Oca seed tubers

For my Masters dissertation last year I did some research into gardeners who choose to grow unusual edible crops. I settled on two species to investigate, achocha and oca. In the past I’ve written about how to grow achocha – it’s a nice, easy plant and in a temperate climate you should have no problems getting a significant yield. You may have more of a problem dealing with the glut….

Oca (Oxalis tuberosa ) isn’t quite as simple. It’s increasingly easy to get hold of tubers in the UK, with them being marketed as very similar to potatoes – but without the risk of your plants being devastated by blight. But my research showed that many of the people who do try oca find their yields to be very disappointing. So how do you grow oca for the best results?

In its homeland, the Andes, oca is known only as a cultivated plant; it’s closest wild relative is still a bit of a mystery. There are lots of different varieties planted there; they come in different colours, and they’re used for different purposes*. Some have been bred to be eaten raw, some are best when cooked and still others are bitter and have to be processed into a dried starch product before they can be consumed. Some varieties are used more like a fruit than a vegetable. Oca is quite popular in New Zealand, where it’s known as the ‘New Zealand Yam’. Here in the UK you’re likely to have a choice of perhaps three or four varieties, sold by their colour. Who knows what they might have been used for traditionally? We’re not off to a flying start.

The problem is that this species is sensitive to day length. In the UK they only begin to produce tubers when the days begin to get shorter, and they’re frost sensitive. Selecting varieties that might crop here at all is a problem, although there are some amateur breeding efforts underway (and I’m looking at The Guild of Oca Breeders when I say that) to develop a day neutral variety that will find life here easier.

Oca tubers chitting

So, given that this isn’t currently a plant that’s ideally suited to life in the UK, how do you get the best out of it? Essentially, it is grown in the same way as potatoes. You can store tubers in breathable bags until it’s time to plant them out in spring, and if they start to sprout too soon you can leave them on the windowsill to chit in the light. When the risk of frost has passed, plant them out about 30 cm apart, under the soil like potatoes.

Since the tubers form in a similar way to those on potatoes, it’s thought that earthing them up (piling extra soil on top of the plant once the foliage starts breaking through) will help to increase yields. However, you don’t have to worry about sunlight causing inedible green patches on the tubers – they don’t have the toxins you’d find in green potatoes.

You can also eat oca leaves; they’ll have a similar flavour to sorrel, due to the presence of oxalic acid. Removing too many will affect your tuber yield; you might want to set aside some plants for leaf production and some for tuber production, if you want good harvests of both.

In the event of a heat wave, or a drought, you will find your oca suffer. They don’t like hot weather, and will sulk. Provide some shade, and plenty of water, to keep them happy. If blight strikes down your potatoes, the oca will be just fine. Traditional cultivation systems intercrop oca with other plants. Other tubers are a common choice, including mashua, ulluco and even potatoes. Sweetcorn is another option (and would provide some shade). In the Andes, oca is often grown with alliums to ward off pests, but they’re not bothered by much in the UK. They should thrive in a cottage garden style arrangement, mixed in with both ornamentals and herbs.

Research has shown that there’s a positive correlation between leaf size and tuber harvest, so it’s worth feeding your oca to promote leafy growth (or ensuring they are planted in hearty soil). Feeding will also affect their nutritional value, mainly their protein content. Oca tubers are known to contain twice as much vitamin C and calcium as potatoes, with a similar carbohydrate content. Oxalic acid levels are lower in the tubers than in the leaves, and the highest levels found in tubers are 7 times lower than the lowest levels found in spinach – so they won’t cause any problems at all in a varied diet.

Frosty oca

The difference between oca and potatoes becomes apparent at harvest time. Whereas you can dig some varieties of potato early in the season, and maincrops from late summer into early autumn, oca tubers don’t start to form until the nights get longer. And so you wait until the first frosts have killed off the foliage. And then you wait another couple of weeks, and then you dig up your harvest. If early frosts are forecast you can protect your plants with fleece. If you grow them in containers (and, like potatoes, they are reasonably happy in containers if well-fed) you can move them indoors.

Oca container

Eat the best tubers from your harvest and keep the smaller ones for replanting. Some of the varieties in the Andes are left out in the sun for a few days after harvesting to make them sweeter. Will that work in the UK? We don’t know – there’s rarely enough sun at that time of year, and we don’t know which varieties we have 😉 You could try it. Oca can be used in all the same ways as potatoes, and also eaten raw. Their oxalic acid content gives them a slightly sharp flavour, often referred to as ‘lemony’. Carl Legge has developed an exclusive oca recipe for us, to promote his new book (The Permaculture Kitchen), and that’s coming up in the next few days, so keep your eyes peeled.

Oca flowers

Breeding efforts rely on the production of seed, not tubers. Oca isn’t known for flowering and setting seed easily – in fact, you will need two or more compatible varieties for pollination to take place. If you do get seed then collecting it can be tricky – the pods tend to shatter and scatter seed on the ground. However, seedlings will grow from self-sown seed, and can be transplanted. If you collect seed then germination takes around 19 days. You can also root stems in water as another means of propagation. If you develop a variety you like, you can of course then propagate it via tubers from year to year.

So… growing oca is exactly like growing potatoes. Except, it’s not really. But they’re pretty plants, with lovely tubers and who doesn’t like trying new things? If you need oca tubers, the two main suppliers in the UK are Real Seeds (with several different varieties) and T&M. You may also find them via smaller suppliers, local seed swaps or your internet gardening pals. And if you’re interested in unusual edibles in general, have a look at my forthcoming book, Jade Pearls and Alien Eyeballs, which will be right up your street.

If you’ve grown oca, and got a good crop, what’s your top tip for growing them?

*That’s also true of potatoes, by the way. The Andes are a very interesting place if you’re a fan of tubers and unusual root crops.

King, S. R. (1988). Economic botany of the Andean tuber crop complex Lepidium meyenii, Oxalis tuberosa, Tropaeolum tuberosum and Ullucus tuberosus (Doctoral dissertation, City University of New York).