Sweet potatoes are not simply ordinary potatoes with orange flesh. They come from a different plant, need much more warmth and are usually grown from rooted shoots called slips. That distinction matters in Britain, where an exposed summer can be too short or too cool to produce a worthwhile crop. If you have a greenhouse or a particularly warm, sheltered garden, however, growing sweet potatoes is an interesting project.
What sweet potatoes need to grow
The Royal Horticultural Society explains that sweet potatoes crop best in warm conditions, approximately 21–26°C. They thrive in a greenhouse, large growing bag or tub with plenty of sun and room for their vines. Outdoor growing is most dependable in very sheltered places during a warm summer.
Do not expect the same results you get from planting ordinary potatoes in March. Sweet potatoes dislike cold soil, need a long growing period and can spread into lengthy trailing stems.
Start with slips rather than seed potatoes
A sweet potato slip is a young shoot taken from a sprouting sweet potato and rooted before planting. Many specialist suppliers sell slips or small plug plants in spring. They are the easiest starting point for a beginner because you can avoid several weeks of propagating shoots.
You can experiment with producing your own slips from a sound sweet potato, but success is variable, and a supermarket tuber may not be suited to UK growing. It may also carry problems that healthy nursery-grown plants are less likely to introduce.
If buying slips by post, unpack them promptly. They may look slightly wilted on arrival. Follow the supplier's instructions for rooting and potting rather than planting an unrooted slip straight into a cold outdoor bed.
How to root sweet potato slips
Place the lower portion of each slip into a small pot of moist, peat-free compost, leaving the leaves above the surface. Alternatively, some slips can be stood in water until suitable roots form, then transferred carefully into compost.
Keep the young plants warm and give them bright light. A propagator or temporary clear cover can maintain humidity while they establish, but ventilate regularly to avoid fungal growth.
Do not leave delicate slips sitting in saturated compost. Aim for moist growing conditions without waterlogging, and check for fresh root and leaf growth. Once the plants are established, pot them on as necessary so their roots have sufficient room.
When to plant sweet potatoes outside
Wait until early summer, after all risk of frost and when soil and nights are reliably warm. In northern England, outdoor planting may remain a gamble even then. Growing in a greenhouse or polytunnel provides a much longer, warmer season.
Harden off young plants gradually if they have been raised indoors. Expose them to brighter outdoor conditions in stages, but protect them from chilling winds and cool nights.
If you're growing in the ground, choose a warm, well-drained position enriched with suitable organic matter. Avoid planting into compacted, permanently wet soil. In a greenhouse, large containers or growing bags can make the crop easier to manage.
Growing sweet potatoes in pots and bags
Use a generous container with drainage holes and plenty of compost. Sweet potatoes make vigorous vines and need more root room than their young shoots suggest. A tiny windowsill pot may grow leaves but is unlikely to produce a useful harvest.
Position the container where it receives strong light and warmth. Allow enough room for stems to spread, or train them up a support to save floor space. Use secure canes or trellis before the vine becomes difficult to move.
Water the compost well after planting, then maintain steady moisture through active growth. Check bags particularly carefully during hot weather because they can dry out quickly.
Looking after growing vines
Sweet potato foliage grows rapidly when conditions are suitable. Stems may trail well over a metre. You can guide them along the ground or carefully tie them to supports without sharply bending them.
Give the plants regular water in dry weather. The crop wants moisture while growing strongly, yet standing water can damage roots. Water according to compost conditions and the plant's size rather than automatically soaking it every day.
Keep the greenhouse ventilated during intense sunshine. Extremely hot, stagnant conditions encourage some pests, including glasshouse whitefly and red spider mite.
Feeding should be moderate and suited to a root crop. A huge mass of lush leaves is not proof that large storage roots are developing below the compost.
Do sweet potatoes flower?
They can, producing flowers somewhat reminiscent of morning glory, but flowers are not necessary for the edible tubers to develop. Don't be concerned if your plant puts on vigorous leaf growth without blooming.
The harvest develops underground as selected roots thicken into storage roots. Avoid digging around the base repeatedly to check their size. Disturbing young roots can reduce the very crop you are hoping to see.
When are sweet potatoes ready to harvest?
In Britain, sweet potatoes are generally harvested in early autumn before cold weather damages the crop. Exact timing depends on variety, planting date and how warm the season has been.
Use a garden fork carefully away from the plant's base or tip out a growing bag gently. The skins of newly harvested sweet potatoes can be delicate, and bruised or cut roots will not store well.
Don't expect every plant to produce supermarket-sized tubers. A cool summer may result in modest roots, particularly outdoors. That is part of the challenge of growing a tropical crop in a temperate climate.
Curing and storing the harvest
Sweet potatoes are usually cured in warm conditions after harvest to help their skins toughen and improve keeping quality. Specific curing practices depend on the variety and facilities available; ordinary cool outdoor autumn weather is not a substitute for proper warmth.
The RHS notes that home-grown crops in UK conditions often store for only a relatively short period. Check harvested roots regularly and use any damaged ones promptly if still sound.
Store them somewhere dry and appropriate for the crop, avoiding cold, damp conditions that cause deterioration. Do not treat them exactly like ordinary potatoes kept in a chilly shed.
Why are my sweet potatoes small?
The usual reasons are insufficient warmth, too short a growing season, limited root space or uneven watering. A healthy green vine can still have a small underground crop if the weather wasn't suitable for swelling storage roots.
If a plant becomes yellow or wilted while the compost remains wet, inspect drainage. In greenhouses, look for whitefly and red spider mite, both of which can weaken foliage.
Starting with strong slips and providing a warmer position usually makes more difference than increasing the fertiliser dose.
Can sweet potatoes grow in northern England?
They can be attempted, particularly under cover. A Lancashire garden often has cooler nights and a shorter reliable warm season than a sheltered location in southern England, so a greenhouse is a significant advantage.
For a first crop, try two or three slips of a suitable early variety in large containers. You'll discover whether your growing space provides enough warmth without investing in an entire bed of plants.
Planning your first greenhouse crop
Buy healthy slips in spring, root them somewhere warm and bright, then plant into a spacious, sunny greenhouse container when the weather allows. Water steadily, manage the vines and lift the crop before autumn cold causes damage.
Sweet potatoes aren't the easiest UK vegetable, but understanding their need for warmth makes them far less mysterious.
Source
Royal Horticultural Society: How to Grow Sweet Potatoes.