How to Grow Oca, the New Zealand Yam

How to grow oca, a low-fuss root vegetable from South America. This lost crop of the Incas grows edible leaves and up to fifty tubers each. Get a head start growing what I think will become a kitchen garden staple in years to come.

The first time I’d heard of oca (Oxalis tuberosa) was five years ago when they were still relatively new to Britain. Also called New Zealand yams, oca is a South American root vegetable that is grown as a major crop in the Andes. It’s even said to pre-date the Incan civilization. They come in various colors, are around one to four inches long, and can taste both sweet, citrusy, and savory at the same time. I was interested because they were said to be easy to grow, suffer few diseases, and are big producers. They were supposed to be a little like potatoes, and having had past crops affected by blight made my ears perk up.

At the time, none of the major seed companies offered oca tubers, and the one place I knew I could get them was sold-out. Desperate to grow them, I took a chance and bought a few off eBay. They arrived small and shriveled and pretty pathetic – I didn’t expect much from them. These days, it’s relatively easier to get oca tubers (New Zealand yams) from certain nurseries and seed companies.

Oca has a unique and zingy flavor

That first year I grew oca tubers in the allotment. The plants grew without fuss, and throughout spring and summer they shot up fleshy stems covered in clover-like leaves. Coincidentally, oca isn’t related to potatoes. It’s in the oxalis family, so its leaves look and taste like wood sorrel. Tender, green, and with an oxalic acid tang. You can add them to salads and stir-fries.

The flavor of the tubers is unique and can be different depending on type, time of harvest, and cooking method. You eat oca skin and all, and raw or cooked. When raw, they’re crunchy and many have a distinctive lemony kick. Cooked, they’re starchier and have a similar texture to cooked turnip or kohlrabi. The flavor gets sweeter, though, and if you boil or steam them, the tang disappears. The best way to cook them, in my opinion, is to wash them off and roast them in olive oil. They’re delicious served up with other roasted veggies.

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How to Grow Oca

At the end of that first year growing oca, the foliage died back, and I dug up the tubers. I could laugh at how disappointing the harvest was — a couple of decent-sized ones but mainly tiny nuggets of nothingness. I tried again the following year and at harvest still had barely enough tubers to keep for replanting. The next year I planted them in the greenhouse, and they seemed to do a little better. Finally, after years of trying, I gave up on them. I gave away the last of my tubers at last year’s seed swap. I hated doing it but had to admit defeat.

A few months later, I was browsing the garden center and saw something I’d never seen there before. Finally, in 2019, oca had made it to the mainstream. I succumbed to whim and tucked them into my basket to try growing again.

I took those few oca tubers home, potted them up in the greenhouse, and eventually planted them into one of my newly constructed raised beds. Last week, and after eight months in the ground, I dug up the best harvest I’ve had in six years of growing them. Aside from buying tubers from a reputable source, I think I’ve learned a few things about how to grow oca along the way. I hope the tips below will help you successfully grow them too!

Oca Growing Guide

• Suitable for zones 7-9, not frost-hardy
• Prefers well-drained soil
• Full sun in a sheltered spot
• Plant three feet apart
• 6-8 months from planting to harvest
• Leaves and tubers are edible
• No known diseases in Britain.
• Tubers can be a target for rodents

How to Grow the New Zealand Yam

Oca plants produce oca tubers at the end of their life cycle, and you can both eat and replant them. I’ve grown both bright magenta colored tubers and white but prefer the look of the red. The white, yellow, apricot, and pink-skinned types are said to be sweeter though I’ve not noticed a big difference myself. The foliage of all types looks the same – bushy masses of fleshy stems adorned with trifoliate leaves similar to clover or wood sorrel. You can harvest small bunches of these during the growing period, but don’t take too much from any one plant.

Order oca tubers from specialty seed companies or garden centers. One source that I recommend in the UK is the Real Seed Catalogue. Otter Farm stock them as well as a few other places. In the USA, you can get oca tubers from Cultivariable.

Oca Growing Conditions

Oca traditionally grows at high elevations in the equitorial zone. However, the varieties I’ve grown have been acclimatized to the British climate and longer daylight hours. It’s likely that many of the traditional varieties still grown in the Andes wouldn’t produce tubers here at all. They would also be infected with a lot of diseases that our British stock are free from too. So beware and buy locally grown tubers.

