Overview
What sweet potatoes need, how long they take, and whether your summers are warm enough.
Sweet potatoes are tropical vines in the morning glory family, not relatives of the potato. They grow from slips, shoots taken from last year’s roots, and need a long spell of real warmth to swell their storage roots. In hot summers they thrive outdoors; in cooler ones a greenhouse, black plastic or a warm wall makes the difference.
Quick facts
- Botanical name
- Ipomoea batatas
- Feeding
- Moderate feeder
- Temperature
- 21 – 26 °C
- Hardiness
- USDA zone 12a and warmer
- Time to harvest
- 90 – 150 days
- Height
- Up to 150 cm (59 in)
- Propagation
- Cutting
Optimal growth
Keep above 10 °C
Growing details
- Spacing
- 30 cm (12 in)
- Frost sensitivity
- Frost tender
Growing calendar
- Start indoors: March, April, May
- Plant out: May, June
- Harvest: September, October
In the UK. Sprout roots for slips about six weeks before planting; bought slips arrive from late April (RHS, Utah State). In the UK, late spring or early summer. Anywhere, after the last frost, once soil reaches 18 °C (65 °F) (RHS, UMD). In a temperate climate, when leaves yellow, before the first frost (RHS, Clemson).
Is this plant right for you?
- Needs heat
- Frost tender
- Long season
- Good in a greenhouse
- Grows in pots
- Edible leaves
- Stores for months
Good to know
Keep it above 10 °C (50 °F): frost kills it.
Before You Grow
Choose a variety for your season length, and the warmest, sunniest spot with light soil.
Choose a quick variety if your summers are short or cool (RHS, Clemson).
What you need
Slips or plug plants
Bought or home-grown. See slips.
Warmth
A greenhouse, or a very warm, sheltered bed. See growing conditions.
Black plastic or fleece
To warm the soil outdoors. See planting.
Loose, well-drained soil
Ideally in a raised ridge. See soil.
Canes or strings
If you want the vines to climb. See training.
A warm room
For curing the harvest. See curing.
Choose your sweet potatoes
Varieties differ in skin and flesh colour, sweetness, vine length and, most importantly, how long they take (RHS, Utah State). Match the variety to your season.
Short season. ‘Beauregard’ is among the quickest, at 90 to 100 days, with rose skin and sweet orange flesh (Clemson). The RHS recommends it for growing outdoors in the UK. ‘Vardaman’ has a compact bush habit (Clemson).
Long, hot summers. ‘Evangeline’ is sweeter than most but needs a long hot summer (RHS, Gardeners’ World). ‘Jewel’ takes 120 to 135 days (Clemson).
Flesh. Moist, orange-fleshed types are sometimes sold as “yams”; firm white or yellow-fleshed types such as ‘Murasaki’ and ‘Sumor’ store longer and are less sweet (Utah State, Clemson).
Disease resistance. ‘Covington’, ‘Regal’ and ‘Centennial’ resist root-knot nematodes and other problems (Clemson).
Buying. Choose a variety suited to your climate; shop-bought roots may be treated to stop sprouting (RHS, Gardeners’ World).
Sweet potato varieties compared
-
‘Beauregard’
- Days
- 90 to 100
- Skin and flesh
- Rose skin, orange flesh
- Note
- Quickest; best outdoors in the UK
-
‘Vardaman’
- Days
- 100 to 110
- Skin and flesh
- Copper skin, orange flesh
- Note
- Compact bush habit
-
‘Covington’
- Days
- 95 to 110
- Skin and flesh
- Rose skin, orange flesh
- Note
- Resists nematodes and fusarium wilt
-
‘Murasaki’
- Days
- 100 to 120
- Skin and flesh
- Purple skin, white flesh
- Note
- Nutty, flaky; very long vines
-
‘Jewel’
- Days
- 120 to 135
- Skin and flesh
- Copper skin, orange flesh
- Note
- Needs a long season; stores well
-
‘Evangeline’
- Days
- Long
- Skin and flesh
- Orange flesh
- Note
- Extra sweet; needs a hot summer
From Clemson, Utah State, the RHS and Gardeners’ World; Clemson gives ‘Vardaman’ 100 days, Utah State 110. For a short or cool season, ‘Beauregard’.
