Thursday, 12 December 2013

Extending the Oca Growing Season through Frost Protection

The death of foliage from frost signals the end of the oca season, but at this time of year when tuberisation is in full swing, any slight extension of the growing season will have the maximum effect on crop yield. I stress that again; even one extra week without killing frost could easily  have more effect on yield than any other factor over the whole growing period.
But it's important to point out that oca can survive light frosts, and there may be some variation among varieties in this ability.
Furthermore, it has been speculated that cold weather or slight frost damage may contribute to initiate or accelerate tuber bulking.

Most frosts in autumn are radiation frosts, occurring on clear windless nights, often just for an hour or so at dawn. These are capable of being combatted more easily than advection frosts, which are associated with a more fundamental and long lasting atmospheric low temperatures.

So until new varieties with day-length-neutral tendencies come along, here are 20 ways to minimise the risk of crop loss from frost:

Topographic placement and aspect. Avoiding frost pockets, and facilitating cold air drainage away from crops is a well understood method. Less commonly understood, is the effect of the upslope area; depending on the soil surface, this can chill or warm the air flowing down over the cropping area. Bare soil, or hard surfaces will produce warmer conditions than continuous ground vegetation or thick mulch.
South-facing slopes will receive and store higher levels of the sun's energy than north facing, and so have more to radiate during the night.

Mini-scale cold air drainage. Raised beds can protect from slight frosts by allowing cold air to drain away to the lower path areas. Paths should be unobstructed, and run downslope to drain air efficiently, though this may conflict with soil erosion considerations. Beds with high wooden edging are as likely to trap frost as they are to shed it. Growing on supports such as netting, pea sticks, or a tall companion crop may also have some beneficial effect in keeping top growth above the coldest air.
Come the first frost, this oca crop should benefit from its bed being raised by a foot or so above the paths, and from its companion crop of sweetcorn, who's dried stems will be left in situ after harvest to trap radiated warmth. The mulch of rotten hay will prevent the soil from keeping the air warm, so should be removed before frosts are due in autumn.
Downslope air flow. Any areas or rows of harvested tall crops should be cleared away before frosts if they are located where they may obstruct or slow the cold air flow away from the crop bed in question.

Cold Hardening. Exposure to cold conditions is considered by some to increase the crop's frost tolerance, but this effect is apparently nullified by any following period of warm weather, so refrain from adding polythene crop protection if the weather is merely cool. Interestingly, cold hardening of many plant species is associated with the translocation of carbohydrates to the roots, so this may be significant in accelerating tuber bulking.

Nutritional Hardening. There is some research showing that plant nutrition may influence frost resistance. Understanding seems poor, but it's probably beneficial to avoid nitrogen feeds in the period coming up to the frost season.

Sacrificial companion crops. A taller crop grown with the oca can help to hold warmer air close to the ground. Yacon works well, and can usually produce good crops itself before frost arrives, as it is not day-length sensitive. A slight disadvantage is that once frosted, the companion crop looses some of its protective value due to reduced canopy density.
Yacon burned by frost. The Oca underneath is undamaged.

Crop debris from previous companion crop. The dead standing remains of tender tall crops (such as corn, tomatoes or quinoa) grown with the oca during the Summer can provide some slight frost protection if they are left in place.
Cordon tomatoes growing over oca in late Summer. The dead foliage canopy can provide some frost protection later in the year.

Self sacrificial protection. Outermost foliage of oca plants will be killed by the first light frost...
...leaving the rest of the plant unharmed. This is possibly instrumental in boosting the plants' rate of tuberisation. Growing large healthy plants from a full season's growth, or planting at close spacing will result in dense, thick foliage capable of absorbing several frost events before the plant is killed.

Physical covers. Polythene, horticultural fleece, newspaper, netting, straw, or sacking can protect against several degrees of frost. For simple management, they need not be removed between frosts, but simply left in place until harvest. Avoid placing them too early, or the plants' cold-hardening adaption may be compromised.
Scaffold debris netting protecting a bed of oca. After a night of -4°C the plants are limp but not killed. Other root crops in the photo are (from top left clockwise) celeriac, Hamburg parsley, chuffa, skirret.

