A bucket full of Oca tubers

Yesterday I lifted my crop of Oca tubers from the allotment: a good harvest of tubers, nearly a bucket full, weighing a stone (14lb or 6.35kg), from twelve plants.

I started off with just five tubers in spring 2014. I was sent these because I was taking part in a Garden Organic Members’ Experiment, a trial of growing Oca in this country. This time last year I harvested about 1lb (2.1kg) of tubers and I ate some of these and saved the rest to start off this year’s plants. I plan to do the same thing with this year’s crop and gradually increase the amount of Oca I grow.

Oca is Oxalis tuberosa, sometimes known as the New Zealand Yam. (It was introduced into New Zealand in the 1860s.) It is a member of the Wood Sorrel family, the Oxalidaceae. It comes from the central and southern Andes, from Venezuela south to Argentina, where it is grown for its root tubers. In Peru and Bolivia, where it thought the plant was domesticated, it is the second most widely grown root crop after the potato. Its development as a food crop may predate the Inca empire.

Oca is a perennial plant with leaves like a bigger version of its relative Wood Sorrel, Oxalis acetosella, and knobbly root tubers which come in a variety of colours: white, cream, yellow, purple, pink, orange and red, in a varety of patterns and combinations. The tubers vary in the amounts of oxalic acid they contain, and some varieties need to be soaked and/or left out in sunlight before they are edible. My Oca tubers are pink and have a slight acid tang which fades when they are cooked.

Oca can be grown in the UK, where it prefers well drained or loamy soils. The plant is adapted to the short day length of the tropics and needs a day length of twelve hours or less to form its tubers. This means that tubers only start to form in September and a couple of months need to elapse before there is a crop worth harvesting. The first sharp frost will kill off the leaves and then the tubers will continue to grow for a couple of weeks afterwards. I accidentally left a few small tubers in the ground last year and each one produced a respectably sized plant the next year, so it should be possible to keep tubers in the ground over winter, at least in sandy soils. You can cover the plants with fleece or even their own frosted foliage, to protect the tubers.

I start my tubers off in small pots in peat free compost and then plant them out about a foot (30cm) apart in early June after the first frosts. Growth is slow to start with and until August the plants form discrete, round clumps. Later on they start to spread out across the soil. Some varieties have pretty yellow flowers but my variety hasn’t flowered. If it is very dry in August and September, it is a good idea to water the plants.

Oca (left), early August 2015

As tuber growth doesn’t start until late summer it should be possible to plant Oca as late as early August, perhaps after harvesting an earlier crop such as garlic, or interplanting with other crops. The Down The Plot and Growing Oca websites have some good suggestions for timing when to plant. The Growing Oca website also has some tips for increasing the yield. Thompson and Morgan suggest that yields of 1.6lb (0.75kg) per plant may be possible in a good year in the UK.

Unlike potatoes, I don’t bother earthing up my Oca plants. The tubers are shaped rather like “Pink Fir Apple” potatoes but they are not susceptible to potato blight. The leaves are good at suppressing weeds as well. Although the tubers are smaller than potatoes, they have a waxy skin and are easy to clean and I never bother to peel them.

I try to lift my entire crop of Oca and store the tubers in a frost free shed. Potato tubers go green in light and produce the poisonous glycoalkaloid solanine but Oca tubers can be stored in the light. They will start to produce sprouts in the spring, but more slowly than potatoes. They will gradually dry out during the winter, but only very slowly in a cool place.

Oca plant, lifted, November 2015

Update 6th December 2015: This year, as an experiment, a Canadian friend harvested her Oca plants in mid-November. Rather than immediately putting the tops on the compost heap, she left the smaller tubers attached and hung them up in her garage for three weeks. At the end of this time even the tiniest of tubers had grown in size as they absorbed nutrients from the dying foliage. If you have space, especially if growth is stopped by an early frost, this is well worth a try.

Cooking and Eating

Oca tubers have an excellent flavour. They can be eaten raw, when they have a zingy, slightly acid taste. (Leaving them in sunlight for a few days will make them sweeter.) They can be cut into small pieces and boiled for five to ten minutes, parboiled and mashed or roasted in the oven with olive or sunflower oil. The acidity fades when tubers are cooked: boiled tubers taste like waxy potatoes with a hint of lemon and roasted tubers are like a flavoursome roast potato. Oca is good roasted with other vegetables.

I can recommend Oca Salad with Capers and Cornichons and Warm Oca Salad but I haven’t tried Oca Homity Pie yet. Last night I cooked Hugh Fearnley-Whittingstall’s vegeree recipe and substituted the courgettes (now out of season) for Oca. Coated with spices, the tubers combined well with roasted aubergine and red onion.

Oca tubers contain significant amounts of Vitamin C, zinc and vitamin B12 and some varieties are also high in iron. They are a low calorie food too – 100 grammes of potatoes contain about 75kcal but the equivalent weight of Oca contains around 30kcal. Yet more good news is that although they resemble small Jerusalem artichokes, Oca’s storage carbohydrate is starch rather than the inulin found in Jerusalem artichokes, so Oca does not have the same gaseous side effects when eaten.

The leaves and young shoots can be eaten as a tangy green vegetable, though not too frequently because of the oxalic acid they contain. The flowers are also edible and look very pretty in salads. Mature stems can be used like rhubarb.

Further Reading and Growing

If you want to read more, I suggest you look at the Oca Links page of the Growing Oca website. You may also be interested in the Guild of Oca Breeders (GOB), who are trying to breed day neutral varieties of Oca, which will produce tubers much earlier in the year. The first potatoes were late cropping short day plants but intensive breeding has produced potato varieties that crop as early as mid summer – it should be possible to do the same with Oca.

If you live in the United States or Canada, the Cultivariable website offers several varieties of Oca.In the UK, The Real Seed Catalogue sometimes sells Oca, as well as other unusual tubers such as Yacon. Thompson and Morgan sell Oca tubers that look like the variety I’m growing. Although tubers can be expensive, you should only need to buy them once if you grow your own.

The Blooms ‘n’ Food website has some great photos of the different stages of growing Oca.

Oca – Not Okra!

When I say the name people sometimes think I mean Okra, or Ladies’ Fingers, whose slimy seed pods are used in gumbo and many other dishes. This is a very different plant to Oca. Okra, Abelmoschus esculentus, is a member of the Mallow family, the Malvaceae. It is related to Hollyhocks, which I wrote about in my previous blog post.