Sweet potatoes

Sweet potatoes are widely grown in warmer climes and are gaining popularity in the UK, with the introduction of hardier cultivars. They have attractive lush foliage, so are ornamental as well as edible, but they need a warm summer or extra protection, especially in cooler regions, to produce good tubers. Sweet potatoes are rich in vitamin C and can be boiled, roasted or made into chips. The shoots and leaves can be used like spinach. 

Jobs to do now

  • Cut back wayward stems
  • Water regularly
  • Feed fortnightly

Month by month

Grow

Grow

Growing in a greenhouse

Sweet potatoes like warmth and crop best at temperatures of 21–26°C (70–80°F), so are ideal for growing in a greenhouse, either in large tubs, growing bags or a greenhouse border. Transplant them from their pots once they have plenty of roots. The trailing stems can be trained up strings, canes or trellis to save space. When growing in containers, any good-quality compost should be suitable, including peat-free.

Keep plants well watered, and feed every other week with a high potassium liquid feed.
 

Growing outdoors

Sweet potatoes can also be grown outdoors, especially in warmer regions. But they are tender, so keep the young plants frost-free in a greenhouse or on a sunny windowsill until late May or early June, potting them on as they grow.

Choose a warm, sheltered, sunny planting site with moisture-retentive, free-draining soil. Sweet potatoes are vigorous plants with trailing stems reaching 1.5m (5ft) or more, so they need plenty of space.

To prepare the ground, add lots of organic matter, such as garden compost. Then in late March or April, several weeks before planting out, cover the area with black polythene to warm the soil and suppress weeds. 

Once all danger of frost has passed, harden off the plants by gradually acclimatising them to outdoor conditions. Then plant them in slits through the polythene, at least 30cm (1ft) apart.
Cover the plants with cloches or fleece – the temperature lift makes all the difference. Water regularly and feed fortnightly with a high potassium liquid feed.

You can also plant sweet potatoes in large containers, one per pot at least 30cm (1ft) wide, filled with multi-purpose compost. Keep them in a warm, sunny, sheltered spot, and water and feed regularly.

Plant

Sweet potatoes are usually grown from cuttings known as ‘slips’, which are available by mail order. You can also take cuttings from shop-bought tubers, but they may not be suitable varieties for growing in the UK, so are less likely to be successful. Alternatively, you can take cuttings from the tubers of plants overwintered in a frost-free greenhouse or windowsill.

Young sweet potato plants, grown from rooted cuttings, are also available via mail order in late spring or early summer.
 

How to plant ‘slips’

 

You can buy unrooted cuttings or ‘slips’ of varieties suitable for the UK climate via mail order from late April onwards. When they arrive, pot them up 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.

 

How to take your own cuttings

It is possible to take cuttings from shop-bought tubers, but these may be less hardy cultivars that are unsuitable for the UK climate. The tubers are often treated with an anti-sprouting agent too, so if you want to use them, scrub them clean first. Then place the tubers in moist vermiculite, perlite or sand in a warm propagator or airing cupboard to encourage sprouting.

To take cuttings from overwintered tubers or sprouted shop-bought tubers, remove the shoots with a sharp knife when they are 5–7.5cm (2–3in) long. Insert them into small pots filled with cuttings compost, then place in a warm propagator to root.

 

 

Harvesting

You can harvest sweet potatoes after four to five months, once the leaves turn yellow and die back in late summer or autumn, before any frosts.

Lift them carefully to avoid bruising the fresh tubers, which have delicate skin. Any damaged tubers should be used straight away.

Sweet potatoes tubers can be boiled, roasted or made into chips. They are best used fresh, as they are hard to store successfully. They freeze well if blanched or boiled beforehand.

The shoots and leaves can also be cooked like spinach.

Recommended Varieties

Common problems

Glasshouse red spider or two spotted mite
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
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.

Get involved

The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.