Super-duper Tubers!

Oca (top) and Mashua (bottom)

18th December: Well, it might be winter, but here in the heart of the Netherlands the winter weather was a long time coming and we had a long mild Autumn. This meant that a couple of the weird and wonderful South American tubers that we are trialling had time to make a bumper crop, since they begin to make their tubers only when the days begin to shorten. We had one plant of Oca Oxalis tuberosa which grows low and looks like a normal clover-like oxalis plant, and one of Mashua Tropaeolum tuberosum which is from the same family as the garden nasturtium (Oost-Indische Kers in Dutch) Tropaeolum majus. Both plants were sourced locally from ‘De Ommuurde Tuin’ (The Walled Garden)  so we knew they would grow well, but we have an open site as opposed to the lovely microclimate of a walled kitchen garden – we are giving them a rougher trial.

Both plants had begun to die-back weeks ago, and with the ground frosts getting serious alarm bells started ringing because these tubers can only tolerate limited levels of minus soil temperatures and last year we had extremely low winter temperatures. We have left a few in the soil but all our plant material is precious to us since we are still building the garden from scratch, so along with getting to try some in our kitchens we also want to plant some in frost-free conditions to preserve our resources.

The two plants have been growing in the perennial vegetable garden in the hope that they will regenerate from tubers left after harvesting like the Jerusalem artichokes (or Sunchoke) Helianthus tuberosus, thereby fitting into this demonstration garden’s low maintenance concept. This area has the best conditioned soil, is well manured and relatively sheltered.

Oca – Oxalis tuberosa                               H: 50cm  W: 30cm                              Tuber crop weight: 0.6kg                    The oxalis grew at the edge of a bed planted with perennial broccoli and mints, and Jerusalem artichokes close by. It was mounded-up with some extra soil at the end of summer to encourage maximum tuber production and looked quite decorative – promising as an edging plant. We planted it in early summer as a small plant and it put on a lot of growth, was untroubled by pests or diseases and only gave up when the first frosts came.

 We had been told that Oca doesn’t usually crop heavily and that its tubers are a gourmet delicacy. This year’s weather favoured its growth pattern of autumnal tuber production and our one plant produced 600g from an area of ground 30cm x 30cm. The tubers can be eaten raw although not in quantity since they contain high levels of oxalic acid. The traditional Andean practice for both oca and mashua is to expose them to a few days of sunlight before eating them which alters the chemistry and sweetens it. With or without this treatment the tubers are very tasty cooked – opinions, experiments, and recipes are welcomed!

Mashua or Anu                       Tropaeolum tuberosum
H: up to 2 metres   W: 50-100cm    Tuber crop weight: 4.5kg
              Mashua is a trailing crop, but not a perfect climber as I found out while trying to get it to scramble up the stakes and ties of an apple tree. It preferred to ‘grow south’ along the ground in the opposite direction rather then climb. However, like the garden nasturtium it just took a long time to get the idea. Unfortunately we had no display of flowers from it – unfortunate because, like the nasturtium, the flowers are edible as are the leaves.

Our mashua was placed under a newly transplanted apple tree to act as a pest barrier. Nasturtium is a known protective companion plant for apple trees so we extended the principle, growing it on one side and its cousin the mashua on the other, shading the tree roots and repelling caterpillars and other insects. Polyculture planting with mashua in Ecuador includes: alternating strips of Oca, faba beans, and barley; or mixed with maize and squashes.

Crops can exceed 2kg of tubers per plant and in our case we seem to have got a bumper crop at 4.5kg, even without hilling soil around the plant (which traditionally happens at the beginning of the growth season and at the onset of flowering). The wet summer, mild winter, and fertile soil are probably contributary factors to the high yield. The plant scrambled a metre high with some initial persuasion and sort of ‘mounded’. All in all it covered a ground area of 50cm x 50cm, with dense foliage of pretty, lobed leaves.

The tubers are used cooked, have a peppery flavour and can be an aquired taste – we haven’t yet distributed the crop between members to get any feedback. Ken Fern from Plants for a Future says: ‘The flavour can be improved somewhat by freezing the tubers after they have been cooked, they are then considered to be a delicacy by many people. We have also noticed an improvement in the flavour if the tubers are harvested after they have been frosted, though if the frost is too heavy they can damage the tubers.’ We’ll let you know, but so far its reputation as an anaphrodisiac is causing some problems – it was fed regularly to armies by the Incas to help them ‘forget their wives’. My boyfriend has refused a tasting session, arguing that (to paraphrase) however small the effect may be, no man wants his ardour dampened! I read different things about whether it affects testosterone levels, but it has been shown in tests on rats that constant concentrated levels of mashua have a temporary effect on sperm production, so this may not be the best potato substitute if you are trying for a baby…


  • Plants for a Future:
  • Grau et al. 2003. Mashua: Tropaeolum tuberosum Ruiz & Pav. :Volume 25 van Promoting the conservation and use of underutilized and neglected crops. International Potato Center, Lima, Peru; International Plant Genetic Resources Institute, Rome, Italy.


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