The best option in a cold area is to plant them as soon as the likelihood of frost has passed, and then plant again up until December.
Varieties differ in their ability to remain in a marketable state through winter. The Ndou, which I recommend, has a long life when not actively growing, and can be harvested and marketed when active growth resumes.
Where severe frost is likely to occur before the crop can be harvested, draw a ridger through the land to pull enough soil over the crop to protect it.
The problem with delaying the harvesting of earlier plantings is that many tubers become too large for the market. They still make good eating, but are visually unacceptable and would therefore be used only for processing.
A frost-free area allows for a far longer planting season. Sweet potato can handle the heat as well.
As I have often mentioned, there is no specific fertiliser programme for each vegetable; the fertilisation required depends on the nutrients available in the soil at the time.
The ‘rule’ is straightforward: increase the mineral content of the elements in short supply and get them into balance with one another as much as is practically possible.
Next, add nitrogen fertiliser as necessary to achieve a healthy colour and growth. When the fertiliser has been applied and worked in, ridge the land, placing the ridges about 1m apart.
You are now ready to plant.
The runners of the sweet potato plants should be approximately 30cm long. If they’re much longer in the nursery, break them off at the desired length. You may have read or been told that a runner with a growing tip is superior, but the difference is negligible, so don’t worry about it.
Using a notched stick to plant runners
Place the runners crossways on the beds about 30cm apart. If they are still brittle and turgid, leave them to wilt slightly and become supple, so that they don’t snap when pushed into the soil.
A practical way to plant sweet potato runners is to take a stick of about 30mm in diameter, remove the bark and cut a notch into one end.
This end can be tapered down to the notch.
Now place the notch over the middle of the runner and push the runner gently into the soil. The two ends will be left protruding from the soil, and the new growth will start from them.
The runners will take root fairly rapidly. Don’t worry if the leaves on the runners die off; there is enough moisture and energy in the runners to initiate new growth.
The soil can be dry or moist but should not be wet. It should be loose enough so that the runners can be pushed in without being broken.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.