Versatile and becoming increasingly popular, sweet potatoes are well worth trying outdoors in milder areas - or in a glasshouse or polytunnel elsewhere. Even in mild regions, indoor growing will produce more reliable crops.
Sowing
Plants are best grown from cuttings or slips ordered from a mail order supplier.
You can try growing them from shop-bought tubers, but you can't plant them like ordinary potatoes as they don't grow in the same way. Many shop-bought tubers are also treated with an anti-sprouting agent, so give them a good scrub to clean them first.
Then place in moist sand in a hot propagator or even in the airing cupboard. Once the sprouts/shoots are 5-7.5cm (2-3in) long, they can be removed as cuttings and potted up into small pots of cuttings compost and placed in a warm propagator to root.
Growing
Plants are ordered as cuttings or slips, delivered from late April onwards. Pot the cuttings immediately on receipt into small individual pots of multipurpose compost. Should the slips be unrooted, simply cover the pots with a clear plastic bag or place in an unheated propagator until roots appear.
Grow the plants on in a frost-free, well-lit spot until early June. In mild regions, sweet potatoes can be planted outdoors after a period of hardening off. They require a highly fertile but light, preferably sandy, soil. If your soil is not naturally sandy or free-draining, plant into ridges 15-30cm (6-12in) high, spacing plants 30cm (12in) apart, with 75cm (2.5ft) between rows.
Ideally, plant under a cloche or fleece tent. Alternatively, grow in a glasshouse in large tubs, growing-bags or the glasshouse border. Whitefly and red spider mite can cause problems on foliage under cover.
Sweet potatoes crop best at temperatures between 21-26°C (70-80°F). Keep well watered, feeding every other week with a high-potassium liquid feed.
Harvesting
Tubers take from four to five months to mature. They can be lifted from the end of August, but it is usually better to leave them until the leaves begin to yellow and die back.