Growing Sweet Potatoes in the UK

Growing sweet potatoes in the UK isn’t simple, but it needn’t be complicated either. This guide gives straightforward advice and fail-safe instructions on getting the most out of that delicious tuber.

Everyone has that one crop they’ve just never had any luck with. For me, it’s sweet potatoes, but this year I had a gigantic crop by stepping back and looking again at the nature of the plant.

Sweet potatoes are tropical plants, and in no way naturally suited to UK climates, and even though their tubers are hardy down to -9°C, they prefer it if you put a little more effort in.

In their native spaces, they will grow similarly to Jerusalem artichoke, just expanding and duplicating year on year underground without any support. Sadly, they’re not as well behaved. Artichokes grow vertically with very little support, but sweet potatoes can trail across areas of 2m²if left to their own devices.

The key questions to ask about growing sweet potatoes in the UK are “how do I keep sweet potatoes warm?”, “how do I train sweet potatoes?”, and “how do I get sweet potatoes started?”. And you’re in the right place, because I’ve discovered the answers over years of mistakes, and finally got it right this year!

Getting started with sweet potatoes in the UK

To start with, let’s dispel the myth that sweet potatoes are the same as potatoes. They grow underground in a similar fashion, but the plant is completely different, and the process is very different.

There is no need to chit a sweet potato. Instead, we create something called a sweet potato slip. The slip is just a rooted cutting grow from the tuber itself, similar to spring Dahlia cuttings. There is also no need to hill up sweet potatoes, they grow underground, and can’t develop the poisonous green patches like potatoes when exposed to light.

Most importantly, they don’t develop blight. Once you’ve got the basic recipe for growing sorted, they are very easy to manage and provide heaps of ornamental benefits, as well as gluts of edible leaves.

When to start growing sweet potatoes in the UK

You can get started with sweet potatoes by creating the slips in early November, as long as you’re happy treating them like a young house plant through winter. This way you’ll have a head start next year.

If you don’t have the space indoors, you can start the slips off in a greenhouse or cold frame in late January or early February. It doesn’t make a big difference on cropping times, as they will generally crop around 4-5 months after planting in their final location.

If you buy your sweet potato slips rather than prepare them yourself, you can usually find them in garden centres ready to plant out in March/April, but it’s much more cost-effective to prepare your own.

When to plant out sweet potatoes

Depending on where you live in the UK, deciding when to plant out sweet potatoes in their final position can be tricky. Too early, frost will kill the plant. Too late, they won’t have enough time to grow tubers.

As a rule of thumb, late April is safe to harden plants off, as long as night-time temperatures are reliably above 10°C. To prepare your plants for their final position, start by moving them outdoors for a few hours every day for one week. After a week of this, they should be ready to spend the rest of the year in the garden.

Plant your slips, or young plants if you’ve bought them ready grown, at the same level in the soil as they are in the pot.

Sweet potatoes grow close to the surface but should never poke out. It won’t cause any damage to the plant or tuber if they do but is a clue that the plant isn’t anchored well enough in the soil to support a tower of growth above. If this is the case, just add another inch of soil or compost to help anchor the plant.

Soil conditions

Sweet potatoes like free-draining, moisture-retentive, slightly acidic soil. Essentially, this can be any peat-free compost but if you’ve got a good batch of leaf mould and garden compost at home, that will be perfect, and if you’re worried about acidity, add a handful of fresh pine needles to the surface (once they turn brown they’ve done their work and you can remove them).

For container gardening, mixing through well-rotted manure can be a basic boost to soil fertility, too.

There are plenty of soil layering methods, which will work incredibly well for raised beds without the need to dig over, but if you’re at all worried about where to plant them in a rotation, just add manure to the surface of the bed over winter will give next year’s sweet potatoes a brilliant boost.

One more crucial tip is that sweet potatoes hate tap water. They react badly to chlorine, so try your best to let your tap water sit before using it or stick to rainwater.

Timeline:

Start sweet potato slips indoors November-February (slice your tuber in half and place in water)
Separate slips for the tuber 2-4 weeks after preparing the tuber – or when the shoots are 8 inches long. Place these in water for 1-2 weeks until roots appear.
Pot each slip into its own 9cm pot, and keep warm and moist until mid-late March
Plant in final position when all risk of frost has passed, and night-time temperatures are reliably above 10°C.

