🍠 Sweet potato

Sweet potatoes are a tropical crop that will crop in a warm, sunny UK summer, especially in a greenhouse, polytunnel or on a south-facing plot. They need a long, warm season, so they reward gardeners who can provide shelter and plastic mulch.

They are grown from rooted shoots called slips, not tubers. The pretty heart-shaped leaves are edible, and the trailing vines make good ground cover. Choose early varieties bred for short seasons, such as Beauregard or Georgia Jet.

Varieties to try

Beauregard (orange flesh, early)Georgia Jet (very early, reliable)Murasaki (purple skin, white flesh)Bonita (white flesh, mild)Erato Orange (compact)

Buy slips in spring, or grow your own by suspending a sprouting sweet potato half-submerged in water on a warm windowsill from February. After 4–6 weeks, shoots form; twist them off with a few roots and pot them up individually in compost at 20°C.

Plant out in early June, once the soil is warm and frost has passed, 40 cm apart on ridges about 20 cm high, covered with black plastic mulch to trap warmth. Plant in a greenhouse border or large 30 cm pots for the most reliable results.

Top tips

• Use a warm windowsill for slips.
• Make ridges and warm the soil with black plastic.
• Plant out in June, never earlier.
• Choose short-season varieties.

Water regularly while the plants establish, then ease off as tubers swell; overwatering gives lush vines and few roots. Feed lightly with a high-potash fertiliser, since too much nitrogen promotes leaves, not tubers. Keep plants covered with fleece on cold nights.

Vines can be trained up a support or allowed to trail, but do not let them root along the stem, which makes lots of small tubers. Slugs, wireworm and vine weevil are the main pests. Keep plants warm: growth stops below 10°C.

Top tips

• Do not overfeed with nitrogen.
• Lift stems that root along the ground.
• Protect with fleece on cool nights.
• Water less as tubers swell.

Watch out for

Slugs and wirewormVine weevilCold damageRot in cold, wet soil

Dig sweet potatoes in late September or October, before the first frost blackens the vines, or when the leaves yellow. Loosen the soil with a fork well away from the plant, since the skins bruise easily, and lift the tubers carefully.

Cure them for 10 days in a warm (25–30°C), humid place such as an airing cupboard or a greenhouse, which heals skin scratches and sweetens the flesh. Brush off soil rather than washing before storage.

Top tips

• Lift before the first frost.
• Handle gently; skins bruise.
• Cure warm for 10 days.
• Do not wash before storing.

Roast sweet potatoes in wedges or whole, bake them like jacket potatoes, mash, or add to curries, soups, stews and traybakes. The flesh is sweet and creamy, and the skin is edible once cooked. The young leaves can be steamed like spinach.

Cured tubers keep for 3–5 months in a dry place at around 13–16°C, never in the fridge, which spoils the flavour. Cooked sweet potato can be frozen mashed or in chunks.

Top tips

• Never refrigerate raw sweet potatoes.
• Store cured tubers at 13–16°C.
• Roast skin-on for extra flavour.
• Cook the young leaves too.