Recently there has been a huge interest in growing sweet potatoes and to a lesser extent some of the other less commonplace ethnic root crops. Some of these are even confused with the true sweet potatoes Ipomea batatas which are quite closely related to Morning Glory. Rather unexpectedly these were introduced well before our now ubiquitous ‘Irish’ potatoes but failed to really catch on. (As effectively likewise failed Jerusalem artichokes and several other, remarkably unpalatable, roots introduced as potential food crops, even Dahlias were tried!)

Both sweet and ‘spud’ potatoes hail from the Americas and from the warmer areas, however ‘spuds’ were from mountainous regions so their plants proved that little bit less tender and easier to grow in colder countries. As importantly for their success is it’s considerably easier to store spuds for long periods whereas sweet potatoes need heat treatment first. And finally- spuds for the next crop can be simply stored over winter to use as seed tubers or sets. However sweet potatoes are difficult to grow the same way and it’s better to over-winter late summer rooted slips or to force a tuber to produce shoots for potting on.

Thus for many reasons sweet potatoes were ignored for centuries despite their sweet and pleasing flavour. Now newer varieties and better methods make them a more practical crop, and a valuable one for they’re not only expensive to buy but nutritionally richer than spuds, the yellow and orange varieties being packed full of carotenes.

As said it is best to over-winter small plants under cover in frost free conditions, not dissimilar to those for Pelargoniums. These small plants can be brought on with more warmth in late winter to produce slips to pot on. Or alternatively many stored or purchased tubers can be forced. If these have been over-heat treated or sprayed with a sprout inhibitor they may not sprout however most supermarket sweet potatoes do sprout if potted in a just moist, gritty compost, on bottom heat. Once the shoots are finger length or so they can be detached from the tuber and potted on separately. (Left attached they may rot when the tuber decays and a mass of congested shoots are unproductive so need separating anyway). The young plants enjoy the same conditions as young tomato plants and can be treated rather similarly though no disbudding or nipping out of side-shoots is necessary.

You then have two choices to make. In or out, and pot or soil. In or out is easy- if you have the space grow them indoors as they need as long a season as possible to give big crops. If you start early then indoors you can have some tubers to eat by mid summer and many more produced until the light levels fall in late autumn. Outdoors after the frosts have finished there is little difficulty until the frosts return though crops will be lighter unless you can arrange warm conditions.

It is another choice to decide on container growing or in the bed or border. This is most important as although sweet potatoes are in many ways easier when planted out this may then make for a worse crop. This is not unconnected with temperature as they are unhappier in a cold soil than a warm pot. However even if the soil is well warmed it is also likely to have many pests. And sweet potatoes can be so damaged by slugs, millipedes and so on that although they crop heavily in some years this is almost entirely unusable even after heavy peeling and cleaning. But a container can be kept pest free and thus give a much cleaner crop. So it’s more sense to grow them in large containers of good compost, especially as these are also possible to keep warmer from earlier in the season. However there are other very good reasons for growing sweet potatoes in containers; these can be moved. Thus they can be put outdoors once summer has warmed up and brought back in before the frosts return thus giving a longer growing season and a bigger crop than in the ground. Sweet potatoes are also deep rooters and not as easy to detect as spuds thus you may inadvertently leave some crop un-dug, whereas in a container all is easily found. And as importantly they can be also used to store the crop. Sweet potatoes once dug do not keep well without heat treatment which is fiddly. Thus they are best left undisturbed in their containers with their foliage allowed to dry off and to die down in autumn. The containers with the crop safe in dried out compost can be moved to a frost free place and then used fresh until well into the new year.

However, it is possible to get good crops in the ground in either the indoor or outdoor bed or border if it is pest free and well warmed. (Previous and concurrent baiting with carrot and potato chips, slug pubs and sprouting grains will reduce numbers.) Much depends on a warm summer as sweat potatoes do so poorly in the cold and extra cover even under cover helps especially at the start. Most commercial cloches and cold-frames are simply too small as these are vigorous plants that need a long season and lots of space to make plenty of haulm to feed the crop. In or out I find it worth planting through black plastic sheet, woven preferably, and covered with another transparent sheet held up on sticks. Otherwise sweet potatoes are not particularly demanding as to soil or compost though a thirsty crop especially as the haulm enlarges. Now this is where a little special attention is required. If their haulm is allowed to touch the soil or compost it will readily root (handy in late summer for making slips) but in so doing will waste resources making lots of roots which become useless mini-tubers (indeed more like a vegetable spaghetti). So it is better to tie the haulms up strings, nets or posts and away from the ground or pot -and this of course also exposes it too much more light and so further aids growth.

Incidentally the haulm has edible leaves, these can be cooked as a spinach (I really cannot recommend this very highly, but it can be.) And often you will get flowers, pretty and usually purplish much resembling bindweed, these with the dark glossy foliage make the vines quite attractive enough to be used for decoration, say around a patio or even in a conservatory.

There are many varieties. The most reliable are the smaller purple skinned ones with cream or yellowish harder drier flesh. However the bigger, buff reddish orange skinned ones with orange flesh are sweeter moister and much more rewarding. These are sometimes referred to as Carolinas’, Jewel is a good variety of these. I find this sort best washed, wrapped in foil and baked whole- the skin slips off once cooked, so delicious with lots of butter…..

Now sweet potatoes are sometimes confused with yams. These are Dioscoreas, most come from Africa and Asia, and there are a huge number of species, varieties and local variants. Given similar treatment to sweet potatoes and with even more warmth most can be grown here. They can store better longer than sweet potatoes but unfortunately are even more vigorous climbers and with an even longer season before they crop so they are really too hungry for warm indoor space for economic cultivation under cover, and just too tender for much success outdoors. Still they are worth trying for curiosity.

Some small yams such as the Chinese or Cush -cush can have a whole tuber planted much as for a spud. For the bigger ones the top from a healthy yam can be planted to get shoots which can be grown on separately though they can be left to form a stool. These shoots need support and as said take a lot of space. Once the tubers have formed, nine months to a year or more they can be dug and then stored hung in an airy warm frost free place. (Some sorts are most difficult to extract cleanly having weird shapes, and if in the border can be very deep to dig up.) The yellow are not dissimilar to Swede, the white more resemble spuds, but all yams are rather bland and need spicy food to make them palatable. Unfortunately yams have little food value other than as pure carbohydrate fuel and I found them easy but with poor returns for the space.

Cassava or manioc (Manihot), is sometimes seen in ethnic shops. These are fat tuberous roots from a shrubby plant also packed full of starch, commercially extracted you know this as tapioca. It is usually grown from stem cuttings and seems hard to start from the ‘roots’ though apparently these can be forced to sprout. Popular in hot countries as it is tough, reliable and immune to locusts it is relatively slow to crop and the common bitter varieties require special pre-treatment before consumption. I have failed with this but you may succeed though as it becomes more than head height it needs some space.

However I have had more frequent success with Eddo, dasheen or old cocoyam (Colocasia). This has large arum like leaves and makes a stunning decorative plant anyway. The tubers are botanically corms and are the part eaten, one big one with dasheen or several smaller offsets with eddoes. The skin may irritate some so is best peeled with gloves, the flesh is white, mostly starch and again with little flavour. I find these remarkably easy to grow in any moist compost in a warm place, and as they only get waist high can be conveniently grown in large buckets under cover. These can even go out for the warmest months.

Finally- you may also come across Tanni or new coco-yam (Xanthosoma) which resembles eddo in almost every way including cultural, but as this gets much taller, more than head height, it is less suitable for growing here under cover.