1.2- Species Oxalis tuberosa – New Zealand Yam + Overview Oxalis tuberosa(Oxalidaceae) is a perennialherbaceous plant that overwinters as underground stem tubers. These tubers are known as uqa in Quechua, hispanicized oca, as New Zealand yam and a number of other alternative names. The plant was brought into cultivation in the central and southern Andes for its tubers, which are used as a root vegetable. The plant is not known in the wild, but populations of wild Oxalis species that bear smaller tubers are known from four areas of the central Andean region. Oca is one of the important staple crops of the Andean highlands, due to its easy propagation, and tolerance for poor soil, high altitude and harsh climates. Oca was introduced to Europe in 1830 as a competitor to the potato, and to New Zealand as early as 1860. In New Zealand, oca has become a popular table vegetable and is simply called yam or New Zealand yam (although not a true yam). It is now available in a range of colours, including yellow, orange, pink, apricot, and the traditional red. + Local cultivar names Oca-growing communities often name varieties based primarily on tuber morphology and secondarily on flavor. For example, common names may include ushpa negra (black ash) or puka panti (red Cosmospeucedanifolius). Great inconsistency of nomenclature has been reported within and among communities.
2- Characteristics of the Species Oxalis tuberosa - New Zealand Yam or Oca
2.1- Description + The plant Oxalis tuberosa is a perennial plant growing by 0.3 m (1ft) to 0.5 m (1ft 8in). The plants are clover-like on top and the roots are like little fat fingers 40-200mm long. Reddish/pink is the usual colour, but they also come inyellow, white, or purple. They adapt well to poor growing conditions and do well in silty loams. + The leaves The Oca’s leaf is cloverlike. It is edible and can be make great additions to salads. The leaves contain oxalic acid, which gives them their sharp flavour. Perfectly all right in small quantities, the leaves should not be eaten in large amounts since oxalic acid can bind up the body's supply of calcium leading to nutritional deficiency. The quantity of oxalic acid will be reduced if the leaves are cooked. + The tubers The oca tubers, in particular, is astounding. Tubers range from 25 to 150 mm in length by 25 mm in width;skin and flesh color may be white, cream, yellow, orange, pink, red, and/or purple and distributed in range of patterns. + The flowers The flowers are hermaphrodite (have both male and female organs) and are pollinated by Insects (e.g., generaApis,Megachile, andBombus). The Oca’s flowers are trumpet-shaped, edible and can be make great additions to salads.
2.2- Origin and Distribution + Origin Oca is native to Bolivia and Peru where It appears to descend from two wild ancestors in Bolivia, but has undergone so much genetic alteration due to human intervention and continued selection over the past 1,000 years that the chemistry of the cultivated plants is quite distinct from the wild forms. + Distribution Oca is planted in the Andean region from Venezuela to Argentina, from 2800 to 4100 meters above sea level. Its highest abundance and greatest diversity are in central Peru and northern Bolivia, the probable area of its domestication. In New Zealand, where oca has been cultivated as a commercial crop under the name “New Zealand Yam” (although not a true yam) since the 1860s, there are quite a few named varieties, and there is a continuing program to develop better ones. It is these New Zealand varieties that we are beginning to see in U.S. produce markets because they adapt most readily to our growing conditions.
