How to grow sweet potatoes and when to harvest them

Sweet potatoes 'Beauregard' and pumpkins
Sweet potatoes 'Beauregard' and pumpkins Credit:

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 or; 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. 

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