Sweet potatoes are hardly ever attempted as crops, which is surprising. They give remarkably large crops and are rich in vitamin A, and some varieties make gorgeous flowering climbers into the bargain. They're considerably easier to grow than many other greenhouse crops and do not suffer much from most of the usual pests and diseases. Indeed I just do not understand why they are not more widely grown.
Sweet potatoes prefer open free draining composts but will grow fairly well in almost any soil mix. They need warm conditions to start with but later on they do not need as much warmth as say melons, indeed they can be kept outside for the summer. Unfortunately although they will grow outdoors the frosts come before they have had time to make a decent crop so they really need to spend the autumn months under cover. If they are grown in big pots or containers they can go out when the weather warms up and then months later be brought indoors as the weather chills. One whiff of frost though and the haulm and tubers rot, be warned!
They tolerate the diffuse light of my double polythene covered tunnel growing alongside melons and cucumbers but do not relish the high humidity those other plants thrive in. In my greenhouse I used to find my sweet potatoes grew well on the north wall in the shade of the tomatoes which filled all the sunny side but it was hard to keep the soil warm enough for them in late autumn. In my double polytunnel this is much easier, and this means as they are always growing I can have fresh roots anytime.
Ideally sweet potatoes are grown starting with young plants of over-wintered rooted cuttings from the previous year's runners! This is without doubt the best method but is obviously impossible the first year. So as no useful cultivars I've heard of come from seed you have to start with a supermarket tuber. You also need to choose your variety. There are many sorts of sweet potatoes, Ipomoea batatas, to say nothing of the yams which are other plants entirely.
The Sweet potato was introduced ahead of the now ubiquitous spud of Raleigh. Many different crops were being tried and rejected. The Jerusalem artichoke from North America was popular for a while, as was the Dahlia -introduced for it's food value and not as a flower. Sweet potatoes were grown experimentally all over Europe but were soon replaced by the spud except in the warmest countries. There are now many sorts of sweet potatoes just as there are many sorts of spuds. The smaller ones with a yellow or whitish flesh and a red or purplish skin I find easiest and these flower with pretty purplish columbine like blooms. These sweet potatoes are quite dry and mealy and are often imported from Southern Europe. However I prefer the larger orange varieties which come from North America such as Jewel. These are much sweeter, moister and better textured when baked.
The Spud probably superseded the Sweet because the tubers of the former could be more easily carried through long winters while the latter needed to be over-wintered as small live plants, or cuttings at least. Sweet potatoes are not like 'Irish' potatoes, they do not form the same sort of dormant tuber but more of a swollen root remarkably like carrot in the orange varieties. They are usually 'heat cured' for storage, and as they are prone to rots then supermarket tubers may be treated with anti-rot and/or anti-sprouting chemicals so need careful washing.
Fortunately though, with patience, a sweet potato root can usually be coaxed to sprout; though the warm humid conditions in a good propagator are almost essential. The shoots sometimes spring from an exposed part of a root but seem to come easier if it is buried in moist gritty sand. Once the shoots are a couple of inches long then these should be detached and potted on separately. If the shoots are left attached to the old tuber they often rot away with it and thus they survive better on their own.
The young plants are quite vigorous and if kept warm soon need potting on and rapidly require canes to wind their shoots around. It is important not to let the shoots trail on the ground or soil, or onto the surface of the compost in other plant's pots. Sweet potatoes root wherever they touch and although this strengthens the plants they also form small tubers from each rootlet which wastes resources. Ideally only one stem grows from the ground and will form one main and usually several subsidiary root tubers. Trailing stems laying on the ground will root everywhere and produce many mini-roots which are not that desirable.
To prevent the stems rooting they are either wound up around canes or tied up strings. In either case they are then more exposed to the sun and get more light than they do on the flat and this helps increase yields. Other than watering no attention is needed apart from tying in the somewhat rampant growth. Watering is very important but feeding is not, it seems good crops come from plants grown in fairly poor soil well drained yet well supplied with moisture. Too much fertiliser tends to make lots of leafy growth -which is eaten in some countries as it is edible when cooked and rich in protein, and good to fatten cattle.
The tubers can be taken from midsummer if the plants were started earlier enough. Modest roots are often considered best as bigger sweet potatoes are thought to become tough -but I find it depends on variety and growing conditions as much as size. Unless frosted the plants do not naturally die back like spud plants so need to be dried off and dug in late autumn if they cannot be kept frost free and growing with a temporary cover. Ideally the plants are kept alive as long as possible then the tubers are available fresh for longest. Once dug or detached from their haulm they soon decay unless heat cured. Thus pot culture is ideal as it makes the plants transportable. Of course the bigger the containers the better results, I use plastic coal sacks doubled up for strength and light exclusion.
If plants in containers are under frost free cover they can be kept just alive all winter as long as their compost is barely moist. If they are dried, or chilled, to the point where the stems wither and fall off then either use the tubers immediately or dry them off thoroughly to prevent them rotting. Do not forget to take cuttings or root the tips of some growing shoots in the autumn to produce the plants for next season, these can be kept alive on a sunny windowsill indoors until the days lengthen again.
And when you are cooking those delicious sweet potatoes nothing goes better than a ginger sauce. Which can be home grown as well. Ginger is more difficult than sweet potatoes though, it needs to be considerably warmer!
Ginger must be grown under cover, and it is happier under plastic than glass as if direct bright sunlight follows the usual long dull overcast period then ginger is apt to scorch and the leaves wither and yellow. It seems to be safest kept like Gardenias with full but diffuse light. I find it does well in my double skinned plastic tunnel and even there prefers to be in the dappled shade amongst other plants.
Ginger needs starting early in the year. A piece of fresh ginger from the supermarket should be selected that has some fat hard green buds on the ends of the fingers. These should be detached from the hand, dried for a day or so then potted up in gritty compost and put in a warm light place. Ideally a heated propagator will be used as it is getting the early plants growing that is the difficult bit. As the leaf blades shoot and grow new ones start then the ginger can be potted on. into larger pots. By the end of a good growing season bucket sized pots are required. The compost needs to be kept moist but barely so if the temperature is low.
As each blade grows the stem at the base thickens, once it has achieved a goodly size the whole stem can be cut off the roots, peeled and turned into candied stem ginger (see box). Alternatively the stem can be left to mature. In a warm bright conservatory ginger can sometimes carry on growing throughout winter but more often it dies back in late autumn. When the old stems start to wither and die back dry the plant off and knock it out of the compost. A large hand or two of root ginger will have formed in the pot. This can be dried further for longer storage but is now too tough to use for candying. Traditionally the root was scraped, completely dried and coated in lime before shipping to us from abroad.
Kept in a warm light frost free fairly dry place a ginger root and it's buds should happily survive until early spring when the next crop can be started. Grown on it's own in a large bucket in a good season I produced several pounds of ginger from one plant alone. Most summers I get less. Even so it's still worth growing for the fun.
Make a strong sugar syrup boil and drop in the peeled and cut up chunks of stem ginger, take off heat and allow to cool. Next day strain off syrup and repeat process with new syrup. Day three repeat again. The three strained off syrups can be combined and used as ginger syrup for flavouring. The now less fierce chunks are packed in a glass jar and covered with more hot syrup then sealed and matured for a month or more before using.