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by Steve Peters
Oxalis tuberosa Oxalidaceae (Wood Sorrel) Family
In its native Ecuador, Peru, and Bolivia, Oca is the second most widely consumed root crop behind the potato, but this bright and colorful fingerling-shaped tuber is almost unknown to most North Americans even though it holds the potential to be a significant food crop throughout the world. Oca is easy to propagate, tolerates a wide range of soil and climatic conditions, and is delicious and nutritious. In New Zealand it has been grown since the 1860’s and is known widely as the New Zealand yam. In Mexico, where it has been propagated for nearly two centuries, the tubers are eaten raw and spiced with salt, lemon, and hot pepper.
Diverse and nutritious
There are probably hundreds of distinct varieties of Oca, displaying a multitude of hues ranging from yellow, orange, red, pink, and white, although only a few are available outside its native region. The roots are crisp and moist with a pleasant sour flavor when eaten raw, because of a fairly high concentration of oxalic acid. Tubers exposed to sunlight for several days have increased glucose content and become sweeter. Steaming lightly brings out a distinctive tart taste balanced with raw nutty overtones. Oca tubers can also be baked, fried, or boiled. Traditionally the tubers are often dried, extending storage life and making a convenient, healthful snack food.
With a similar nutritional value to potatoes, Oca is recognized as a staple survival food. The tubers contain 70-80% moisture, 11–22% carbohydrates, 1% each of fiber, fat, and ash, and a significant amount of calcium and iron. The green tops of the bushy, compact, 1–2 foot high plants have clover-shaped leaves that are also edible. Their tangy, tart flavor makes them a great addition to salads.
Easy to grow, but needs long fall days for tuber production
Oca prefers full sun in a nutrient-rich, sandy loam soil, although it will tolerate low fertility and a wide pH range from 5.3 to 7.8. It grows best under moderately cool conditions and will survive very mild frosts. When temperatures exceed 80º F, however, the plants begin to show signs of heat stress such as wilted leaves. Therefore in warmer climates it is best to grow plants under partial shade.
The greatest limitation for widespread use of Oca is its inability to produce full-sized tubers without certain climatic and day-length requirements. Most varieties do not begin to form tubers until the day-length has shortened to 12 hours (around mid-September in North America). Two to three additional frost-free months are needed for full tuber development. In most of California Oca will survive as a perennial, since winter temperatures rarely go below freezing. The milder regions of the Pacific Northwest also are optimal. Oca is typically grown there as an annual, and can sometimes even survive under winter soils to re-sprout again in the spring. In the central and northeastern U.S, however, nearly frost-free protection is required during the fall, because hard frosts will kill plants long before good-sized tubers are formed. Furthermore, Oca is not even recommended for the hot, humid regions of the southeastern U.S. because the tubers become susceptible to rotting under these conditions. Future adoption of Oca in many temperate regions of the world will require day-neutral or long-day adapted varieties to overcome the day-length problem.
If you can grow a potato…
Oca is propagated much like potatoes. Plant each seed tuber about 12 inches apart and 2 to 3 inches deep in the early to mid-spring after the possibility of hard frosts has passed. For an earlier start in more frost-prone regions, start indoors and transplant out after the average last frost. Oca can also be grown to maturity in 5-gallon containers. Adequate moisture after mid-September (the time of tuber initiation) increases tuber size significantly, and a later harvest will ensure larger tubers. Mounding soil around each plant will also help produce higher yields. Plants will tolerate light frosts, but if temperatures drop below 28° F for more than a few hours the leaves will die back completely. This is the time to harvest your crop. Average yields per plant are approximately 2–3 pounds under good conditions. If tubers are harvested carefully to avoid bruising or cutting, they will store for several months in a cool (35–45º F), dark place. Seeds of Change offers a few distinct varieties which will each have slightly different flavor and texture. Some varieties will form tubers earlier than mid-September, and some will be more productive. Keep track of which varieties do best for you and try saving some of these tubers for subsequent plantings.
We are pleased to offer this exciting new crop for 2009 and realize that there is still much to be learned about its cultivation and adaptability. Please share your experiences with us so we can share the information with other growers.
Steve Peters
Seeds of Change Product Development Manager
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