Gardeners' Questions: 'Can I grow sweet potatoes in my back garden?'
- Credit: Inga Nielsen
Dear Martyn: This year I have an idea that I want to grow sweet potatoes rather than the normal spuds that I have grown for years, but I am really struggling to find any plants. Looking through the catalogues I see there is a range of varieties available but they all seem to be sold out and they appear to be quite expensive compared to traditional potatoes. Could you give me some of your excellent advice on how to grow them from tubers? And can you just plant the ones you can buy from the supermarket? J Burns
Dear Reader: 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.
Plants are best grown from mail order cuttings or taken from plants overwintered in a frost free greenhouse or windowsill. Sweet potatoes cuttings are not in fact cuttings, when rooted they are 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 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. They need to have a temperature of around 20C to start to sprout.
Remove the shoots with a sharp knife when they are 5-7.5cm long and pop them into small pots of cutting compost, rooting them in a warm propagator. Treat cuttings from overwintered plants in the same way.
If grown outdoors, sweet potatoes need moisture-retentive, free-draining soil, in a sheltered, sunny position (they are particularly happy in rich organic sand). Prepare the ground as necessary with lots of good garden compost. 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.
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You can also 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. 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. Lift carefully to avoid bruising. Tubers rot if frozen and are hard to store, so consume sweet potatoes promptly.
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.