Where sweet potatoes grow best
Sweet potatoes are tender, tropical plants that need heat, full sun and regular water (RHS).
Temperature. They crop best at 21 to 26 °C (70 to 80 °F) (RHS, Gardeners’ World). Below 10 °C (50 °F), plants and roots suffer chilling injury (Utah State, UMD), and frost kills the top growth (UMD).
In cooler climates such as the UK, grow them in a greenhouse, polytunnel or cold frame. Outdoors they only crop well in very warm, sheltered spots, and give smaller harvests (RHS, Gardeners’ World).
Sun. At least six hours, better eight to ten (UMD); the warmer and sunnier the spot, the bigger the crop (RHS).
Space. Vines reach 1.5 m (5 ft) or more and root where they touch the ground; let them sprawl or grow them up supports (RHS, Illinois).
- Best growth
- 21 to 26 °C (70 to 80 °F)
- Plant into soil at
- At least 18 °C (65 °F)
- Season
- 4 to 5 months
Soil
Loose, well-drained, loamy or sandy soil is best. In heavy clay, roots come out smaller and misshapen (Clemson, UMD).
Preparation. Fork in about two bucketfuls of compost or well-rotted manure per square metre (RHS). Utah State keeps compost to about 2.5 cm (1 in) and uses a soil test to guide fertiliser.
Ridges. Plant on a raised ridge 20 to 25 cm (8 to 10 in) high: it drains and warms faster, gives the roots room, and makes lifting simpler (Illinois, Utah State, UMD).
Warm it first. Outdoors, cover the ground with black plastic, cloches or fleece for a few weeks before planting (RHS, Gardeners’ World).
Planting
Root slips in spring, warm the soil, and plant out well after the last frost.
Warm the soil under black plastic or fleece for a few weeks before planting (RHS, Gardeners’ World).
When to plant sweet potatoes
Plant well after the last frost, once the soil has warmed to 18 °C (65 °F) (Clemson, UMD). In colder soil they stall and the roots suffer.
In the UK, slips arrive from late April. Pot them up and keep them warm indoors, then plant out in late spring or early summer, under cover if possible (RHS).
In the US, as soon as the soil warms after the last frost, for the longest warm season (Illinois). Black plastic lets you plant up to ten days earlier (Utah State).
- Warm soil first.
- Well after the last frost.
- Harden off first.
Growing and rooting slips
Sweet potatoes grow from slips: shoots taken from a sprouting root (Clemson). Buy them, or grow your own from a healthy root about six weeks before planting.
-
Pick a root
A healthy, medium-sized root from last year’s crop or an organic one; scrub shop-bought roots, which may be treated against sprouting (Clemson, Gardeners’ World).
-
Sprout it
Suspend it half in a jar of water on toothpicks, or lay it in moist sand at 21 to 27 °C (70 to 80 °F). Shoots appear in about two weeks (Clemson, Utah State).
-
Take the shoots
At 5 to 15 cm (2 to 6 in), cut or twist them off at the base (RHS, Clemson).
-
Root them
In a jar of fresh water, which takes about ten days, or straight into small pots of moist cuttings compost, burying most of the stem (Clemson, Gardeners’ World).
-
Grow on warm
Bought slips too: soak a day or two, pot up, and keep warm and humid under a clear bag in bright light, potting on as they grow (RHS, Gardeners’ World).
How to plant sweet potatoes
A good slip has four or five leaves and a healthy root system (Utah State).
-
Harden off
Over a week or two, stand plants outside by day and bring them in at night, so they get used to outdoor conditions gradually (RHS, Gardeners’ World).
-
Cut holes in the plastic
About 30 cm (1 ft) apart along the ridge (Gardeners’ World).
-
Plant
Potted plants at the depth they were growing (RHS); bare slips 7.5 to 10 cm (3 to 4 in) deep (Clemson). Space them 30 cm (1 ft) apart in rows 90 cm to 1.2 m (3 to 4 ft) apart (RHS, Utah State).
-
Water generously
Firm in and keep watered while they establish (RHS, Utah State).
-
Protect
Keep plants under cloches or fleece outdoors in cool areas (RHS, Utah State).
