I first heard of yacon a couple of years ago. I was sat by the fire on a very cold winter's evening, beer, pencil, paper and the TV for company, scribbling the coming year's wishlist. The presenter of a gardening programme was visiting a nursery in southern England and being treated to what sounded like an unmissable taste: an underground "pear".
I rang the nursery next morning: engaged. I rang every five minutes for the next two hours before I finally got through. Sold out that morning, I was told. No matter. I immediately ordered for the following year, and have been growing this beguiling tuber ever since.
Fresh out of the ground yacon is very much like a baking potato to look at. However its flavour is a little strange for what you might expect from an underground tuber – it's like a sweet cross between early apples, watermelon and very mild celery, with a touch of pear. Mildly flavoured raw when first dug, it's the texture as much as the taste which sets yacon apart. The tubers have that fine texture of water chestnuts. They don't quite collapse as such – they've more resistance than that – but, like a very fine sorbet, they do sort of give in.
Yacon is also refreshingly juicy. "Yacon" means "water root" in the Inca language and its tubers were historically highly valued as a wild source of thirst-quenching refreshment for travellers. The liquid can also be drawn off and concentrated to produce yacon syrup. As with Jerusalem artichokes, yacon tubers are rich in an indigestible sugar – inulin – meaning that the syrup they form has all the sweetness of honey or other plant-derived sweeteners like maple syrup, but without the calories.
Yacon also benefits the bacteria in the intestinal tract and colon that boost the immune system and aid digestion. This potential as a dietary aid and as a source of sweetness for diabetics has led to yacon being grown more widely, especially in the USA.
Yacon tubers (Smallanthus sonchifolius) can be red, orange, yellow, pink and purple but most of the more colourful ones are found only in South America, where yacon originates. The rest of us are likely to find only white varieties.
Yacon is a perennial plant, so once you have planted it, so long as you look after it, you will have it forever.
Yacon is pleasingly easy to grow in most soils where there is reasonable rainfall and moderate heat. The plants do require a long season to grow – forming their tubers in autumn – but anywhere that parsnips and Jerusalem artichokes thrive will suit yacon perfectly well.
You can either buy plants or if you know someone who has them you can divide the crown including the smaller roots that grow above the main tubers.
Yacon can be slow to get growing in spring but quickly puts on lush, leafy growth through the summer to a height of 2m, occasionally a little more once established. It flowers some years towards autumn, but it's what's happening under the surface that's of most interest.
Nose below the surface in late autumn and you'll see that yacon produces two sets of roots – the large edible tubers that act as the energy storage facility for the plant, and the smaller propagation roots (resembling Jerusalem artichokes) which grow just under the soil surface and are the seeds for the following year's growth.
When you lift your yacon plants to harvest the tubers, cut the stems back to about 10cm long and store the crowns covered in damp compost in a cool frost-free place where they won't dry out.
In early spring plant the crowns into large pots and wait for shoots to start growing from each small tuber. Split the crowns into individual shoots with their tubers attached and plant into smaller pots.
Yacon plants are quite sensitive to temperature, so plant them out when you would tomatoes, a metre or slightly more from their neighbour, in a sheltered, sunny spot. Any compost you add to the planting hole and watering through dry periods will ensure good growth throughout the season.
Yacon is very rarely troubled by pests or diseases, but they are hungry plants so either add much compost and/or rotted manure between growing seasons or move their growing site altogether.
Yacon tubers develop into autumn, and as the frosts approach it's worth putting a little straw around the plant to protect the tubers. The leafy growth is withered by the cold – as soon as this happens, use a long fork to gently lift the tubers. It helps to have another person pulling on the stems of the plant at the same time to get the whole plant up.
Snap the large tubers from the crowns. They're crunchy, tasty and refreshing immediately, but a few days in the sun can add to their sweetness.
Yields can be variable – in the first year I had around six tubers the size of very large baking potatoes per plant, in the second year considerably more.
A cool, dry shed or garage is perfect for storing yacon tubers until you're ready to eat them. They may well sweeten a little over time, and (if you're lucky) they can last many months in storage.
Yacon has a crunchy texture, slightly reminiscent of water chestnuts, and a sweet flavour, so it's rather good simply peeled, sliced and eaten as a snack.
It's great in salads too, though its tendency to brown means that you should add it at the last minute, once everything else is assembled and ready to be dressed, or sprinkle with a little lemon juice to prevent it discolouring as it's peeled (and do peel it, the skin can be a little bitter).
