Yacon (Smallanthus sonchifolius) is, of the Andean "Lost Crops" I have grown, the least trouble and the most successful. It seems to like growing here. Picture the bastard offspring of a Jerusalem artichoke and a dahlia on steroids, with big triangular leaves and yes, even in our indifferent climate, big tubers and that sums up yacon. You cut off the big dahlia tubers and replant the Jerusalem artichoke rhizomes the following year. Lift them carefully though, because those big tubers are brittle and make an audible crack if extracted carelessly, which will delight naughty children of all ages, but does nothing to prolong their keeping qualities. As for storing them, I find they keep quite well in a cool room as long as their skins are dry.
Give it a good sunny position and good soil and both the plant and the tubers can get seriously big. Stick it in a greenhouse as Neil and Gill Faiers of Bangors Organic B&B did and you'll need a machete to get through the door and a JCB to harvest it. This is only a partial jest. Their plant, well over two metres high, looked ready for a walk-on part in Day of the Triffids and was frankly, a bit intimidating. Funny, but I don't remember them thanking me for the small offset I gave them a few months earlier.
The tubers, unlike the starchy tubers of oca, ulluco and potatoes, are crisp and watery, with a sweetish flavour. If you leave them to shrivel a bit, the sweetness is accentuated and unlike most other root crops, you can eat them raw. In the Andes this curing process is carried out in the sun; sunlight tends to be in rather short supply in the late autumn here, but they'll shrivel on a windowsill. They also do a good job as water chestnut substitutes in stir fries. I know this has echoes of the iguana, crocodile, bullfrog tastes-like-chicken syndrome, but it happens to be true. They keep a pleasant crunchy quality even after cooking. This sweetness is a function of their high fructo-oligosaccharide content. In fact yacon is supposed to be the best producer of these in the plant kingdom.
Fructo-oligosaccharides, henceforth abbreviated to FOS, are credited with all sorts of benefits. They are composed of individual glucose molecules linked to multiple fructose molecules in such a way that they pass undigested through the human digestive system and into the colon. Here they are gleefully consumed by your resident probiotic bacteria. So, it's a buy one, get one free situation: a sweet taste, yet negligible effect on blood sugar levels and a powerful boost to your gut's good guys: a prebiotic for your probiotics.
The combination of its low glycaemic index, beneficial effects on gut flora and high antioxidant levels have led to yacon being aggressively marketed as another of the indespensible superfoods all health conscious citizens should be consuming. I laughed out loud on spotting recently, in a somewhat down market purveyor of health foods, yacon and goji berry muesli. My guess is the endorphin release I experienced as a result probably outweighed the benefits of eating it. It was a darn sight cheaper anyway.
As fascinating as I find the discussion of a plant's therapeutic benefits, I can't help feeling a four letter word rising in my throat - hype. Anyway, you won't need to be gulled by all their propaganda, you'll soon be growing yacon successfully in your own garden, with a smug, self-satisfied smirk on your face.
I made some quite successful pickles by cutting the tubers into dice-sized chunks and soaking them in brine, adding a bit of whey to get those lactobacilli working. The trick is to keep the chunks weighed down so that anaerobic fermentation occurs. The result is a sweet and sour pickle. It might be fun to shred a bit into sauerkraut at same time as the cabbage is being chopped.
Of the two variteies I've grown, 'morado', which came from Eilif Aas of Norway/ Peru, has a purplish skin with a purplish cast to the leaves and produces smaller tubers with yellower flesh than the standard variety, whose real name is unknown. There's a difference in flavour too, although I can't quite pin it down. There are also varieties with purple spotted flesh. In fact I had one of these many years ago, but, unfortunately it succumbed to rot before it even put forth a shoot.
Something I haven't tried yet, but would like to, is making yacon syrup. Ivan Manrique, Adelmo Paaraga and Michael Hermann of CIP (Centro Internacional de la Papa) have produced an excellent booklet on this: Yacon Syrup: Principles and Processing. This will tell you all you need to know (and maybe a bit more) about making yacon syrup on a large scale. Seeing as my attempts at beekeeping have always yielded less honey than the bees required in sugar to stay alive, a source of sweet syrup could be very useful.
The leaves have been touted as a blood sugar stabilising tea. I tried this, but found the flavour a bit too resinous and overpowering. I suppose you could cut your Darjeeling with a bit, then try it out on some unsuspecting tea heads.
For a plant so big and butch, the flowers are a bit of a disappointment. They are small yellow or orange sunflower jobs. I have never managed to produce any fertile seed from my plants, although the Japanese, who have taken to yacon in a big way, have bred new varieties such as one called 'Sarada Otome'.
I saw plenty of plants growing there in 2007, often in home gardens. The leaves were certainly different from the plants I am familiar with. With uncharacteristic restraint, and in deference to Nippo-British relations, I eschewed the delights of an al fresco yacon meal in any number of gardens as I cycled by.
Anyway, even if you don't eat the damn thing, you should certainly put it in your garden as a summer bedding feature. It's a big bold plant that looks just as good as a lot of the other stuff being promoted for tropical style gardening. Impress your friends by darting into the undergrowth and returning with a huge yacon root which you then proceed to peel and devour.