Q What's the best way to buy sweet-potato plants?
A 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. For the Which? Gardening magazine trial, 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 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.
Caption: Well-rooted sweet-potato plants do best
Q How do I plant sweet potatoes?
A 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 organic matter and add a general-purpose organic fertiliser. Sweet potatoes need plenty of water, so a trickle-irrigation system is useful if you have it.
Give outdoor plants plenty of space to grow. They need at least 45cm between plants and 1m 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.
Caption: Plant through landscaping fabric
Q How do I care for sweet potatoes?
A 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.
You should liquid feed your plants, which put on a great deal of growth and need lots of food. We tried several brands and found little difference between them, but there was a clear drop in yield for plants that weren’t liquid fed.
Some say to remove flowers, but our plants produced too few flowers to notice any difference in yields.
Q What pests and diseases affect sweet potatoes?
A Plants grown outside are mostly pest and disease free, although you should keep an eye out for slugs, snails and caterpillars.
In a polytunnel, they’re likely to suffer from white fly and red spider mite, so use a biological control or spray with a Westland Resolva Bug Killer for whitefly. Use a fatty acid or pyrethrum-based spray on red spider mite.
Q How do I harvest sweet potatoes?
A Sweet potatoes should be harvested as soon as the frost has touched the leaves, or the foliage starts to die back. Remove all the foliage and then dig out the tubers or empty the pot. You may find the roots and tubers have wrapped around one another.
Caption: Dig up sweet potatoes once the frost has touched the leaves in autumn
Q How do I store sweet potatoes?
A 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.
Caption: Store sweet potatoes in a cool, dry place