2.3- Biological characteristics + Climate requirements Oca needs a long growing season, and is day length dependent, forming tubers when the day length shortens in autumn (around March in the Andes). In addition, oca requires climates with average temperatures of approximately 10 to 12 °C (ranging between 4 and 17 °C) and average precipitation of 700 to 885 millimeters per year. Oca requires short days in order to form tubers. Outside the tropics, it will not begin to form tubers until approximately the autumn equinox. If frosts occur too soon after the autumn equinox, the plant will die before tubers are produced. + Soil requirements Oca grows with very low production inputs, generally on plots of marginal soil quality, and tolerates acidities between about pH 5.3 and 7.8. In traditional Andean cropping systems, it is often planted after potato and therefore benefits from persisting nutrients applied to, or left over from, the potato crop. The Maca tuber can grow in even higher altitudes of 4,000-4,500 meters (13,123-14,763ft), where the plant is cultivated on cold, windy plains. + Propagation Propagation by seed is possible but is rarely used in practice. Sexual propagation is complicated by several factors. First, like many other species in the genus Oxalis, oca flowers exhibit tristylous heterostyly and are consequently subject to auto-incompatibility. Furthermore, on the rare occasion that oca plants do produce fruit, their loculicidal capsules dehisce spontaneously, making it difficult to harvest seed. Oca flowers are pollinated by insects (e.g., genera Apis, Megachile, and Bombus). Oca is usually propagated vegetatively by planting whole tubers. Harvest the tubers in late autumn after the frosts have killed off top growth. Store in a cool dry frost free place and plant out in April. Basal cuttings in spring. Harvest the shoots with plenty of underground stem when they are about 8 - 10cm above the ground. Pot them up into individual pots and keep them in light shade in a cold frame or greenhouse until they are rooting well. Plant them out in the summer.
2.4- Nutrition Oca is one of the highest vegetable sources of carbohydrate and energy. They are a good source of pro-vitamin A (beta carotene), and also contain potassium, vitamin B6 and small amounts of fibre. Yellow-orange coloured varieties indicate the presence of carotenoids; whilst red skins and red specks in flesh indicate the presence of anthocyanins. The table to the right displays the nutritional content for fresh and dried oca. Oca is a valuable source of vitamin C, potassium (included in value for ash), and iron. It also provides some protein, with valine and tryptophan its limiting amino acids. Cultivars vary greatly in nutritional content, so these measures should be taken only as approximates. It is also high ranks from the nutritional point of view. Nutritional value per 100 g of edible stem tuber of Oca
2.5- Health benefits of Oca Oca is a highly productive perennial plant with waxy, brightly colored tubers that are perfect as a season-extending crop. It is an excellent source of carbohydrates, phosphorus and iron, as well as essential amino acids that promote the health and proper function of muscles, organs, nails, hair, skin and more. Oca is one of the highest vegetable sources of carbohydrate and energy. They are a good source of pro-vitamin A (beta carotene), and also contain potassium, vitamin B6 and small amounts of fibre. Potassium may help to maintain healthy blood pressure. Dietary fiber from vegetables helps reduce blood cholesterol levels and may lower risk of heart disease. Folate (folic acid) helps the body form healthy red blood cells. Women of childbearing age who may become pregnant and those in the first trimester of pregnancy need adequate folate to reduce the risk of neural tube defects and spina bifida during foetal development.
3- The Uses of Oca
3.1- Food Uses of Oca Oca is a highly productive perennial plant with waxy, brightly coloured tubers that are perfect as a season-extending crop. It is cultivated primarily for its edible stem tuber, but the leaves, flowers and young shoots can be eaten as a green vegetable also. Add oca’s flowers and shamrock-shaped leaves to salads. Mature stems can be used similarly to rhubarb. Andean communities have various methods to process and prepare tubers, and in Mexico oca is eaten raw with salt, lemon, and hot pepper. The flavour is often slightly tangy, but there is a considerable degree of difference in flavors between varieties and some are not acidic at all. Texture ranges from crunchy (like a carrot) when raw or undercooked, to starchy or mealy when fully cooked. Oca tubers contain oxalates (as a member of the oxalis family) as is evident when the plants are growing, with their distinctive oxalis leaf shape. Although gardeners need not worry as these Ocas are not invasive. Oxalates, concentrated in the skin of the tuber, are reduced if the harvested tubers are exposed to sunlight (tubers do not go green like potatoes). This process also sweetens the taste. Oca tubes tend to have a slightly tangy lemon taste. The flesh is firm but juicy and crisp when eaten raw or lightly cooked, and becoming more starchy if fully cooked. The tubes don’t require peeling when eating Oca raw - just wash them clean, and they can be sliced to add a hint of a lemony zest to salads. There are many ways to cook oca, including baking, boiling, steaming, roasting, and stir frying. Alternatively cook them in the same way as potatoes - boiled, baked, grilled or fried. They also make an excellent addition to winter soups and stews. The T&M Oca loses its skin colour on boiling and turns a more cream colour and loses any lemon flavour, becoming more ‘nutty’ in taste. The tubes contain over 70% water but are nutritionally rich with carbohydrate, calcium and iron. You can even pick some of the fresh leaves in summer for their tangy, lemon taste which adds that bit of zest to a green salad.
