to 10-12 inch and promote slip hardening under direct sunlight. The warmer conditions in greenhouses and tunnels promote sprouting and growth for an early slip production; however, slips are less sturdy for field planting than those from open beds.
Expect yields of 500 to 1,000 slips per 50-lb bushel of seed-roots in 10-15sqft of bed area. For field planting, 10- to 12-inch long sturdy slips with two to four leaves are best. Cut the slips (do not pull) from the beds at 1 to 2 inch above the soil line to minimize the risk of carrying soil pests and diseases (Fig. 4). Small growers can use a hedge trimmer, knife or sickle to cut slips, but there are slips harvesters used by large sweet potato growers. Cut the slips and put them in boxes to move them to the field. Select for quality and plant them in the field within 2 day after cutting to avoid loss of reserves and roots that have emerged while waiting to plant. Slip strength and quality influence root quality, which influences the root potential to develop into storage roots. Secondary weak roots are thinner without the potential for storage root development, so they grow into what is known as "pencil roots".
Sweet potato is cold sensitive and should be planted after danger of frost is over and the soil temperature at 4 inch-deep is >65°F (>18°C). In Missouri, sweet potato planting varies because of the temperature differences between north and south. In northern areas, growers start field planting the first week of June, but in southeastern areas, planting may start the last week of May. Check the weather forecast to make sure there is no risk of freezing before planting.
REVISED: January 7, 2021