Sweet potatoes are best grown from cuttings, which are not, in fact, rooted and technically called 'slips'. You would normally buy them via mail order from late April onwards. When they arrive, pot them immediately into small pots of multi-purpose compost. Keep the compost moist, using tepid water. Cover the pots with a clear plastic bag or place them in an unheated propagator, until they root.
Shop-bought tubers: These can be used but will be less robust cultivars ill-suited to outdoor growing. They are often treated with an anti-sprouting agent, so scrub them clean before planting.
Place tubers in moist vermiculite, perlite or sand in a warm propagator or airing cupboard to encourage sprouting.
Remove the shoots, with a sharp knife, when they are 5-7.5cm (2-3in) long and pot them into small pots of cutting compost and root them in a warm propagator. Treat cuttings from overwintered plants in the same way.
Outdoors: If grown outdoors, sweet potatoes need moisture-retentive, free-draining soil, in a sheltered, sunny position (they are particularly happy in organic rich sand). Prepare the ground as necessary.
Use black polythene, to warm the soil and suppress weed growth. Lay the polythene over the soil several weeks before planting, from late March or April as the soil starts to warm up.
Grow the plants on in a bright, frost-free position in the greenhouse or on a sunny windowsill, until late May until early June, potting on as necessary.
‘Harden off’ before planting outdoors, in slits through the polythene.
Cover with cloches or fleece – the temperature lift makes all the difference.
Indoors: Grow sweet potatoes in a glasshouse in large tubs, growing-bags or the glasshouse border, transplanting from the pots once they have produced plenty of roots. The foliage can be trained up string, canes or trellis. Any good growing medium is satisfactory, including peat-free types.
Sweet potatoes crop best at temperatures between 21-26°C (70-80°F).
Keep greenhouse plants well watered, and feed every other week with a high-potassium liquid feed.
Overwinter plants in a frost-free greenhouse or windowsill.
Tubers take four to five months to mature and are best lifted once the leaves turn yellow and die back.
They rot if frozen and are hard to store, so consume sweet potatoes promptly.
Lift carefully to avoid bruising.
Sweet potatoes can be boiled, roasted or cut into chips; the shoots and leaves can be cooked and used as a spinach substitute.
‘Beauregard Improved’: Similar to the tubers on supermarket shelves. Tubers have distinctive salmon-orange flesh which is well flavoured and sweet.
‘T65’: has red-tinted skin colour and creamy white flesh. Reliable and vigorous, wtih good sized tubers.
‘Georgia Jet’: Early maturing, productive and reliable throughout the UK. Flesh is deep orange and very tasty.
‘O’Henry’: A compact variety, tubers develop as a cluster beneath the foliage. Ideal for containers.
Glasshouse red spider or two spotted mite: Leaves become mottled, pale and covered in webbing, on which the mites can be clearly seen; leaves also drop prematurely.
Remedy: They thrive in hot, dry conditions, so mist plants regularly. Use sprays based on soft soap, plant oils or extracts. Use the biological control Phytoseiulus persimilis in the greenhouse.
Read more information on Red spider mite
Whitefly: small white flies suck sap and excrete sticky ‘honeydew’ over the plant, encouraging the growth of sooty mould.
Remedy: Use biological control Encarsia formosa in the greenhouse, sticky traps or sprays based on plant oils or soft soap.
Read more information on whitefly