Care
Water steadily while roots form, then ease off before harvest.
Water most in the first two months, while plants establish (UMD).
Care by growth stage
Establishing
Drought sensitive: first 50 to 60 days
Water regularly and weed. See watering.
Vines spreading
Feed: once in early summer
Train or trim the vines. See training.
Roots swelling
Water: 2.5 cm (1 in) a week
Keep watering evenly. See watering.
Last 3 to 4 weeks
Splitting: no late water
Ease off. See watering.
Harvest
Frost: dig at once
Lift before frost and cure. See harvesting.
Watering sweet potatoes
Sweet potatoes are thirsty while establishing and swelling their roots; short of water, the roots won’t swell (RHS). They are most sensitive to drought in the first 50 to 60 days (UMD).
How much. At least 2.5 cm (1 in) a week, soaking the soil 15 cm (6 in) deep (Clemson). Plants in containers may need watering daily in hot weather (RHS).
How. Sink a pot beside each plant and water into it (RHS). Water in the morning so leaves dry by night (Clemson).
Not too much. Soggy soil causes rot and long, stringy roots (UMD, Illinois).
Before harvest. Stop watering for the last three to four weeks: late water splits the roots (Illinois, Utah State).
Mulch holds moisture and suppresses weeds (RHS).
Feeding sweet potatoes
Sweet potatoes have a medium nutrient need (UMD).
In the ground. Prepared, composted soil shouldn’t need more feeding (RHS). US extensions base feeding on a soil test and side-dress once in early summer, before the vines cover the ground (Utah State, Clemson); UMD side-dresses only on light, sandy soil, if needed.
In containers. A high-potassium liquid feed such as tomato feed every two weeks through summer (RHS, Gardeners’ World).
Training and trimming vines
Sprawl or climb. Vines can sprawl over the ground or climb a wigwam of canes about 1.8 m (6 ft) tall, a trellis, or strings in a greenhouse; tie some stems in to get them going (RHS).
Trimming. Sweet potatoes don’t need pruning, but you can trim vines that outgrow their space (RHS, Utah State). Gardeners’ World pinches out the tips of long stems to make plants bushier.
Eat the trimmings. Young shoots and leaves cook like spinach (RHS, UMD).
Weeds. Weed until the vines cover the ground; after that they shade out most weeds (RHS, Illinois).
Sweet potatoes in containers
Large containers and grow bags suit sweet potatoes, especially in a greenhouse (RHS).
Container. At least 30 cm (1 ft) wide, one plant per pot, in peat-free multi-purpose compost enriched with well-rotted manure or compost (RHS, Gardeners’ World).
Position. A greenhouse, cold frame, or a very warm, sunny spot against a wall with trellis for the stems (RHS).
Problems
Small or split roots, rot in store, and a few pests, mostly under glass.
Common sweet potato problems
Match what you see to its likely cause.
Small crop
- Cold
- Short season
- Dry soil
Not enough warmth, water or growing time (RHS). Grow under cover and choose a quick variety.
Split roots
- Late water
Heavy rain or watering late in the season, after a dry spell. See watering (Illinois, Utah State).
Long, stringy roots
- Wet soil
Too much water or poor drainage (Illinois). Grow on ridges.
Misshapen roots
- Heavy soil
Clay soil (Clemson). Improve with organic matter or grow on ridges.
Roots rot in store
- Chilling
- Damage
Frosted vines, soil below 10 °C (50 °F), bruised skins, or cold storage (Illinois, UMD, Clemson). Use damaged roots first, never store them, and check stored roots regularly (RHS, Gardeners’ World).
More problems with sweet potatoes
Common pests
Red spider mite and whitefly
Under glass. Red spider mite causes mottled leaves with fine webbing: raise the humidity to stop it breeding (Gardeners’ World). Glasshouse whitefly are tiny white sap-feeders under the leaves (RHS).
Wireworms
Tunnel into the roots in some gardens (Clemson). ‘Centennial’ resists them (Clemson).
Root-knot nematodes
In warm-climate soils. Grow resistant varieties and rotate (Clemson).
Mice
Burrow into ridges and eat the roots. Check regularly for signs of them and control them if needed (Illinois).
