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Grow your own Sweet Potatoes in the UK

Sweet potatoes are increasingly found on the UK dinner table having emigrated here from America where they are a traditional Thanksgiving vegetable.

These vitamin c rich vegetables are a welcome variation in our gloomy winter diets packing a nutrition and a calorie punch; sweet potatoes are 50% more calorific than the good old garden variety spud, however considering they are traditionally mixed with butter, cream, sugar and on occasion marshmallows to create the eponymous sweet potato pie, I’m not sure anyone is really counting.

Sweet potatoes also make a delicious alternative to regular potatoes when making wedges and mash and is a wonderful meat substitute in Winter curries alongside its faithful companion: the chickpea. Interestingly, the leaves can also be used as a substitute for Spinach.

Although sweet potatoes originate in Central America, growing a good crop here in the UK is perfectly feasible with a little motivation and that most premium of garden aspect, the warm and sunny site.

Sweet potatoes do very well in greenhouses so if you are blessed with one of these, it is perhaps best to consider setting aside a piece of this prime real estate for them, either in a large tub or in a border and save your external sunny position for another crop.

Be not disheartened however, an outside container or south facing bed will yield enough sweet potatoes for them to have a positive impact on a gardener’s self -esteem.

Potatoes they are in name but in nature they actually hail from the Ipomoea family which we associate more with Morning Glory or Spanish Flag than with edibles. They in fact do not follow many of the potato rules for cultivation either, but edible tuber they are, so potato we call them.

The start of the many dissimilarities between potatoes and sweet potatoes is that they are not grown from entire sprouted or ‘chitted’ seed tubers as with regular potatoes of the Solanum family but from ‘slips’ or rooted cuttings, grown from a parent potato and then removed and grown on as with a traditional cutting.

These slips can be purchased online from reputable seed merchants or at good sized garden centres, however if you are new to sweet potato growing or don’t mind spending slightly more money on your starting stock, then Super Plugs from online retailers are a solid option for you. Super plugs will be healthy, well rooted and actively growing small plants and buying them in this form removes the need for early tending or having a young population of slips clogging up your windowsill, but it’s entirely up to you.

Sweet potatoes can be considered and cultivated as a perennial vegetable, so your initial investment of time and money will reap years of tubers if treated correctly and stored well.

One good quality potato saved from a previous harvest or that of a friend will produce three to five slips. Each slip in turn will become a plant which will yield five to ten potatoes depending on your variety and how the season has progressed for you.

It is worth noting at this point that if you intend of create slips from a supermarket sweet potato you may need to prepare yourself for failure. They are often sprayed with substances to retard sprouting and you don’t need that sort of chemical infused disappointment in your life.

So, let us assume you have a prime, organically grown tuber in your hands, of solid and trustworthy providence, proceed as follows….

1. Place your tuber in a box of vermiculite, perlite or sand. I like to sieve the sand from the children’s sand pit at the end of the summer to reuse it for these purposes. If you have no sand pit or are particularly afflicted with cats, steer clear of this and get some perlite.

The perlite, vermiculite or sand needs to stay moist and cover the tuber, it must also be kept warm at this point in an airing cupboard, warm propagator or on a windowsill near a radiator.

2. If your box has a lid (a plastic storage box is ideal), then lightly place it over the top, if not, cover the box with cling film. This serves two purposes, one to retain some warmth and the other to create condensation which will help your growing medium stay moist.

It will take about four weeks to see sprouts so this is a good post-Christmas, pre-new year activity as by the end of January you’ll be motoring along towards your new sweet potato growing season, a nice distraction from the dismal weather and an exciting glimmer of the year to come at a time when gardening task are thin on the ground.

3. Once your sprouts are 5 to 8 cms or 2 to 3 inches long, you can cut them from the parent potato with a sharp knife and pot them on in to clean pots of cutting compost, John Innes No 1 or similar.

You need to remember which end was cut from nearest the parent as this is the end that will root in the compost. If you cut all the sprouts at once it is easy to lose track of which end is which, so you can either cut at an angle the end which is to be in the compost or be extra vigilant when performing this task.

Some sprouts may already have some of their own roots forming at this point, but it is by no means common so don’t get overly concerned either way.

