As anyone who has tried can confirm, overwintering kale in the far North is challenging. In what follows I relay historical research on traditional growing techniques published in Norwegian, as well as my application of the findings to growing kale in harsh conditions in Norway.
In researching hardy kale most of the focus seems to be on finding hardy perennial landraces and improving them genetically. Hardy genes are a good place to start, but it has become more clear to me that although S.T.U.N. (Sheer Total Utter Neglect), might work further south, keeping even hardy plants alive as far north as Norway requires C.A.R.E (Comparison, Attention, Research, Emulation).
Looking at the way in which kale was grown in harsh climates and learning from the generations who survived due to techniques they developed over generations of trial and error is as important as maintaining the genetics. Sadly this kind of knowledge is disappearing along with the heirloom landraces.
Kale and Cuban Oregano in the south facing window.
Over the past few years I have developed stewardship over a number of kale landraces including Shetland Kale. The best report I have found is a Norwegian text, the title of which translates as “Historical Greenhouses: growing kale for humans and beasts in Shetland” by Amy Lightfoot.
What is spectacular about this text is that it combines the details of how protective structures were built and situated in the harsh landscape, while also relaying the descriptions of the effects of the structures on protecting plants. In brief, kale was sown in stone enclosures called crubs (krobb) in late summer. On average a crub was 10m2 and held 1000 plants. As Amy describes: “The purpose of a crub was to produce slow-growing, hardy seedlings for planting in the kale yard the following year. Crubs protected them from grazing animals and rabbits, and even more importantly from wind and frost.” As I will explain later, crubs also critically, and counter-intuitively, protect from the sun.
I have tried to emulate these techniques in Norway on our small farm by overwintering kale in our greenhouse. So far this seems to be working quite well and what I have found is that some of the details of the report by Amy Lightfoot are perhaps the most important.
Moving Kale and Collards to the greenhouse to protect from frost.
When applying the findings from the report to my farm I can confirm that during the middle of the winter, protecting against wind and cold are the most important factors. Greenhouses do an excellent job here, maybe even better than a crob. However, as the report shows, “the worst damage happens in late winter or early spring. If the sun grabs hold of a frozen plant, it almost cooks the plant. If a plant is in the share of the stone crob it manages better”.
I found this counterintuitive insight to be of critical importance. Exposing frozen kale to direct light in the spring might just be the one factor that growers in the far north get wrong. As margins of error get thinner the further North you grow, one instinctively overwinter plants in areas protected from the wind, such as south facing walls or in greenhouses, thinking that the extra sun and warmth such spots provide will enable them to survive the winter. Understanding that warmth is good, but direct sunlight is bad means one has to find a balance between the suns warmth on the one hand, and frost and humidity on the other.
In my greenhouse I have copied techniques from Eliot Coleman’s The Winter Harvest Handbook in which he describes that adding a floating row-cover to crops in a greenhouse adds another 2-3 degrees of warmth and protection from evaporation. I found not only this to be the case but a floating row-cover also gives a bit of protection from direct sunlight, which as we have seen can burn kale in the spring.
It is now late winter and we have had a few days of fairly strong direct sunlight. On such days the temperature can be minus 10 Celsius outdoors, but plus 10 in the greenhouse. It seems that the kale under floating row-covers manages to not get burned by the direct sunlight, while the kale that is exposed to full sun is showing signs of sunburn. I have now set up a sunscreen to protect the kale or moved them into the shade. I hope this will be enough to keep them alive till conditions are good enough for them to put on new growth.
Another benefit of keeping kale protected using these old methods relates to humidity. The greenhouse is considerably dryer than the exposed outdoors (as I would imagine a crub would be to a lesser extent). My experience is that kale that has too much mulch around the stem rots as it thaws. In the greenhouse I mulch around the kale but keep the stem totally dry and free of mulch to avoid rot at the base of the plant. If the plant is covered in ice at the base, it is less likely to survive the cycles of thaw and freezing that we experience for months on end in Norway.
It’s early years yet in experimentation with kale growing in the North. I am thankful that Amy Lightfoot took the time to record and pass on the knowledge from the old growers in Shetland.
You can find the Norwegian text here:
Lead image: Shetland kale and Tree Collards in the south facing wall, after overwintering in the basement.
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