Oca (a relative of the weedy Oxalis) is rather easy to grow, and relatively free from pests and diseases*. It doesn’t like frost, and starts tuberising when the days get shorter in September, so their main requirement is protection, at both ends of the growing season: when fronds start to grow in the spring and while decent sized tubers are formed in the autumn.
You can plant (even rather shrivelled tubers, or just the sprouts coming out of them, removed and used as cuttings) in pots in the spring, on a windowsill or in a greenhouse, covering for good measure. For example, if you use 9 cm pots of peat-free compost, planting just under the surface, a clear recycled yogurt pot will fit perfectly on top, keeping the compost surface moist, and mice from chomping on the tubers. Soon, pretty trifoliate leaves emerge.
When the risk of frost is over, oca can be planted out in well cultivated soil, rich in organic matter. Plants bush up, so leave at least 30 cm between them for air to flow, and to provide enough room for the tubers to grow.
Growing and flowering
Oca does not require much maintenance, but it’s best kept watered in very dry spells.
Also, keep an eye out for the flowers, which are rather pretty! If your oca does not flower (commercial varieties are less likely to), you are still in for a crop of tubers, so no worries. But if you get the flowers, and your different plants come with different stylar morphs**, you may even get seeds and breed your own new varieties!
You can tell your oca is making seeds if the flower calyxes don’t drop when poked, after the flower has died away. Oca seed pods are rather explosive, so, if you want to collect seeds, it’s best to envelope them – literally – in glassine. Otherwise, look out for seedlings on a wet day next year: they have round leaves and a reddish tinge on the underside of the cotyledons.
Anyway, back to tubers. They won’t start to form until the days get shorter in the autumn, so, once the risk of frost looms again (depending on your region and local climate), cover your oca in fleece (bracken and bubble wrap can also be used) and try to keep the foliage alive as long as possible, so the tubers grow bigger. You may find some aerial tubers form too, on the stems, out of the ground. Nothing to be concerned about: you can use them for planting again next year, if they survive in storage.
Once a hard frost kills your foliage, dig out your tubers. If left long in the ground, they are likely to be too much of a temptation for slugs and rodents.
Curing and storing
Oca (like rhubarb) contains oxalic acid, which makes it acidic and which, in big quantities, is poisonous. To make your oca sweeter and reduce the oxalic acid content, keep the tubers in the light (mind the frost!) for a week or so. Then store, like potatoes, in paper bags, and enjoy!
Like potatoes, you will find oca chits readily in storage, which usually is a paper bag (in a frost-free place). But chitting does not affect the edibility of the tuber, and it is not necessary to growth.
Eating your oca
We ask our members to boil oca for tasting purposes, but here are some more elaborate cooking ideas by Carl Legge
More about growing oca:
Based in the US
* although voles and mice do like to chomp on them, and slugs do tunnel through, given the opportunity
** oca flowers come with three types of arrangement of the sexual organs, stigmas and anthers. The length of the style, which carries the stigma, can be short, medium and long. The anthers are arranged on the remaining two layers (medium and long, short and long, short and medium). Here’s a exemplified drawing. Fertilisation only happens with pollen from flowers of different morphs.