Positioning Sweet Potatoes in the UK

Knowing that heat is important, there are three choices for positioning sweet potatoes in the UK. Each comes with its own soil conditions, but all follow the same basic principle; plenty of organic matter and well-drained, moister retentive conditions.

Growing sweet potatoes in containers

There are a lot of factors for growing sweet potatoes in containers, and it is by far the best and most reliable option for growing them in the UK, but it comes with more work than the rest.

It’s best to create a micro-climate in the pot itself. The recipe is based on layering to keep heat production going throughout the season and is based on the hot-bed method for growing tropical fruits in the UK, pioneered by Gilbert White, the 16th Century father of ‘six quarters’ seasonal planting, and passionate earthworm advocate.

Essentially, place a layer of straw over a layer of fresh or part-rotted manure, creating the conditions for decomposition, and add at least 6-8 inches of compost on top of that. Plant directly into the compost, and the manure will keep it warm.

Remember to keep them climbing. One tall tepee per pot should be sufficient, making sure to tie in the vines regularly and harvest edible leafy greens from the plant as often as possible to stop it from getting top-heavy.

Growing sweet potatoes in raised beds

Raised beds aren’t ideal for growing sweet potatoes, but if you get the timing and soil structure right, sweet potatoes can become a wonderful addition to your garden year after year. The essential thing to remember is that these tropical plants need plenty of warmth – so make sure to choose the sunniest raised bed in your garden and delay planting your sweet potato slips until the weather has warmed up considerably – mid or late May is best.

For a detailed guide on spacing, planting depth and soil preparation, check this article on growing sweet potatoes in raised beds.

Growing sweet potatoes indoors

Sweet potatoes can be treated as a perennial house plant. Just prepare your slips and pot them straight into a pot. Train them up a trellis, a tepee or an obelisk for a unique pillar of edible foliage, and simply leave one tuber in the soil for the next year when you’re ready to harvest.

Sweet potatoes are tropical plants, so like most UK houseplants, heat is more important than sunlight. If they can get at least 4-6 hours of sunlight a day they’ll be happy. Direct sunlight is best, as its photosynthesis creates the sugars in your tubers, making them perfect for sweet baking as well as savoury. Constant heat and water give reliable crops of healthily sized tubers.

The only difference between sweet potatoes and most houseplants is that you want to keep watering regularly. Similar to tomatoes, they don’t mind too much, or too little, as long as it is constant.

Sweet potato varieties for UK gardeners

If you’re using a sweet potato from a supermarket to propagate you’ll need extra heat compensations because supermarket sweet potatoes have wracked up some serious air miles, as most are grown in much warmer climates. For these varieties, I’d advise growing indoors, or in a greenhouse, or as a minimum, against a warm wall.

Growing supermarket sweet potatoes needn’t scare you though. They are just as reliable as long as you can keep them warm, and grown in a container indoors you’ll get amazing crops, but also a truly stunning perennial house plant that can return year on year.

If indoor growing isn’t an option, and you don’t have space for a greenhouse, you need to buy some UK bred tubers. You can buy these from most garden centres, the same way you buy seed potatoes. These need preparing still but are specifically bred to cope better with UK weather.

The variety in these tubers is astonishing, but these three are all reliable even in the coolest parts of the UK (we grew Erato Orange outdoors this year with at least 20 tubers from one planter):

Erato Orange, as the name suggests is similar to the well-known orange-fleshed Beauregard, which fills up most supermarket shelves.

Erato Gusto is a white-fleshed, nuttier variety, with a less intense sweetness.

Erato Violet is a purple-fleshed strain, which keeps its colour after cooking. The tubers are smaller, which is common for purple varieties, but it makes up for it with flavour and more vivid flowers.

If you’re looking to grow these as an experiment and see the crop as a bonus rather than a goal, Sweet Potato ‘Blackie’ is a highly ornamental variety with beautiful black leaves and violet flowers. Its leaves and tubers are all edible, but not worth growing for food as the flavour is bitter, and its tubers are tiny. But if you want a show stopper for a hanging basket, it’s perfect!

Everything you need to know about growing sweet potatoes in the UK, covered

And that’s that. Sweet potatoes shouldn’t be seen as complicated, you just need to get the basics right and you’ll have a generous crop of sweet potato tubers to eat and store.