Tuber
How to grow sweet potatoes
Ipomoea batatas
Sweet potatoes are a tender, trailing crop from the bindweed family, unrelated to ordinary potatoes and grown from rooted shoots called slips, not from tubers. In the UK they need all the warmth you can give them: plant in late May or June in a greenhouse, polytunnel or large pot, or outdoors through black plastic under fleece in the south. Keep nitrogen low, feed potash from July and lift before the first frost.
- Hardiness
- Tender, needs a long warm season
- Position
- Full sun, warm; under cover or fleece in the UK
- Soil
- Light, free-draining, fertile
- Ideal pH
- 5.5 to 6.5
- Water
- Moderate, regular while vines establish, less as tubers form
- Feeding
- Moderate feeder
Sowing and planting sweet potatoes
WhenPlant slips late May to Jun under cover or fleece; harvest Sep to Oct before frost
Starting off
Grown from unrooted slips bought in April to May: pot each into a 9 cm pot, keep at 20 to 25 °C under a bag until rooted in 10 to 14 days, then pot on and plant out after the last frost in late May to June under fleece. Raise your own slips by standing a tuber half in water at 20 °C in February.
Spacing and planting
Plant slips 30 cm apart in rows 75 cm apart on a 20 cm ridge, or through black plastic mulch that warms the soil; bury the slip to half its length so it roots along the stem. One slip fills a 40 litre pot or 45 cm tub in a greenhouse or polytunnel, the better bet in the UK.
Varieties worth growing
- Beauregard Improved
- The standard for British gardens: orange flesh, copper skin, and the most reliable to bulk up in a short season.
- Carolina Ruby
- Dark red skin and deep orange flesh, with attractive purple-tinged foliage. Crops well under cover.
- Bonita
- Pale skin and cream flesh that is drier and less sweet, closer to a floury potato when baked.
- Evangeline
- Orange-fleshed and among the sweetest, for baking and roasting. Needs a warm spot to do well.
- Murasaki
- Japanese type with purple skin and white, nutty, dry flesh that holds its shape when cooked.
Sweet potatoes month by month
Timings are for central England. Allow two weeks later in the north and on cold, heavy soil.
- January
Nothing to do. Check stored tubers.
- February
Order slips. To raise your own, stand a tuber half in water at 20 °C.
- March
Twist off home-raised shoots at 10 to 15 cm and root them in pots of moist compost in warmth.
- April
Bought slips arrive from late April. Pot up at once and keep at 20 to 25 °C under a clear bag until rooted.
- May
Pot on and keep warm. Lay black plastic over the outdoor bed. On an exposed allotment, give them the most sheltered bed and weigh the fleece down well. Plant in the greenhouse or polytunnel at the end of the month.
- June
Plant outside through the plastic after the last frost and cover with fleece. Water regularly while plants establish.
- July
Start potash feeding. Train stems up strings or canes under cover to save space. Remove fleece outdoors in hot spells.
- August
Keep feeding and watering steadily. Ventilate under cover on hot days.
- September
Stop feeding early in the month. Lift from late September as the leaves begin to yellow.
- October
Lift everything before the first frost. Cure for 7 to 10 days in warmth, then store.
- November
Store at 12 to 15 °C in the dark. Use damaged tubers first.
- December
Check the store. Set aside a sound tuber to start slips in February.
Feeding sweet potatoes
A moderate feeder that turns nitrogen into vine. Give a light base dressing at planting, then potash from July to swell the tubers, and stop in early September.
Moderate, leaning to potassium for tuber formation; too much nitrogen gives all vine.
Feeding plan
Planting, late May to June: beds
Fruit & Vegetable Fertiliser 4-5-6
70 g/m², the bottom of the label range, worked into the ridge before the plastic goes down. At 30 cm by 75 cm spacing that is about 15 g per plant.
Planting: pots
Fruit & Vegetable Fertiliser 4-5-6
5 g per litre of compost, the lower label rate: 200 g for a 40 litre pot. No more balanced feed after this.
July to early September, every two weeks
1 g per litre of water, 10 g in a 10 litre can, watered onto the root zone. In a 40 litre pot, enough of the solution to wet the compost through.
Monthly, June to August
1 g per litre of water as a foliar spray or drench. Optional; it supplies trace elements, including the boron that helps prevent cracked tubers.
