Introduction & 2008 Growing Season

Oca (Oxalis Tuberosa) is a promising root crop, rarely grown in the UK. I’m interested in it because it seems to offer potential for efficient bi-cropping and intercropping with more conventional vegetable crops.

I'm using this blog to share information gained from cropping experiments, as there seems to be little information currently available on this aspect of the crop, certainly in UK conditions.

I will not go into basic cultural requirements as this has been well covered elsewhere. I would recommend the following as good sources of information:

www.downtheplot.com/oca.php
www.realseeds.co.uk/unusualtubers.html (who also sell seed tubers).
www.pfaf.org/database/plants.php?Oxalis+tuberosa
www.wildchicken.com
thevegetablegarden

Certain properties of Oca make it suitable for filling vacant gaps in the four-dimensional space of a highly productive, low-work vegetable-growing system (the fourth dimension is time!). These properties are:

• Apparently tolerant of competition from other crops. 
• Harvested during winter, when there are few other fresh vegetables available. 
• Occupies little space until late summer. 
• Makes maximum growth late in the season when space is more available. 
• Acts as an effective moisture-retentive living mulch during late summer. 
• Acts as a protective ground cover during early winter. 
• Described as being unaffected by pests and diseases.

My Style of Gardening.

I'd describe my gardening style as: organic, intensive, urban-salvage, no-dig. I’m particularly interested in overlapping crops, where one is sown or planted out before the previous one has matured. This requires careful timing to minimise competition, but can be far more space-efficient than simple bi-cropping, catch-cropping, or companion planting alone.

An example of this intensive method is shown below. A bed with ripening spring-planted onions has recently transplanted sweetcorn estblishing between them. Once the onions are lifted, Winter squash will be transplanted between the corn. The squash and corn will be cleared in time to sow an over winter green-manure crop.

With a green-manure crop also preceding the sequence, the soil has been protected for the entire year, and produced three crops during the season, all without digging.

First Experiences with the Crop, 2008

Oca (centre) fighting it out with peas, beans, corn and lettuce. Mid June.

I had read all I could about Oca, so knew what to expect in terms of ultimate space requirements and timing. I planted the seed tubers in pots, then spaced them either side of a 4ft wide legume bed consisting of a 2mtr high central net, with climbing French beans and tall peas. Either side of the central divider was planted with alternating sweetcorn, and the Oca plants, with lettuces early in the season just for good measure.

4th December. All other crops have been cleared from the bed, and the first light frost has cut down the foliage.

Tubers are forming, but they are still tiddlers.

A week later (12th Dec). The stems have died back. I decide to lift one plant, but the tuber size is still disappointing.

(1st January) The stems have been fully killed off by more frost. Harvest time! ...

... And the tubers have doubled in size

So What did I learn?

The single most important way of increasing the yield is to delay harvesting long enough. While lifting the tubers I noticed that, even on the 1st of January, the stems which were below ground were still slightly green and fleshy, so I suspect the tubers might have continued to swell if they had been left still longer. This is much later than is recommended elsewhere, but in this urban environment, frost comes late, which favours the crop's productivity.

As suspected, competition with (or from) other crops is not great, making it ideal for bi and tri-cropping, and crop-overlap. 

The moderate yield per area does not justify it as a monoculture crop, by my standards.

Contrary to received wisdom, it does suffer from at least one pest - there was some damage to exposed tubers late in the year - probably pigeons, crows, or ring-neck parakeets. But no signs of any disease.

The plants ultimately require a lot of space. Two rows on a 4ft bed obstructed the adjacent paths by late summer.