Yacon also has a delightful tendency to absorb sauces and dressings, which make it a fantastic vehicle for other flavours. Try it grated with carrots in a mustardy vinaigrette with a handful of sunflower and pumpkin seeds, or in the traditional South American fruit salad, salpicón. Combine peeled, chopped yacon with chunks of pineapple, chopped papaya and mango and dress in freshly squeezed orange juice and a spritz of lemon.
You could also use yacon instead of apples in a Waldorf salad. Just peel and dice the yacon and toss it in lemon juice to stop it from going brown, then combine it in a bowl with chopped celery, some raisins and walnuts. Dress with mayonnaise thinned with a little sour cream and serve immediately on crisp lettuce leaves.
Yacon makes for a juicy, refreshing munch in the garden – just dug and brushed free of soil, or washed if you have water to hand. Otherwise get your tubers into the kitchen and try them with a little lemon juice and honey sprinkled over.
And don't waste an opportunity with the leaves – they make a delicious wrap, in much the same way as vine leaves or cabbage leaves do, for any number of fillings.
Yacon and green bean salad
Crunchy yacon can be grated into salads, julienned and tossed into stir fries, roasted with other root veg or steamed. It adds pleasing bite to this flavoursome salad – serve it as it is as a main course, or wrapped in little gem lettuce leaves as a starter.
Serves 4 (or 8–10 as a starter)
Juice of ½ lemon
1 medium-large yacon
300g green beans, topped (and tailed if you like, but the tails look rather nice)
15g sesame seeds
250g cooked chicken breast, torn into long shreds
Small handful of Vietnamese or regular coriander (Vietnamese coriander has a slightly hotter, more peppery flavour), stalks removed and roughly chopped
Small handful of mint, stalks removed and roughly chopped
For the dressing:
2 tbsp chunky peanut butter
1 tbsp Chinese rice vinegar
1 tbsp soy sauce
1 tsp sesame oil
1 small red chilli, halved, deseeded, membrane removed and finely diced
Juice of 1 small lime
Little gem lettuce leaves (optional)
Fill a bowl with water and add the lemon juice. Peel the yacon and cut it into batons, about 5mm x 5mm x 50mm, tossing them into the lemony water as you go to stop them from turning brown. Bring a pan of salted water to the boil, add the beans and boil until they just lose some of their crunch, about 2 minutes. Drain and refresh under the cold tap. Pat dry with kitchen paper.
In a small frying pan, warm the sesame seeds over a medium heat until they just begin to turn golden. Tip them onto a plate to stop them cooking any further.
In a small bowl, whisk together all of the ingredients for the dressing until smooth.
Drain the yacon and combine in a large bowl with the green beans, chicken, sesame seeds, coriander and mint (save some of the herbs and seeds for garnishing the salad). Pour over half of the dressing and toss the salad with your hands until everything is well combined. Serve on a platter, trickled with the rest of the dressing and the remaining seeds and herbs or wrap in little gem leaves and serve as a starter.
Yacon and blue cheese salad
It's not easy to improve upon the famously fabulous combination of walnuts and blue cheese but the addition of yacon, with its succulent sweet crunch, really lightens and freshens this deliciously different lunch.
Serves 4 as a starter
Small handful of shelled walnuts or pecans
Juice of 1 lemon
1 medium-large yacon
Handful of salad leaves
180g blue cheese, such as Dorset blue vinney, roquefort or gorgonzola
For the dressing:
1 tbsp apple balsamic vinegar
Pinch of flaky sea salt
3 tbsp extra-virgin olive oil
Preheat the oven to 180C/Gas 4.
Spread the walnuts or pecans out onto a baking tray and toast in the oven for 8–10 minutes, shaking halfway through, until lightly coloured – keep an eye on them to ensure they don't burn.
Fill a bowl with water and add the lemon juice. Peel the yacon, cut into slices and toss into the lemony water to prevent them from discolouring.
In a small bowl, whisk together the vinegar with the salt before adding the olive oil a little at a time, whisking all the while until smooth. In a bowl, lightly dress the salad leaves in a little of the dressing and divide between 4 plates.
Arrange the sliced yacon on top, crumble over the blue cheese, then trickle over the rest of the dressing. Scatter the nuts over the top and serve immediately.
• This extract is taken from A Taste of the Unexpected by Mark Diacono (Quadrille, £20)