Common diseases
Black rot
Sunken lesions on slips and roots, which decay. Don’t plant slips with black spots on the lower stem; grow resistant varieties, rotate and don’t overwater (Clemson, Utah State).
Scurf
Dark blotches on the skin. Skin-deep: the roots are fine to eat but store less well (Illinois).
Fusarium wilt
Grow resistant varieties such as ‘Covington’ or ‘Regal’, and rotate every two years (Clemson).
Harvest & Use
Lift carefully before frost, cure the roots warm, then store cool but never cold.
Harvesting sweet potatoes
Harvest when the leaves turn yellow, and always before the first frost (RHS). That is four to five months after planting in the UK (RHS); Clemson gives 90 to 120 days. Check size from about 80 days, as overgrown roots can split (UMD).
Early pickings. From late summer, dig into the side of the ridge and take a few roots without lifting the plant (Illinois, Utah State).
Lifting. Cut off the vines first, then loosen the soil with a fork well away from the plant and lift gently. The skins bruise easily; use damaged roots first (RHS, Clemson, Illinois).
Frost. If frost hits the vines, cut them off and dig at once: rot spreads from dead vines into the roots (Illinois, UMD).
Yield. About 0.5 to 1 kg (1 to 2 lb) per plant (Utah State).
- Leaves yellowing.
- Before the first frost.
- Handle gently.
Curing and storing sweet potatoes
Curing heals nicks and turns some starch to sugar (Clemson). Don’t wash the roots (UMD).
Dry. Leave them on the ground for two or three hours, or in warm sun for a few hours (Illinois, Gardeners’ World).
Cure warm. Ideally 29 °C (85 °F) and 85 to 90% humidity for a week or two. At home, use the warmest room, often the kitchen, for 14 days, or a warm greenhouse for about 10 days (Clemson, Gardeners’ World). Nothing happens below 21 °C (70 °F).
Store cool, never cold. UMD stores at 13 to 16 °C (55 to 60 °F) and 85 to 90% humidity; Utah State at 10 to 13 °C (50 to 55 °F), dry. Spread them in a single layer with airflow. Below 10 °C (50 °F), including the fridge, they develop off-flavours and rot (Clemson, UMD).
How long. US extensions store well-cured roots for three to six months (Utah State, Clemson); UK-grown roots usually keep only a few weeks (RHS). Check them regularly.
Propagation
Making slips and overwintering cuttings for next year.
Keeping your own plants
Slips from stored roots. Keep a few healthy roots through winter and sprout them in spring. See slips.
Overwintered cuttings. In summer, root cuttings of the vines in water, pot them up, and keep them frost-free through winter to plant out the next spring (Gardeners’ World).
Healthy stock. Use slips from clean, unspotted plants (Clemson).
More to Explore
Short answers, guides that pair well with sweet potatoes, and the sources behind this one.
Frequently asked questions
What are sweet potato slips?
Shoots taken from a sprouting sweet potato, sold unrooted or lightly rooted. See slips.
Can I grow sweet potatoes from a shop-bought one?
Sometimes, but it may be treated against sprouting and may not suit your climate.
When should I plant sweet potatoes?
After the last frost, once soil reaches 18 °C (65 °F). See when to plant.
Can I grow sweet potatoes in the UK?
Yes, best in a greenhouse or polytunnel; outdoors only in a very warm, sheltered spot.
How long do sweet potatoes take?
About 90 to 135 days depending on variety; ‘Beauregard’ is among the earliest. See harvesting.
Why did my sweet potatoes split?
Water or rain late in the season after a dry spell. See common problems.
Can you eat sweet potato leaves?
Yes, young shoots and leaves cook like spinach.
Should I store sweet potatoes in the fridge?
No, below 10 °C (50 °F) they spoil. See storing.
Related guides
Sources
- How to grow sweet potatoes (opens in a new tab) Royal Horticultural Society
- Sweetpotato (opens in a new tab) Clemson Cooperative Extension
- Sweet potato (opens in a new tab) University of Illinois Extension
- Sweet potatoes in the garden (opens in a new tab) Utah State University Extension
- Growing sweet potatoes in a home garden (opens in a new tab) University of Maryland Extension
- How to grow sweet potatoes (opens in a new tab) BBC Gardeners’ World