Oca (New Zealand Yam)

Plant tubers once frost danger has passed

Plant whole tubers a few inches deep as soon as your last frost has passed, setting plants 40 to 60 cm apart in rows 80 to 100 cm apart. Give them full sun and well-drained soil, and expect sprawling growth that fills that spacing by late summer. Oca is a short-day plant that runs a seven to eight month cycle and puts on nothing but leaves and stem until the days shorten. Every week of early growth is stored capacity for the tuber bulking that only starts in autumn, so a late start costs yield you cannot make up later.

Save your own seed tubers, and renew the stock when yields slide

Hold back the soundest tubers from the best plants at harvest to replant the following spring, storing them cool, dark and frost-free. If yields fall off over several years with no change in your growing, buy fresh certified or tissue-cultured stock rather than replanting your own again. Oca is propagated almost exclusively from tubers, so the health of your planting stock carries forward with no seed generation to reset it. At least one virus is known in oca and others are suspected of damaging the crop, and virus-free material is reported to give higher yields — a tired clone will not recover from feeding or watering.

Watch for vine weevil, the pest most likely to trouble oca

Through the growing season, check for notched leaf margins on the foliage and for grubs in the root zone and in the tubers of any plant you lift, paying closest attention to container-grown plants. Oca is otherwise generally untroubled by pests and diseases and needs no spray program. Oca may be susceptible to vine weevil, whose adults notch the foliage and whose larvae tunnel roots and tubers where you will not see them until harvest. Beyond this, Andean growers report that few pests or diseases bother the crop, so scouting is the whole program.

Cover the foliage against the first autumn frosts

As the first frosts arrive, throw a frost blanket or low tunnel over the plants on cold nights and pull it back on mild days. Keep the tops alive as many weeks as you can before you give up on them. Oca tolerates cool weather but does not survive being frozen, so a single hard night ends the crop. Because the tubers only bulk during the short days of late autumn, the foliage you keep alive through those weeks is the crop, and plants killed at the first light frost yield almost nothing.

Dig only after the tops are killed, never early

Leave the plants undisturbed until frost blackens the tops, then dig the whole root zone carefully and wide, as the tubers sit on stolons well out from the crown. Digging a plant to check on it in summer will show you nothing but roots. Oca is a short-day plant that forms no tubers under long photoperiods, and only begins bulking once the days shorten past the autumn equinox — then needs several more weeks. The crop is made in the last stretch of the season, so an early harvest is not an early crop, it is no crop.

⚠ Oca tubers contain oxalic acid, reported between 1.2 and 51.3 mg per 100 g fresh weight; even at the higher levels this poses no threat to human health at normal eating quantities.

Sun-cure the dug tubers to sweeten them

Spread the washed tubers in a single layer in full sun for several days before eating or storing them, bringing them in if hard freezes threaten. Taste one before and after, the difference is the whole point. Freshly dug oca is sharp and lemony from its acid content. Sun exposure converts that acidity toward sweetness, which is why Andean growers sun-dry the crop before roasting or boiling it, and it also deepens the tuber colors.