Casual covering. Crop debris collected from nearby spent crops, such as pea haulms, corn stalks, weeds, etc can simply be thrown over the crop, then left until harvest time. N.B this is not a soil mulch, which would reduce soil radiation; it must be placed over the crop foliage to hold radiated warmth around it.

Water spraying. Popular with commercial fruit growers, this relies on latent heat of fusion released during the freezing of water sprayed over the crop. Automated systems are de rigueur for this method, but for really precious crops, some folk may be willing to get up in the middle of a freezing night with the hose! There is an account of using this method on oca crops here. It's strange and counterintuitive, but having the foliage plated in ice really can stop it freezing.

Air disturbance. Some commercial fruit growers use giant fans, or even low flying helicopters to mix air layers, and so prevent crops freezing. Rather an expensive method!

Added thermal mass. Laying bricks, concrete slabs, containers of water, etc on or near the crop beds will increase thermal capacity.

Soil density. Soil with a high ratio of voids (air pockets) will hold less heat than solid soil, and will give up that heat more slowly due to the insulating effect of air, so avoid cultivating the bed, especially as the frost season approaches, to keep the soil dense, and maximise heat storage capacity.

Soil colour. Dark colours are more efficient absorbers and emitters of radiated heat. Building up soil humus levels to darken it, or dusting the surface with soot are effective and traditional. Small amounts of charcoal might work, but there is  a danger that the effect is counteracted by its insulating properties.

Soil surface area. Soil surface with corrugations or 'lumpiness'  has a greater surface area, so is able to deliver heat faster than a smooth soil surface. Raised beds or ridged rows also score here, having a greater area than flat beds.

Soil surface insulation. Any mulch present will greatly reduce radiated heat from the soil compared to bare soil. Rake back mulches when frost threatens. Peaty soils are also poor heat radiators.

Weed Management. Weeds under the crop are undesirable as they reduce radiation from the soil, however weeds taller than the crop are protective, holding warmth under them.

Soil wetness. Water has a higher heat storage capacity than the mineral content of soil, so wet soil will have more heat to radiate to the air. If soil is dry in early winter (as if!) extra watering will help protect against frost.

Active heating. Some fruit orchards used to be protected by smudge pots, or even heaps of burning tyres, but this has fallen out of fashion/become illegal, to be replaced by large propane or petrol burners. Obviously none of these methods are particularly sustainable, or justifiable in current times. The use of manure-fueled hotbeds may work, but controllability and reliability of effect is poor, and the labour involved huge. If heat has to be added artificially to a crop, in my mind, it's a sign that it's the wrong crop for the location.


  1. I think that there is another interesting (although possible less controllable) element, which may have something to do with the growth quality of the plants before tuberization begins. We started harvesting the 160 plants in our lower oca bed the last week of November and got 310lb. Meanwhile, I know people who harvested later with much poorer yields, even though their plants were grown under similar conditions and were comparably mature. We didn't even have time to wait out the usual post-frost bulking period, but got a lot of tubers in the large, 3"-5" range (there's the makings of joke in there somwhere. An oca, a carrot, and a parsnip walk into a bar…)

    Anyway, it seems that it is possible to get reasonably fast bulking once tuberization begins, but I'm not sure what the contributing factors might be.

    As for next year's frost protection, it's helicopters or nothing for us!

    1. There are so many variables that might be an influence; soil nutrients, moisture levels, or the specific conditions of the cold period leading up to killing frost. We are in the zone where we cannot just google the answer.

  2. very interesting. And shur many factors for good harvest, but earth ist very important. I see it on my harvest. Hardy and wet and tubers are not good like on the other side...
    Regards Cordula

    1. Yes, I think deep well cultivated soil has a significant positive effect on yield.

  3. I am in Los Angeles, CA. Thank you so much for your wonderful dedication to ocas. I planted my first ocas this year before I found you. I could have done so many things better!!! Anyways, can you tell me what kind of tomatoes are those featured with your oca? thanks!

  4. Hi Anonymous, if you mean the blues, they are 'OSU Blue'. The variety has since been refined and released commercially as 'Indigo Rose' and should be easily findable in the States.

    1. thank you so much!!! you are just amazing!!!!!