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How to grow sweet potatoes and when to harvest them

Sweet potatoes 'Beauregard' and pumpkins
Sweet potatoes 'Beauregard' and pumpkins Credit: gapphotos.com

It couldn’t have taken long to think up the name “sweet potato” for Ipomoea batatas. Although an easy subtropical plant to grow, in our cool UK climate a few tricks and a long hot summer are needed to form the large, vitamin C-rich tubers.

Start sweet potatoes around March or April, but forget what you know about normal spuds. Raising them is more akin to dahlias – plants are grown from tuber shoot cuttings in spring, called “slips”. You can even begin by using shop-bought sweet potatoes (they must be organic to be free of growth-suppressant chemicals).

Slice in half and place cut side down in a shallow tray filled with ½in (1cm) water. Place on a heated propagator or a windowsill above a radiator. Keep the water topped up and shoots will sprout. When 6in (15cm) long, snap off shoots at the base and stand them in a glass of water to root, potting into multipurpose compost once roots have formed.

If this sounds like hard work, simply order slips from unwins.co.uk or marshalls-seeds.co.uk; purple ‘Molokai’ and orange ‘Beauregard’ are lovely. When the risk of frost is over, move plants to a sheltered spot to adjust to life outdoors. A week or so later, plant into final positions. If you have a polytunnel or glasshouse, they perform well with extra heat.

I don’t – so instead I add 3½in (10cm) of compost to form a raised row in sun on my allotment, spacing plants every 1ft (30cm) along this. Planting on a bump aids drainage and, with it, warmth. It’s a good idea, though not essential, to build a wigwam of canes for the plants, which are vines, to grow up. As a bonus they make attractive climbers and can be grown singly in large pots on a sunny patio or balcony.

Most sweet potatoes take roughly 100 days of growth to mature. The best of a new harvest will usually come along in November, 3 or 4 months after planting. Their leaves should, at the very least, be turning yellow.

After harvesting, you can save and store new slips for next spring's planting season. 

Find Jack’s blog at jackwallington.com. Follow him on Twitter @jackwallington and on Instagram at @jackwallingtongardendesign