Oca versus Frost

I always like to have a nice photo to illustrate a post, but this time, viewers of a nervous disposition should look away.

The photo shows Oca tubers slightly, partially, and completely damaged by the recent period of sub-zero conditions. It seems that the frost has penetrated 50 to 70 mm below the soil surface, causing quite a few casualties - around 20% of tubers had to be discarded. Affected tuber flesh looses its colour and crisp texture, taking on the appearance of clammy rubbery white maggot flesh. Within a day or two a characteristic smell of decay appears, and the maggots are on the way to becoming biological soup.   Some might attempt to turn this into a delicacy, but not me.

According to Lost Crops of the Incas... " Farmers mound dirt over the base of the plants to encourage stolon formation". But stolons aplenty appear without earthing-up, so maybe a more important reason is to protect tubers from frost once the stems have died back.

Hard or prolonged frost is unusual here in West London, and I consider this year's cold snap a rare event, so do not plan to routinely earth-up Oca crops in the future. Quite apart from the extra work involved, it would be awkward in most bi-crop situations, and it goes against my preference for minimum-tillage cultivation.

Anyhow, plenty of tubers escaped damage...

Underground Oca

Frozen soil for the last week has put a halt to lifting tubers, but the last plant I lifted from semi-frozen ground came up so cleanly that the root system was almost intact. This gave a really good view of the root extension and structure.

Notice the original planting tuber, which is still firm and unrotted. I've always thrown these aside (they do not look appetizing) as I would when lifting potatoes, but I'm curious to see if they resprout in Spring, and perhaps take advantage of that existing root system to get a flying start. Looks like something else for me to try this coming season. If anyone out there has tried it, I'd love to know. (update 1/3/10, The original tubers, which I temporarily heeled in with a view to replanting, have in fact now rotted. Some sources state that oca is a perenial, others an annual. My observations would sugest that Oca is not a true perennial, but a plant-replant annual. End update)

When thinking about intercropping, it's simple enough to see if crops are competing above ground, but this got me searching out my well thumbed copy of Robert Kouriks classic read Designing & Maintaining Your Edible Landscape Naturally which contains some eye-opening scale drawings of various vegetable root zones in the section 'Intercropping Below Ground - The Shape of Roots', which are helpful when visualising the amount of competition between plant root systems. From observation of the plants I've lifted, Oca roots seem to have a similar range to tomatoes - 3 feet radius by 4 feet deep, but with most of the fibrous rootball close to the planting tuber. The fleshy roots, one of which grows from each eye, extend further and send out a more sparse network of fine root hairs which are difficult to follow in the soil, so it's hard to say exactly how far they spread.

But most importantly - those fleshy root stems are STILL alive despite the devastation to the plant above ground caused by days of continuous sub-zero temperatures. My hunch is that tubers continue to expand even this late, by drawing sap from these root stems.

The experiment here will hopefully confirm this.

Oca Weights & Measures - for Oca Anoraks Only

I need an objective method of assessing crop productivity so that different cultural methods and varieties can be impartially compared with each other. Weighing the tubers is the most obvious way to do this, but a single 100g tuber is far more desirable than 100g of tiddlers. Clearly weight alone should not be the yardstick.

So I will classify tubers as 'large' or 'small' as defined by whether they will go through a 25mm diameter hole, and weigh each group separately. Tubers with a minor axis diameter of 25mm are perfectly usable, but much below this, and they become a faff to lift and prepare.

This grading does not take as long as I expected, since only the borderline tubers actually need to be checked with the gauge.

My 'Oca Productivity Index' will reflect the fact that the small tubers are about half as useful and desirable as the large ones thus:

OPI = (weight of large tubers in kg) + ½(weight of small tubers in kg)

So the plant I lifted yesterday gave 1407g, of which 867g were large, and 540g small, giving an OPI of 1.14

This figure is convenient because it corresponds roughly to the weight (in kg) of useable tubers, and can be used 'per plant', 'per m²', etc.

Sometimes tubers grow 'daughter tubers'. No doubt plant physiologists will have a name for this, but I call it a damn nuisance, and I expect cooks do too. Anyway, for the purposes of weights and measures, all daughter tubers are snapped off (this can be done quite cleanly) and assessed separately. Otherwise the OPI will be flattered.

In case it shows up some significant factor, for each plant I am also recording the following:

• Number of tubers, large
• Number of tubers, small
• Weight of largest tuber
• Length of largest tuber

The photo at the top of this post shows the tuber that tops the table so far at 77g and 111mm.

2009 Crop - Harvest Results

31st December. It's 25 days since the first light frost, and 16 days since harder frost completely killed the foliage.

Having made the decision that the crop is ready to harvest, and having lifted the first plant, two things immediately sprang to mind. Firstly, it's a heavy crop, definitely more productive than last year. And secondly, a nagging doubt that the stems are not all as dead as they could be, and consequently the plants could still be transporting sap to the tubers.

It's possible to go too far with the waiting game, but you don't know where 'too far' is until you go there, so rather than lift all the plants now, I've decided to lift them individually at intervals of a day or two, weighing each plant's tubers, to see if there is any increase in the yield-per-plant over the next couple of weeks. If I'm right, it should plateau-off at some point, and this will give an indication of the optimum harvest time.

Weighing the tubers from the first plant was a pleasant surprise. 795g of large, and 405g of small tubers, giving a total of 1200g. (Definition of large and small tubers here). Annoyingly, I did not weigh last year's harvest, but I would estimate this year's to be almost double.

Update: 28/1/10 Results of the waiting game:
It started off so well, the crop from the first three plants supporting my hunch completely, but then the weather put a spanner in the works by freezing the ground for a couple of weeks. This caused the local wildlife to get extra hungry, some of whom developed a taste for Oca tubers. The result of this was an unknown quantity of tubers being scratched up and carried away, and the experiment was ruined.

Anyway, here are the limited results of the experiment, which allow some conclusions to be drawn:

Plant 1 (16 days after killing frost) 1200g  (OPI= 0.99)

Plant 2 (17 days after killing frost) 1217g  (OPI= 1.01)

Plant 3 (19 days after killing frost) 1407g  (OPI= 1.14)

Then came a period of snow and frozen conditions when no plants were lifted until:

Plant 4 (26 days after killing frost) 1155g  (OPI= 0.98)

More freezing weather with starving crows, rats, and feral ring-necked parakeets helping themselves:

Plant 5 (34 days after killing frost) 717g  (OPI=0.62)

finally, abandoning the experiment, I lifted all remaining plants. Weights include damaged tubers:

Plants 6 to 12 (43 days after killing frost) average yield 1038g (OPI not calculated)

Tubers exposed and damaged by birds.

Conclusions

• Tuber weight may reach maximum at about 20 days after killing frost, or possibly even later. However the sample size here is so small that the data is not statistically significant, and no definite plateau was identified. It is quite possible that different temperature conditions would produce a different result.
• Average yields of over 1kg, and peak yields approaching 1.5kg per plant are achievable in the south of the UK without using fertilisers, protected cropping, or labour-intensive cultural methods, even when bi-cropped.
• Even if buried, tubers can be damaged by penetrating frosts, or raided by hungry birds, which can nullify any benefit of waiting for the purported optimum harvest time. Where freezing conditions are expected, it may be worth the extra work to earth-up, or provide other physical protection.  
• The primary bi-crop (tomatoes) did not show any noticeable variation in yield relative to the control bed (which had the same spacing, but with French marigolds instead of Oca). N.B. the tomato yield was not weighed, so this conclusion is based on subjective judgement.