Now we wait for your sprouts to root and start putting on top growth, so keep them warm and moist as you would any of your overwintering plants. If you have a propagator then super, a heated mat is also perfect for rooting cuttings but as always if you have a warm spot and a handheld mister then you can keep them sufficiently warm and moist to have success. It is worth noting that when growing on cuttings that putting them above a radiator is too warm and can be very drying so move them to somewhere warm but away from a direct heat source.

Whether you are growing on your sweet potato plants in a greenhouse or outside, it is going to be essential to harden them off before moving them to their final growing position. Sweet potatoes crave warmth and will be uncomplaining of spending a few hours outside in 16-18c during a sunny April day, but they won’t thank you for anything colder and a chill wind may well leave you empty handed. You can’t be cavalier with them at this time so if you are uncomfortable outside in a t-shirt, then it is not the time to be hardening off your sweet potatoes.

A firm hardening off program covers a 7 to 10-day period of increasing the plants exposure to the outside world until they are ready to go to their final sites.

Apparently, an American rule of thumb is that you plant your sweet potatoes out the day after May Day which translates well enough to most parts of the UK if keeping one eye on the forecast and night time temperatures. If you are growing in a greenhouse and the forecast is promising for both day and night-time temperatures, then you can transfer outside a week or so earlier but as always under caution.

If you are planting in to containers, put the kettle on, your tasks are brief and unchallenging. All you’ll be needing is a large, clean tub with requisite drainage holes, crocks in the bottom and lifted up from the ground on feet or blocks. Your compost must be very rich so if you have faith in your homemade mix then by all means use it, however a John Innes No3 is a sure-fire win. Besides, whatever growing media you use, you’ll be supplementing it fortnightly with a general-purpose fertiliser.

As an aside, once you have potted up in to the greenhouse, do ventilate well, warm is good but sweet potatoes shouldn’t be boiled until they have been harvested.

One tub per plant is the norm as they need as much space as you can give them, so be prepared to turn over some sizeable part of your greenhouse or your sunny outside space to your tubs.

You will eventually need to tackle the top growth in your containers as it does have a sprawling habit. Growing up strings in a greenhouse or tucking the growth in to a tripod of canes in other situations is the best way of dealing with the stems, making everything neater and keeping it all off the ground. Now container people, your work is done, eat your biscuits over there.

Should you not be in possession of a greenhouse or you have decided against container growing then your plan of action will be somewhat different. You will still need to start out with very fertile soil, so a plot that has been animal or green manured over winter is the ideal spot, and as the tubers need an easy space in which to expand, well dug, free-draining and friable soil is essential. Sandy or loamy soil is the best kind of soil for this venture so if you are on clay or suffer greatly for rocky, stony soil then it will be better for you to grow your potatoes in to rows of mounded compost so the plants instantly have a head start, and more room to stretch their legs. This does require using up an amount of compost from your own heap or buying in compost which I appreciate can be expensive, but on the plus side it will make your harvesting significantly simpler and ultimately improve your soil.

Raised beds are also an excellent option as you have a great deal of control over your soil quality and you should suffer less from compaction, unless you are negligent bed stomper and then shame on you. It goes without saying that all areas should be free from weeds.

One more point on outdoor growing before we move on to the happy topic of harvesting, and I refer you back to the plants sprawling habit. You won’t need any more than 60cm of top growth to successfully achieve tubers below ground so when the stems get to this length you can pinch out the tips to prevent the stems taking over your patch. This practise will also go some way to deterring aphids who enjoy nothing more than young tips.

You may also do this with containers and greenhouse growing of course but if you are growing up strings and supporting canes then the space issue is not so important. The aphid comment however, still stands.

Let’s get on to harvesting as that’s what we are all here for and after a season of sweet potato cultivation you probably deserve it.

Sweet potatoes take approximately 16 weeks from planting out to the point that the foliage starts to die back, turn yellow and generally feel sorry for itself. If you planted out in late May to early June as is advisable in our climate then this takes you comfortably to the end of September to early October.

You don’t need to harvest at this point unless you have plenty and want to get going on the eating, as they will keep quite well in situ and continue to grow in size for a little while longer, out of sight, underground.

This is the point you must watch for your first frost warnings, as a frost on your tubers late in the season will kick start rotting and all of your ventures will have been for nought.

