Cultivating and Harvesting
Cultivate at least 2-3 times with a rolling cultivator to apply fertilizer and to control emerged weeds after transplanting. Cultivator gangs are set to throw dirt on the sides of the ridge and leave a 2-inch band across the transplants untouched. To control weeds, herbicide can also be sprayed during cultivation. After sweet potato vines have grown down the sides of the ridge it is best not to cultivate anymore because the vines suppress weed growth, and yields can decrease when the vine is injured.
Storage roots continue to grow until the leaves are killed by frost. Depending on the cultivar, roots develop to marketable size in 90 to 120 days after transplanting. Time of harvest is often determined by digging up a few representative plants and determining the percentage of roots in the size classes. Normally, harvest begins when most of the roots are in the No. 1 size class, as No. 1 sweet potatoes command the highest price. When to harvest can best be determined by sampling or digging a couple of rows in each field.
When tops of the plants turn black after the first frost, it is imperative to harvest as quickly as possible. However, partial or complete freezing of the foliage is not likely to damage the crop unless the temperature of the soil around the roots falls below 55 degrees for several hours. Roots chilled below 40 degrees overnight may develop internal breakdown in storage or may develop hardcore. In harvest systems where the vines would tangle in the harvester, vines are cut before harvest. Vine killing in hot, wet weather or in poorly drained soils may result in anaerobic conditions and subsequent souring of roots either in the ground or in storage, so roots should not be left in the ground for long periods after the vines are killed.
Sweet potato roots are turned up on top of the ground by a side angle disk plow and partially exposed to aid the workers in picking and sorting.
Sweet potatoes are very susceptible to damage at harvest; therefore hand-harvest is preferred over mechanical harvesting. Harvested roots left in the sun at temperatures above 90 degrees sunscald in 30 minutes. Scalded areas turn purplish-brown and are more susceptible to storage rots. In very dry soil, the root periderm, or outer layer of skin, becomes more fragile and easily abraded, or “skinned,” on the hard soil clods during harvest. Sweet potato roots do not have a thick protective outer layer of cells such as that on white potato tubers. Any abrasion can lead to rots in storage. Skinning injury in dry soil can be avoided either by waiting for rain or by irrigating the field before harvest. Rough handling by workers also tears the delicate skin. Workers should wear gloves and avoid dumping the roots roughly into the container.
Skinned areas can become dark and sunken and surrounded by a narrow brown border. These scars offer opportunities for storage rot pathogens such as Fusarium to enter the root. Curing roots after harvest allows the periderm to reform, reducing subsequent storage damage. Skinning also takes place in packing and shipping to markets so packing lines should be designed to reduce injury.
To harvest, the field rows are usually plowed with a modified disk or moldboard plow with a spiral attachment. Roots are then hand harvested and graded in the field. Sweet potatoes can also be dug by a chain digger or a riding harvester that conveys the roots to a sorting crew using a harvest aide. Potato harvesters are sometimes used to harvest sweet potatoes but damage is usually unacceptably high.
After harvest, the sweet potatoes are transferred to 20-, 40-, or 60-bushel bulk containers and are either packed and shipped immediately or put into a controlled environment storage facility for curing.