What you need to know about sweet potatoes – Part Four

Harvest Maturity

INDICES: Sweet potatoes do not mature as fruits do, but continue to grow as long as conditions allow. Harvest time is determined by market price, expected total yield, and root size. Sweet potatoes should be harvested when the majority of roots have reached the desired size for the intended market. This typically requires between three and three and half months from the time of transplanting.

Average root size in a field can be estimated by removing the soil around several randomly selected plants. Sweet potatoes will continue to enlarge if left in the ground, but root diseases and insect damage typically increase with the amount of time the roots remain in the soil.

Harvest Method
The sweet potato vines should be cut off at the soil level prior to the intended harvest date. During the dry season, the vines should be removed three to seven days before digging. During the rainy season, the vines should be left intact until just prior to harvest. Vine removal helps to toughen the skin of the root and facilitates harvesting. The vines can be removed manually with a scythe or machete, or mechanically with a rotary mower.

After vine removal, the sweet potato roots can be dug by hand or by machine. Manual harvesting of sweet potatoes typically involves the use of a metal spade, pick, or fork which is used to loosen the soil and undercut the roots. Care must be taken to avoid cutting or injury to the roots. The roots are then lifted out of the ground, separated from the main stem, and temporarily left on top of the soil or put directly into a field container. The roots should be handled gently to avoid skinning and bruising. Freshly dug sweet potatoes have a very thin and delicate skin that is easily removed. Skinned areas of the root surface are open wounds and become entry sites for bacterial and fungal pathogens. Skinning also lowers the appearance and attractiveness of the root. Workers should be advised not to throw or step on the roots. A range of mechanical harvest devices also exists for sweet potatoes.

These include moldboard ploughs, middle buster ploughs, and single or multiple row diggers. Mechanical harvesters require the vines to be removed prior to digging. Moldboard ploughs turn the soil and roots over on top of the ground and produce the least amount of physical damage to the roots. However, they leave many roots covered by soil that makes them difficult to recover. Middle buster ploughs (usually 30-35 cm size) with broad wings, may be used, although they tend to damage the roots and scatter them on both sides of the row. It is essential to operate the plough accurately on the row, at the proper depth, and at the correct speed.

Single or multiple row diggers undercut the roots and use a rod conveyor chain to separate the soil from the roots. The roots may be placed back on top of the ground or conveyed up the chain to a sorting crew riding on the harvester in the sophisticated models. The sweet potatoes are detached from the main stem by hand and graded by size or quality into field containers. Mechanical diggers can cause considerable root skinning in very dry soil or at high operating speeds. The chains should be padded to reduce bruising. A one-row chain-type digger is ideal for harvesting small plots and typically harvests 1 to 1.5 hectares per day at a harvest speed of 1.2 km/hr. Roots should be field graded by size and quality at the time of harvest. The rigorousness of grading depends on the demands of the market and the amount of variability in root size and quality. Select out sweet potatoes that are badly bruised, cut, severely mis-shapened, or insect damaged.

Severely damaged or unmarketable roots should be put in a separate field container away from the sound and marketable roots. Roots less than 2.5 cm (1 in) in diameter (strings) are generally discarded. The marketable roots should be gathered off the top of the soil as soon as possible after harvest. Sweet potatoes are highly susceptible to sunburn damage if exposed to intense or direct sun even for periods as short as a half hour. The damaged areas of the skin remain permanently discoloured and are very susceptible to postharvest decay. In order to avoid sunscald, the harvested roots should be put in field containers as soon as possible and covered with vegetation or stored away from direct sunlight exposure.

Sweet potatoes should never be thrown or left in open piles in the field. The roots should be gently dry brushed to remove excess soil prior to curing. Cotton gloves are typically worn to facilitate field cleaning of the roots.

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