Winter is coming (at least, up in the hemisphere where I live it is), and thoughts are turning to vegetable garden season extension. Winter season extension in the vegetable garden is about finding the warm spots, the frost pockets, the materials with sufficient thermal mass, just enough thermal inertia, and perhaps a little structural intervention in the form of cold frames, row covers, low tunnels, or high houses.
Warm Spots and Frost Pockets
Warm spots face south, sure, but they are also protected from the dominant winter winds, which in most of the
temperate northern hemisphere blow in from the north and west. A little bit of a ‘roof’ helps hold in heat too.
The sun is coming in at a greater slant now, so a pine tree (which will hold its leaves through the winter) that overhangs the site slightly will increase the number of frost free nights for your remaining vegetables.
Frost pockets are downhill from everything else. On an open slope, the cold air keeps rolling down, but if there is anything to block its advance, it pools up right there. (This was likely the overriding factor in the trees chosen for the courtyard of the Bibliotheque Mitterrand. )
The lowest point in the yard, that’s obvious, but also the uphill side of shrubbery and fences- even if they face south! The ideal situation for winter season growing without any built structures is a south facing slope with a U of trees opening on the downhill side.
As the cold air sinks, the warmer air will rise up the slope. Not only will the trees divert the frost to roll away from the garden site, they’ll catch that warmer air as it rises, and will protect the site from the winter winds.
Materials for Season Extension
While I will cover this further in another post about apartment farming and the microclimates of balconies, the basics are this: you need thermal mass, but without thermal inertia. Clay bricks have thermal mass, but not too much thermal inertia, which means they will absorb heat all the day and then return it for the bulk of the night. The opposite would be marble walls. Marble has mass: will absorb heat, but it is totally thermally inert: it will never ever give that heat back.
Earthen walls (adobe, rammed earth, etc) also have thermal mass, but somehow they are always a little cooler. I suspect this is a humidity issue. Wood lacks thermal inertia- it’ll absorb and return heat beautifully, which is thermal mass, but it lacks enough thermal inertia. About 30 minutes after sundown, wood has essentially finished spending all solar heat it took in all day.
Structures for Garden Season Extension
There are a variety of structures, mostly distinguished by height and materials. Greenhouses are often glass and definitely heated (to above 50 degrees). Cold frames, being generally wood and glass construction, are like tiny unheated greenhouses.
Floating row covers are fabric strips intended to help hold in the heat of the soil rather than letting it dissipate. When people throw sheets and blankets over the tomatoes in the fall, this is what they are up to. The downside is that frost that comes into contact with the upper surface will transfer its freezing effects through to anything touching the underside of the cover. The better bet for anything beyond the most minimal season extension is a low tunnel.
Low tunnels are like row covers, but they don’t float. Instead, they are held up by short (3 feet / 1 meter, give or take) high hoops. They also aren’t fabric, but more likely a thick poly plastic. High houses are high tunnels, anywhere from 8 to 12 feet or more tall. Definitely big enough to walk into, almost always made of poly plastic over bent metal hoops, and never heated, certainly not above 45 degrees.
There are a lot of design considerations for these things, more each time you go up in size, but starting with a basic cold frame is entirely reasonable and very very doable. Tomorrow: two versions of how to build a cold frame.
One final note: all of these structures require ventilation. That’s really actually the biggest problem. They are so good at holding heat that they end up baking the plants inside. You’ll need to plan in some leaks, even if it means getting set up by you before you head out for the day and then plugged up for the night when you get home.