How to grow sweet potatoes and the best varieties

Sweet potatoes are hugely popular, with many claiming they’re a healthier, tastier alternative to potatoes. 

This should make them top of any gardener’s list of must-have crops, but they’re notoriously tricky to grow in the UK’s cool, short summers.

The experts at Which? Gardening magazine grew and tasted 10 varieties of sweet potatoes to find you the best ones to grow. We also share our advice how to plant sweet potatoes and grow them in the UK.

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How to grow sweet potatoes in the UK

Buying and caring for slips or young plants 

Sweet potatoes are sold in several ways: as slips – shoots taken from a sprouted tuber; ready-rooted slips/cuttings (also called plug plants) or larger young plants in 7cm pots. We bought ‘Beauregard’ in all sizes and found it’s worth buying plants that are ready-rooted, as they were faster to get started. 

Unpack and pot up the sweet potato plants as soon as they arrive using a Best Buy compost for raising young plants. Slips also need prompt attention. Soak them overnight in a glass of water and pot into long tom pots, burying the stem up to the lowest leaves. Keep the pots damp, but not wet, and leave them in a warm greenhouse or on a windowsill until all chance of frost has passed.

Discover how to buy the best greenhouse

Planting sweet potatoes in the ground or in pots

Outdoors, we tried planting on a ridge or on flat ground that was either covered or not covered with landscaping fabric. The highest yields were from plants grown on a ridge of soil, around 30cm high, covered in landscaping fabric to keep the heat in, let the rain through and keep the weeds at bay. Covering the plants with a cloche when they’re first planted also helps.

Prepare the ground by digging in a 10cm-thick layer of soil improver, such as garden compost, and add a general-purpose organic fertiliser. Sweet potatoes need plenty of water, so a trickle-irrigation system is useful.

Give outdoor plants plenty of space to grow. They need at least 45cm between plants and one metre between rows, as they put on huge amounts of foliage, which trails over the ground.

If you opt to grow in pots, use a Best Buy compost for containers and a Best Buy controlled-release fertiliser. You’ll need a very large pot – ours were 30L – and again, it’s useful to install an automatic-watering system if you can.

Discover our Best Buy garden irrigation systems

Training

We found that plants growing in pots were easier to manage and produced a bigger crop if the foliage was tied onto a wigwam of bamboo canes. Staking didn’t really improve crops grown outside.

Feeding

You should liquid-feed your plants, which put on a great deal of growth and need lots of food. 

Try our Best Buy liquid feeds.

How and when to harvest sweet potatoes

Sweet potatoes should be harvested as soon as the frost has touched the leaves, or when the foliage starts to die back. Remove all the foliage and then dig out the tubers or empty the pot. You may find that the roots and tubers have wrapped around one another. Sweet potato tubers store very well in a cool, dark place, although it’s a good idea to eat up any smaller tubers first, as they can dry out quickly.

Common sweet potato problems

Red spider mite and whitefly

In a polytunnel, they’re likely to suffer from whitefly and red spider mite, so use a biological control or spray with Westland Resolva Bug Killer for whitefly. Use a fatty acid or pyrethrum-based spray on red spider mite.

Read more about whitefly

Read more about red spider mite

How we test sweet potatoes

The Which? Gardening magazine experts ordered the 10 most widely available sweet potato varieties as slips (shoots taken from a sprouted sweet potato) or ready-rooted slips/cuttings (also known as plug plants).As soon as they arrived, we potted them up into tall ‘long tom’ pots and kept them in a warm greenhouse until all chance of frost had passed. We planted all 10 varieties both outside in the ground and in a polytunnel. 

Outside in late May, we dug in organic matter, applied a general-purpose fertiliser and installed trickle irrigation pipes. We then covered the beds in landscaping fabric to try to increase the temperature of the soil before planting. We planted five plants of each variety through a slit in the fabric. 

In the polytunnel, we filled 30L pots with a Best Buy peat-free compost for containers and a general-purpose feed. We grew three pots of each variety and put a wigwam of bamboo canes in one of the three pots, tying in the foliage as it grew, to see whether supporting the growth would help.

We harvested all the varieties in early November, when outside plants had received their first light frost. We then cooked and tasted every variety.