Friday, 1 January 2010

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.

  • 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.


  1. Interesting that your killing frost was much later than ours here in Cornwall. Microclimate is everything. I hate frost pockets.

  2. Oca seems to me to be particularly well suited to urban (allotment) sites. The heat island microclimate of a city is ideal to give the late killing frost date. Likewise coastal locations.

  3. Thanks for the post, I"m waiting on my plants to harvest. I have 2 rows that are 100' long. Any suggestions on saving seed for following year?

  4. Hi Anonymous
    Sounds like you have a fair few plants. Are you talking about saving true seed, or seed tubers?

  5. What an exciting experience!/Hilarious! Delightful! True!/wonderful stuff! thank you!

    Growing Plants

  6. Thanks for this. I wonder what happens when there is not frost at all? When I lived in Central London there were often years with no frost, let alone penetrating frost.

  7. Great article, can't wait to find my own OPI!