New Zealand yams prefer growing in temperate climates that have a shift in daylight lengths from the autumn equinox to winter. They sense when the daylight hours shorten and that prompts them to begin forming tubers. That means that outside of their native regions, they grow best in northern Europe, Britain, New Zealand, and the Pacific Northwest of the USA and Canada. If you’ve successfully grown them in other places, please let us know as a comment below.

In my experience, oca hasn’t been a fan of the moist, clay soil in my allotment garden and prefers well-drained above all. Oca tolerates poor soil, and a friend who threw a couple of tubers in the dry corner of her polytunnel has confirmed this. A scoop of compost as mulch will probably give you a better producing plant though. If you’re growing them in containers, and they do grow well that way, plant them in equal parts of soil and compost. I’ve not had huge harvests in containers though, compared to growing them in the ground.

How to Plant New Zealand Yams

Although some growing instructions direct you to chit them – leaving them out to sprout roots before planting – this is entirely unnecessary. You can either plant tubers direct in the soil or singly in six-inch pots once it’s warmed up in spring. Early to mid-April here in the northern hemisphere.

Once all danger of frost is past, plant those grown in pots in the garden, spacing them about three feet apart in all directions. Planting them closer can reduce their yield but I do it anyway to save space. I tend to plant them about two feet apart.

After the New Zealand yam plants are in their final positions, keep them well watered and give them plenty of warm sunshine. That’s all they need to grow lush and green. Each plant will grow into a small bush about two feet tall and up to three feet in diameter. You don’t need to earth the plants up at all, but they do love a good mulch. It’s full of nutrients and keeps the soil moist.

Oca tubers form in the autumn

One big mistake I’ve made with oca is planting them in exposed positions. The foliage will grow in windy places but not to the size you’ll get if they’re in a more sheltered spot. I think the size of the foliage in autumn is essential too. Oca only begins growing tubers once we approach the autumn equinox, and the days begin to shorten. The foliage helps pump nutrients into the forming tubers, and in cases where my plants have been small, the tubers are small too. It’s an observation rather than scientific study, but that’s my two cents.

When to Harvest Oca

Harvesting oca tubers happens once the foliage has been killed off by hard frosts. The tubers stop growing then, and you have a small window of time to harvest them if you’re expecting truly cold weather. Though the tubers are hardy, and can survive in the ground over winter, they do grow close to the surface of the soil. If the ground freezes, you could lose your crop.

Here on the Isle of Man, we can have few hard frosts so it’s a little bit of a guessing game as to when to dig them up. The absolute earliest time you can dig up your oca harvests is eight weeks after the autumn equinox. It’s better to wait ten to twelve weeks though, so for us that means mid-December.

Wait as long as you can, but if you see little tubers forming on the stems feel free to pick them off. They’ll be damaged by frosts anyway, and you’ll get a little taste before the real harvest. You’ll also see tiny tubers forming in areas where the stem touches the ground when you are ready to dig your harvest. If you have a mild winter, these tubers and any left in the ground can survive and regrow again next year.

How to Store Oca

To dig up the tubers, gently lift the plant with a fork. The tubers grow close to the surface so you shouldn’t need to dig too much. Expect to get up to fifty tubers in sizes ranging from marbles to pinecones in optimal conditions. The tubers cling to the plant on long white roots, and most will come out that way. Smaller ones might break off, and it’s worth rummaging around in the soil for them. They’re like potatoes in that way and each little tuber may grow into a new plant next year.

Clean them while they’re still damp from the soil. A swish in a bucket of water or a spray of the hose cleans them up easily at this point. After they’re cleaned, store in a bright place, preferably in the sun for a couple of weeks. This helps the flavor to sweeten. You can then store them in a cool but frost-free place for many months. A cupboard, counter, or fridge work equally well. I wouldn’t recommend leaving them in a shed or greenhouse though, since rodents will have away with them.

Lastly, don’t forget to save some of the best oca tubers to grow again next year, repeating these same growing instructions. Though the tubers are sometimes expensive to purchase, once you have them growing, you should never need to buy them again. Unlike me!