Sweet potatoes

Introduction

A sweet potato contains around one-and-a-half times the calories and vitamin C of the ordinary garden potato. It is widely grown throughout warmer climates and is now gaining popularity in the UK using hardier cultivars and growing under protection in cooler regions. It can be boiled, roasted or cut into chips; the shoots and leaves can be cooked and used as a spinach substitute.

Seeding

Pot cuttings: Plants are best grown from mail order cuttings or taken from plants overwintered in a frost free greenhouse or windowsill.

They can be grown from shop-bought tubers but these may be less robust cultivars ill suited to outdoor growing. They are often treated with an anti-sprouting agent too, 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.

Growing

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.

Planting

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.

Troubleshooting Growing Problems

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 biological control in the greenhouse.

Whitefly: Small white flies suck sap and excrete sticky ‘honeydew’ over the plant, encouraging the growth of sooty mould.

Remedy: Use biological control or sticky traps in the greenhouse.

Harvesting

Tubers take four to five months to mature and are best lifted once the leaves turn yellow and die back.

Lift carefully to avoid bruising.

Tubers rot if frozen and are hard to store, so consume sweet potatoes promptly.

Sweet potatoes freeze well if they are blanched or boiled beforehand.

Eaten fresh sweet potatoes can be boiled, roasted or cut into chips; the shoots and leaves can be cooked and used as a spinach substitute.

Companion Planting

Plant with: beets, broccoli, brussels sprouts, cabbage, carrots, cauliflower, chinese cabbage, chives, garlic, kale, kohlrabi, onions, radishes, strawberries

Recipes