Signs to watch
Metres of vine and tiny tubers
Excess nitrogen or a cold, short season
Never feed nitrogen, use potash and plant through black plastic
Older leaves yellow between the veins on acid, light soil
Magnesium short
Epsom salts or kieserite, not dolomite
Cracked tubers
Erratic late water or boron short
Steady watering and seaweed
Pests, diseases and problems
Slips sit without growing after planting; leaves turn purple or yellow
Cold. Growth stops below about 12 °C, and a cold night soon after planting can set plants back for weeks
Do not plant until June outdoors, warm the soil under black plastic for two weeks first, and cover with fleece or cloches until midsummer. Grow under cover in the north.
Leaves finely speckled and dull, with webbing under them, in a greenhouse
Red spider mite in hot, dry air
Damp down paths and mist the foliage daily. Introduce the predatory mite Phytoseiulus at the first sign.
Small white flies rise from the leaves when touched, sticky deposit below
Glasshouse whitefly
Yellow sticky traps to monitor, and the parasitic wasp Encarsia introduced early.
Holes eaten into the tubers at lifting
Slugs in the soil, or wireworm on ground recently under grass
Lift promptly once the foliage yellows instead of leaving tubers in cold, wet soil. Apply slug nematodes in August. On new plots, grow in pots for the first couple of years.
Tubers gnawed, with toothmarks
Mice or voles sheltering under plastic mulch and dense foliage
Set traps under cover in late summer and lift the crop as soon as it is ready.
Stored tubers develop sunken, dark patches and rot
Chilling below 10 °C, or skin damage that never healed because the tubers were not cured
Handle gently, cure for 7 to 10 days somewhere warm and humid, then store at 12 to 15 °C. Never in a fridge or a cold shed.
Harvest and rotation
Harvest and storage
Lift in late September to October before frost, 16 to 20 weeks from planting, when the leaves start to yellow; fork carefully from the side, as the skins bruise. A plant gives 0.5 to 1.5 kg, more under cover. Cure for 7 to 10 days at 25 to 30 °C to heal the skins and sweeten, then store at 12 to 15 °C in a dry, dark place; never refrigerate.
Rotation and companions
Rotation: bindweed family (Convolvulaceae), unrelated to potatoes, so it slips into a 3-year rotation anywhere warm, best after legumes. Dwarf beans or sweetcorn planted to the north give shelter without shade. Keep away from potatoes, which compete for water and nitrogen, and from squash, which smothers its vines.
Good to knowNeeds a long warm season, grow from slips under cover or fleece and plant out only after the last frost.
Growing sweet potatoes in pots
A large pot under cover is the most reliable way to grow sweet potatoes in Britain. Plant one slip in a 40 litre pot or 45 cm tub, or two in an old compost bag of similar size, using multipurpose compost opened up with a fifth of sharp sand or grit and the 4-5-6 at 5 g per litre. Stand it in a greenhouse, polytunnel or conservatory, or against a sunny south wall from late June. Train the stems up canes. Water regularly through the summer, then let the compost get a little drier in September. At harvest, tip the whole pot out, which avoids the fork damage that spoils tubers lifted from the ground.
Sweet potatoes: common questions
Can you grow sweet potatoes outside in the UK?
Yes in the south and in sheltered gardens in the Midlands, if you plant through black plastic and keep the plants under fleece or cloches for the first weeks. Yields are lower than under cover, often 500 g or so per plant. Further north, grow them in a greenhouse or polytunnel.
Can I grow sweet potatoes from a supermarket sweet potato?
You can, by standing a tuber half in water in a warm room in February and rooting the shoots it produces, but results vary. Shop tubers are often varieties bred for hot climates and may be treated to stop sprouting. Slips of a variety such as Beauregard Improved, selected for cooler summers, are the safer route.
When are sweet potatoes ready to harvest?
They are ready from late September, 16 to 20 weeks after planting, when the leaves start to yellow. Lift them all before the first frost, as cold soil damages the tubers. Cure them for 7 to 10 days at 25 to 30 °C before storing; this heals the skins and makes them sweeter.
Sources
Growing information on this page was checked against these references in November 2026. Feeding rates are Dr Forest label rates.