4- Growing Oca Tubers on the World
4.1- History + In the South America Because the plant plays a large role in stories about the origin of many South American peoples, botanists at Cornell University and the Field Museum in Chicago analyzed the genetics of oca to determine its source. It appears to descend from two wild ancestors in Bolivia, but has undergone so much genetic alteration due to human intervention and continued selection over the past 1,000 years that the chemistry of the cultivated plants is quite distinct from the wild forms. For example, pre-Incan people bred out some of the acids found in the skin of the tubers, altered day length sensitivity and increased the proportion of starches. As a result, there are literally thousands of South American varieties - although most of them don’t have commercial names. Grown primarily byQuechuaandAymarafarmers, oca has been a staple of rural Andean diets for centuries.Of all Andean root and tuber crops, oca is presently second only topotatoin area planted within the Central Andean region.Oca is important to local food security because of its role incrop rotationsand its highnutritional content. In its native lands of Bolivia and Peru, oca is second only to the potato in agricultural importance. Its most common name is Oca, the Spanish spelling thought to be derived fromoqa, a word from the Quechua language indigenous to the Andean region and spoken by the Incas. However, in many parts of South America, other names such asquiba,hibias,timbo,apilla and evenpapa roja(“red potato”) are common, so reading regional cookbooks can be challenging unless you have a South American dialect dictionary on hand. + In North America and Europe Actually, oca is not new to horticulturists - it was introduced to England, the United States and France as a novelty during the 1830s. Known as “South American wood sorrel” (it’s a cousin of the common wood sorrel), it caused such a stir that enthusiasts held oca parties where entire meals were constructed around these fascinating tubers. In North America and Europe, oca tuber is relatively unknown and not cultivated commercially. However, manypeople living in the UK and North America have reported great success in growing ocain their gardens. This is not surprising since oca is known to tolerate poor soil and different climatic conditions. The major challenge vegetable gardeners are facing in these regions is the difficulty to find oca seeds in the stores. However, searching for oca seeds may be well worth the effort - this nutrition-packed, health-promoting tuber boasts a wide range of micro and macro nutrients includingvitamin C, iron, zinc, flavonoids, B vitamins, and fiber. It is also low in calories. + In New Zealand In New Zealand, where oca has been cultivated as a commercial crop under the name “New Zealand Yam” since the 1860s, there are quite a few named varieties, and there is a continuing program to develop better ones. It is these New Zealand varieties that we are beginning to see in U.S. produce markets because they adapt most readily to our growing conditions. In New Zealand, oca are planted in November. About February, when the foliage is about 30cm high, the oca are mounded up. In mid-March the yellow flowers appear and the tuber production begins.
4.2- Varieties and cultivars Oca is a perennial plant extensively cultivated in the central and southern Andes for its edible tubers. This tuberous root vegetable is the second most widely grown root crop behind the potato in Peru and Bolivia. Andean farmers cultivate numerous varieties of oca. Oca diversity may be described with respect to morphological characters, local cultivar names, or molecular markers. It’s hard to generalize about oca’s flavor and culinary attributes, because there are so many kinds: Some are best eaten raw; others are best boiled, baked or steamed. Sun-dried oca can be eaten like dried figs or stewed like fruit. Oca tubers also can be grilled, fried or candied like sweet potatoes. As for flavor, they vary from potatolike, to chestnut-sweet, to apple and celery. ‘Apricot,’ a new variety from New Zealand, is similar in taste to its namesake. Oca’s cloverlike leaves and yellow, trumpet-shaped flowers are edible and make great additions to salads. Sour oca and sweet oca form distinct genetic clusters based on AFLP data. This suggests the possibility of distinct evolutionary histories for each use-category. According to Purdue University, however, oca actually has less oxalic acid than spinach, and one would have to eat nothing but oca to experience any harmful effects. Furthermore, most of the oxalates in oca are water-soluble, which means all you have to do is boil or steam it and pour off the water. And when left in the sun for a few days, several varieties will undergo a chemical change in which some of the acidic elements are converted to sugars. To play it safe, anyone who has an allergic reaction to rhubarb, sorrel, beet greens or spinach, or anyone suffering from gout or kidney stones, might want to avoid oca.
