Sweet potato growers in Missouri can increase production and gain market share by growing their own slips/transplants under high/low tunnels for early field planting. This article discusses results of a study that demonstrated that early field planting with slips grown on-farm under low/high tunnels provide 2-4 weeks earlier start than with slips from open unprotected beds, which resulted in 18% to 82% larger marketable yield. This study was supported by a Specialty Crop Block Grant (Missouri Department of Agriculture and U.S. Department of Agriculture - Agricultural Marketing Service).

Sweet potato is a perennial tropical crop but cultivated as an 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. Therefore, planting should begin when soil temperature is above 65°F and the risk of freezing has passed. In addition, sweet potato storage roots can keep growing in the Fall as long as temperature and soil moisture are favorable. The longer the growing season the larger the yield the crop can achieve. Therefore, an early planting with on-farm produced slips can extend the growing season and the sweet potato crop has the best chance to reach profitable yields.

In our 2-year study of slip production under protected systems, slip harvest and field planting started the last week of May with slips from the low tunnel inside a high tunnel in three Missouri locations (Oregon Co., Greene Co., and Henry Co.). About a week later, slips from the high tunnel without low tunnel were ready to harvest and used for field planting. Depending on location, 2 to 3 weeks after the first planting, slips from the low tunnels were harvested and field planted for storage root production. Finally, field planting with slips from the plots without any protection occurred 3 to 4 weeks after the first field planting. Figure 1 shows the difference in growth of sweet potato plants from the four planting times one week apart (Oregon, Co). The first planting date covered the surface area before the second, third, and fourth planting date. Similarly, the second planting date covered the surface area before the third and fourth planting date, and so on. This earliness in vegetative growth allowed the plants to initiate storage root development earlier than the later planting dates.

Figure 1 Growth difference in field sweet potato plots planted sequentially (about a week apart) in Oregon Co. MO, 2022. First planting was the last week of May (left), second planting a week later (center left), third planting two weeks after the first planting (center right), and the fourth planting three weeks after the first planting (right). Plots were drip-irrigated, and pictures taken a week after the last planting date.

Early field planting allowed a longer growing season for the valuable storage roots to develop. In the earlier planting, sweet potato storage roots developed first and had a longer season to grow by harvest time.

The lengths of the growing period in subsequent plantings were shorter according to the planting dates since all plots were harvested the same day at the end of September. This difference in storage root growth was reflected in the yield (figure 2). The average marketable yield (the sum of large, medium, and small storage roots) and yield of large storage roots from the three locations were different among the planting times. The larger marketable yield (518lb/100ft planted row) was from the first planting time the last week of May, then the second planting time with 405lb/100ft, the third planting with 334lb/100ft, and the smallest marketable yield (284lb/100ft) from the last planting date. Also, the average yield of medium size storage roots was larger in the first planting date than the last planting date.

Figure 2 Yield difference in field sweet potato plots (15ft long) planted sequentially in Henry Co. MO and harvested the last week of September 2021. Sweet potato storage roots were separated by size: large (jumbo), medium (US1), small, and cull. Top left corresponds to the first planting the last week of May, top right corresponds to the second planting a week later, bottom left corresponds to the third planting two weeks after the first planting, and bottom right corresponds to the last planting four weeks after the first planting.

Based on these results, the shorter growing periods due to the delayed plantings resulted in reduced yield (figures 2). Consequently, sweet potato yield is influenced by the length of the growing season. However, scouting the field to harvest when most of the storage roots are of the most desirable size is recommended. In general, the medium size (U.S.#1) is the most valuable, but most direct sale farmers can sell all sizes at similar price. Too long growing season may promote development of large storage roots, so adjusting plant spacing may be necessary to obtain the highest proportion of desirable storage root size. In addition, irrigation plays an important role in growing sweet potatoes, which promotes growth and storage root development. In this trial, sweet potatoes were drip-irrigated in two locations and sprinkler irrigated in the other, which was crucial in obtaining high yields.

In summary, early planting with quality, on-farm grown sweet potato slips available early in the season (last week of May) can increase yield and reduce planting costs, which would improve the economic sustainability of the operation.