Origin. Oca may be one of the oldest Andean crops. Tubers have been found in early tombs on the coast, hundreds of kilometers from its native highland habitat.13 Although wild relatives exist throughout much of South America, the ancestral plant is unknown.
Description. Oca is a compact, perennial, tuberous herb, usually 20–30 cm high, with cylindrical, succulent stems that vary in color from yellow and green to a purplish red. The stems normally rise from the base of the plant. Oca has an efficient plant “architecture” for photosynthesis because of its extremely high leaf area (due to its growth form and leaf angle, shape, and thickness).
Under long days, the stolons grow as above-ground stems; under short days, they penetrate the soil and form tubers. As days shorten, the stolons swell into rhizomic tubers that generally range in length from 3–20 cm and are produced in abundance. As in the potato, tiny scale leaves border the deep-set eyes.
Horticultural Varieties. The Andean Indians recognize about a dozen cultivars and more than 50 distinguishable types. The Colección de Ocas—over 400 accessions—is housed at Cuzco, Peru. There are also major collections at Puno and Huancayo, Peru, and Quito, Ecuador.
Daylength. The common Andean types generally require days shorter than 12 hours to initiate tuber formation; in most cases longer days promote only foliage development.
Rainfall. In the Andes, the crop is grown where annual rainfall is 570–2,150 mm, distributed evenly throughout the growing season.
Altitude. Oca grows near sea level in New Zealand, but in the Andes of Peru, Bolivia, and Ecuador, it is found at 2,800–4,000 m elevation.
Low Temperature. Although oca is resistant to low temperatures and thrives in moderately cool climates, freezing kills back its foliage. However, the plant's tubers have exceptional regenerative capacity.
High Temperature. Temperatures above about 28°C cause the plant to wilt and its leaves to die; resprouting can occur, but tuber production is consequently delayed.
Soil Type. Oca seems indifferent as to soil and is reported to tolerate acidities between about pH 5.3 and 7.8.14 Not unexpectedly, a light, rich soil is best for tuber production.