4.3- Growing Oca Tubers Oca is propagated from tubers, so it’s cloned in a manner similar to potatoes. It prefers sandy soil, partial shade and cool, damp weather. Plant whole tubers in pots in late winter, and once they form healthy vines, transfer them to tubs or into the ground after the threat of frost has passed. Planting at this early date is important because the plants must be well established before hot weather sets in. They don’t tolerate hot sun and a hard drought certainly will kill them unless they’re well watered. The small tubers are best planted individually in a 15cm (6") pots of multipurpose compost during April. As they are frost tender they should be grown on in the greenhouse or on the windowsill. Plant out the small plants when frost risk has past in late May and cover the plants with fleece until established. Each tuber makes quite a bushy plant so allow at least 90cm (36") between plants for optimum tuber production. Some of the stems which rest on the soil will readily root and the plant can grow to a sizeable bush. Oca also make a decorative container plant throughout the summer and up to the first frosts when the foliage will die back. The cultural practice is similar to potatoes. Planting is done in rows or hills 80-100 cm apart, with plants spaced 40-60 cm apart in the rows. Monoculture predominates, but interplanting with several other tuber species, including mashua and olluco, in one field is common in Andean production. Often this intercopping consists of several different varieties of each species. Such mixed fields may later be sorted into tuber types during harvest or before cooking. Alternatively, tubers can be planted directly outdoors in late May. By this time they may well be showing small ‘sprouts’. Plant Oca directly into a shallow drill, about 8cm (3") deep, and cover with soil or compost and a layer of fleece. Remove the fleece from early June or as the soil and weather warms. Plants can be ‘earthed up’ as you would potatoes to give some further growing room and anchorage, although this is not essential. Oca prefers well drained soil and an application of a general fertiliser will be appreciated as the plants grow. A mulch of well rotted compost or grass cuttings around the plant during summer will keep the soil moist and aid the plant’s growth. Water plants well during dry spells, and especially from mid September when tuber initiation commences, as this will promote larger tubers. Earthing up the growing stems as they start to form tubers can increase yields significantly. Watch the video below to find out more about how to grow Oca (New Zealand Yam): Click here to watch part 2 of Thompson and Morgan's 'How to grow Oca' videos. Click here to watch part 3 of Thompson and Morgan's 'How to grow Oca' videos.
4.4- Control pests They have no known pests, at least not in North America, which is a plus for organic growers. However, mice, barn rats and chipmunks are quite fond of oca. 4.5- Harvesting Oca tubers The essential point to remember is that the tubers form and swell as autumn develops, when day length shortens and temperatures fall. Ocas do not form underground tubers until very late in the season. Do not worry at all if the foliage gets frosted - it will die off after a heavy frost. Some plants may form some small aerial tubers on the stem. These need to be picked off before a heavy frost. Do not harvest the underground tubers until the last remnants of foliage has become frosted and died off from late November and into December. The longer you can leave the tubers in the soil the better. Lift the tubers carefully, dry them and taking care not to bruise or damage them, then store in slatted trays or a hessian sack in a cool shed or garage. Tubers do not need covering against the light, and will store happily for several months until ‘sprouting’ commences and some of these can be replanted. 4.6- Crop Yields of Oca Yields vary with the cultural method. Annals from Andean countries report about 7-10 tonnes per hectare for Oxalis tuberosa production. But with adequate inputs and virus free propagation material, oca production can range from 35 to 55 tonnes per hectare. In some trials a wider spacing gave a higher percentage of larger cylindrical, slightly tapering tubers averaging 10cms (4") in length and 35g (1.25 oz) weight. It can be harvested an average of 50 tubers per plant of varying sizes. In a really good growing season, and allowing plenty of autumn and winter days to fully develop the tubers, then 0.75 kg (1.6 lbs) per plant (70 tonnes per hectare) may be possible.