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Sweet potato

Propagating, planting and harvesting sweet potatoes

Planting material

Sweet potatoes are propagated vegetatively through vines (or runners) which are cut up in cuttings, each 300-400mm long. Most other crops are propagated with seed, which comes from the reproductive parts of the plant. Selecting disease and insect free material from a reputable supplier is the first cheap step in reducing spray cost and increasing yield potential.

Once the runners are cut, they must be placed in water. Never expose them to the hot sun as they will wilt and die very quickly. Runners stored in water can last for days, making planting process a little easier. Roots develop on the runners which are below water after 1-2 days. Do not leave the runners in the water for longer than a week.

Runners can be cut into two pieces. The top part or top vines and stem vines. It is a way of doubling your number of plants from the initial purchase. The top vines usually grow faster and provide higher yields. Top vines tend to take better in the soil than stem vines. In heavy soils and very hot climates it is better not to cut the vines in two so there is a better chance of growth. Smaller vines have less energy and have a higher death rate.

using vines from old plants in the field sounds like a good idea but it is not. It might work in the first two years, but after that your yields will decrease and the plants tend to be weaker. they have less resistance to diseases which further decreases yields. Always use the latest fresh vines from reputable growers.

Planting and harvest dates

The availability of planting material and climatic conditions determine when you can plant sweet potatoes in your area. They are the limiting factors. The average growth season is 4-5 months. Premature sweet potatoes are watery and have little sugar and do not taste well. A grower will quickly lose credibility if these sweet potatoes are marketed. Light tubers is also another sign of early harvesting.

Sweet potatoes can be planted anytime of the year as long as the temperature is high enough. The plants are not sensitive to day length.

Unless you are in a tropical area with relative warm winter, it is not recommended to use transplants in winter. Well, not in South Africa except for the low veld where it is possible to some degree.

Early market planting date in lowveld and north SA is January to March, harvest in July (if you are lucky) to November.

Late market planting date August to December and harvest January to May.

In areas where there is frost, the normal planting time is from the last week in November to the second week in December. Harvest from March to end of April.

Some growers try planting early in north facing lands in the first week in October. But that is risky and yields are usually lower.

The biggest threat to quality of the tubers is rain during the harvest period. If tubers are left too long in the soil they will rot and spread disease.

To compensate for low stands in early and late plants, plant vines at higher densities than normal.

Depending on the climate, sweet potato prices are at their highest during September to beginning of February, but it is not always possible to plant the crop in time for these markets. Try various varieties and see if you can get as close as possible.

In frost prone areas growers wait till the leaves died from frost which makes harvesting much easier. the crop can be left in the soil during the winter and once the market is ready, harvested for optimal prices. Do not irrigate during this time, otherwise the sweet potato tubers will rot. The keeping ability of sweet potato varieties differ significantly, so choose the right one when this method is used. You cannot use this method in the winter rainfall regions for obvious reasons.

Sweet potatoes need a growth season of at least 130 days so planting after December is not recommended.

In areas with heavy frost it is risky to leave the crop to overwinter as the soil temperatures can get so low that the tubers freeze, making them unmarketable. Frozen tubers go mush and rot in the soil.

Planting sweet potatoes

Most sweet potatoes are planted by hand because mechanical planters are expensive and difficult to come by. The most common method to plant sweet potatoes is with ridges, with each plant at least 300mm apart. Planting them closer yields smaller tubers. Supermarkets like small to medium tubers, so the plants can be 200mm apart.

In effect, the size of the tuber can be managed by the inline spacing. So read your market carefully. You can also vary the spacing throughout the field to supply different markets.

Crop rotation

Sweet potatoes cannot be grown on the same soil year after year without a buildup of pests and harmful soil diseases. Rotating sweet potatoes with different crops ensures healthy soils and minimum pests. Ideal other crops are maize, wheat oats and some vegetable crops.

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