It is difficult to give general advice on the first frost dates as it is so regional across the many UK microclimates and changeable year on year. In my Surrey back garden, it is on average the week before son’s birthday, the 18th of November but I am vigilant from mid-October.

Once you have decided to lift your tubers you must take the utmost care, as any damage to the tubers from a misplaced garden fork will seriously hinder your storing efforts which are the essential last step in creating the perfect Sweet potato.

Lift on a dry day. I know this is a big ask and it may very well end up being completely useless to wait for something that may never come, especially in November, and the least wet day may have to do.

Start by placing your garden fork 30 cm away from the plant and lift gently at an angle to tease the earth up and avoid doing as much damage as you can. Don’t go about stabbing at the earth as you’ll end up with a kebab on the end of your prongs. Once the surrounding soil is disturbed and you see your tubers, you can get in there with your hands to free them.

If you are harvesting from containers then avoid a fork all together and tip the defoliated tub gently (you may need a strong assistant) on to a tarp and forage carefully with a gloved hand for your treasure.

At harvest, the potatoes will have a thin skin and I’m not being absurd when I say that you must handle them with the utmost care. A large, shallow tray or cardboard box lined with an old pillowcase or tea towel is a safe method of transportation and storage. Lay the potatoes down in one single row and do not stack them as this can create a damp atmosphere, perfect for rotting.

If you have had a harvest on a wet day and there is a lot of mud stuck to your tubers, do resist the urge to pick or wipe them clean as this may take the skin along with it and impact storage and longevity.

Take your precious cargo in its flat, towelled bed, mud and all, in to your house or greenhouse and leave them to dry for 10 to 14 days so the skin has an opportunity to harden and dry and any earth has turned to dust, so it can be gently brushed away with the flat of your hand.

The sweetness of a sweet potato isn’t inherent at the point of harvesting or much after the drying period as a time of curing must pass for the starches in the flesh to turn to sugars. Should you damage a tuber at harvesting it is an interesting exercise to cook and eat it, comparing it to how you might expect a sweet potato to taste. Without curing it is quite different although not unpleasant.

Once your sweet potatoes are dry, skin harder and reasonably free of earth, you must move on to the curing.

Trawling the internet will throw up many horrifyingly involved systems for curing chambers and curing ovens but please don’t fret. As a back-garden grower, you can create a homemade curing chamber which is simply a plastic bag stuck with holes.

Go to town on some carrier bags and make sure that, with a knife, you have made lots of ventilation holes in enough bags to carry your crop. Lay the potatoes in the bags, again only one layer and no stacking. This may mean you put only 3 or 4 in a bag but that’s fine, we all have enough plastic bags lying around. Tie the tops of the bags and leave them in a warm, dry place for about six weeks.

Store them in a cool, dark and dry place such as a pantry or veg rack until required. Be wary of storing the potatoes in a shed or garage if you have rodent issues as the sweeter the potatoes become, the more a mouse will seek them out and the more you will mourn the loss of your sweet crop.

I feel I may have been a bit bossy and involved in this sweet potato edition and I hope it hasn’t put you off trying to cultivate this delicious vegetable. It certainly isn’t for the true beginner but if you have a basic knowledge of vegetable growing it does present a nice challenge and if you choose to distil the advice down and go the easy route to harvest, that being a few Super plug plants in containers with fortnightly feed in a sunny corner, you will yield delicious and impressive results.

Varieties to Consider

The variety you may want to consider if you are container growing is ‘OHenry’ as it has a more compact tendency and the tubers tend to grow more directly beneath the foliage than in other varieties. The OHenry variety is a little different to the sweet potatoes you may imagine as they are white skinned and white fleshed so if traditional aesthetics matter then give this one a miss. However, with curing and storing this variety will be just as lovely and sweet as the more traditional orange sweet potato and can be used in the kitchen in exactly the same ways.

Just as suited to container growing and bed and border growing, the more recognisable variety and probably the more easily found, is the 'Beauregard'. This is the typical sweet potato, thicker skin and orange to salmon pink flesh beloved of the sweet potato pie enthusiasts, as it keeps its colour nicely when cooked.

Do consider when container growing with the Beauregard that you will need a slightly deeper container than with the OHenry as the Beauregard has a deeper rooting tendency and requires a bit of extra leg room.

Happy Growing!

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