Tubers - Big and Few, or Small and Many?

Fruit thinning works for gooseberries, could it work for Oca?

(Above) 6th December, a mass of stem-rooting tubers forming above ground.

Oca tubers form close to the roots of the plant, but also where stems come in contact with the soil - up to several feet away from the original planting location. Seeing this mass of small tubers today reminded me that last year, although the total yield (weight) was fine, a high proportion of the tubers were too small to be worth cleaning and cooking.

So how to maximise the proportion of large tubers?
Remove some exposed tubers while small, so that the plant's reserves are concentrated into those remaining?

Prevent stem-rooting (by keeping stems from contacting the soil) so that only the tubers associated with the original roots form, giving fewer, larger, less dispersed tubers ?

Either of these approaches would detract from one of the crop's great plus-points - it is zero-work. Others have grown Oca in containers, which might limit the stem rooting tubers, but I have not heard reports of larger tubers from this method.

I'll let the crop run it's course this year, but this is something to ponder, and maybe test out next year.


UPDATE. An example of training Oca to grow upright (using canes) here: Thriftyliving

Oh-oh! Oca allelopathy!

Is Oca Allelopathic?

Allelopathy is the ability of a plant to chemically suppress the growth of another.

I've just discovered the excellent Radix blog, which focuses on unusual root crops, and it strongly suggest that dried Oca foliage inhibits germination (of lettuce seed in the experiment).

This obviously is important when mixing and overlapping crops.

But it may not be all bad news. Managed carefully, decaying Oca foliage debris could be beneficial in ensuring a weed-free bed for any following crop, as is the case with several traditional green-manure crops. The real problem would be if the plants roots exuded suppressive chemicals while growing. This would make it highly unsuitable for bi-cropping. So far I have seen no evidence of this, but I will be watching out for it.

What I need to do is an experiment to test for this next growing season. Hmmm.

UPDATE: I have found evidence (here) that Oca roots produce an exudate which suppresses growth in competing plants. However, the chemicals are also beneficial - insecticidal, and pathogenicidal. At least some of the chemicals require UV light to be activated, (so effectiveness is restricted to the surface and first few mm of soil?).

UV light levels reaching the root exudates must be very low once the foliage has formed a canopy.

All interesting to know, but as is sometimes the case, science does not give a clear answer. Will Oca poison or protect its bicrop partner, or something between the two?

Well, it obviously does not seriously harm most crops, from the evidence of my own eyes, and the experience of generations of Andean Indians, who's traditional methods of cultivation include bi-cropping with maize, and mixed planting with other tuber crops.

It seems to still come down to 'suck it and see'.

UPDATE: Details and results of the allelopathy trial here.

2009 Growing Season - Oca with tomatoes

After some consideration I decided to bi-crop with tomatoes as the primary crop.

So ... Action-points for this season:

• Use a more formal multi-crop structure.
• Allow more space for the ultimate size of the foliage.
• Hold out for as late a harvest date as possible.
• Keep better records of dates, spacing etc.

Early March. As previously, I chitted tubers indoors.

Although the tubers saved from the previous season are technically genetic clones, I noticed one tuber sprouted much earlier than the others. Another was a much darker colour. Some mutation does occur, so I have labelled them and will see if the characteristics are passed on.

25th March, planted in pots in an unheated greenhouse.

28th April. Planting out. The plants will be placed in a single row down the centre of a 4ft wide bed (hopefuly giving enough space for the Autumn spread) alternating with the primary crop of cordon grown tomatoes. Spacing is about 20 inches between each tomato, with an Oca between each. Space on either side will be used for spring-sown, fast-maturing crops, which will be harvested before the space is taken by the Oca.

The bed has had an early (sown in February under plastic) green manure crop of mustard. Without disturbing this, 7ft canes are placed ready for the tomatoes, and ...

...the Oca are transplanted in small clearings giving them some protection from any cold wind.

By 9th June, the mustard is long gone and the interplanted tomatoes are knee-high. Direct-sown salad crops are up (here, landcress and spring onions, not visible are module-sown beetroot and lettuce) The tomatoes have received a layer of mulch to encourage stem-rooting and the Oca is still well-behaved.

By 1st August the fast-growing side crops are mostly harvested, just in time for the Oca to spread dramatically.

I was concerned that the lower trusses would be slow to ripen due to shading from the Oca, but they seemed no different from those in the control bed (without interplanted Oca).

17th August. Tomato cropping is in full flow. The single row of oca is providing complete ground cover across the 4ft bed. The tomatoes have been stripped of leaves over their lower third to aid ripening and ventilation. This also lets in more light to the oca.

1st of November. Blight and cold weather have just about finished off the tomatoes. The Oca is thriving in the cool temperatures and increased moisture levels.

Its' happy enough to flower..

.... and show the first signs of tuber formation. Stems in contact with the ground produce thick fleshy roots which form a tuber at the end.

(6th December) A light frost has damaged the outermost foliage. This seems to have stimulated tuber formation. There are huge numbers of tubers forming where the stems are touching the soil.

(20th December) Two weeks later there have been several proper frosty nights. The crucial period for tuber growth is now!

The plant guild offers up an unexpected bonus crop; the Spring-sown land cress has survived the hot Summer under the Oca, and is now making a come-back through the collapsed stems.

31st December. The foliage is well and truly dead. Surely time to harvest!

For harvest results go here

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.