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Controlling Frost Pockets

 
Jesse Philips
Posts: 4
Location: Ontario, Canada
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Hello all,

I've heard, on the topic of starting your homestead with earthworks, that adding texture to the terrain is important for (among other things) "controlling frost pockets".

So, say you had a flat pasture. Easy to break up for rotational grazing, easy to convert to traditional monoculture garden beds/plots, fairly consistent across the whole area. Obviously, water doesn't move well over the area, you aren't creating much in the way of warmer microclimates, etc.

With your earthworks, you're adding texture to the landscape. This creates slope to catch more sun with, swales to move more water with, wet zones and dry zones and edge effect. But what I don't understand is frost pockets. With north facing slope in mostly shade (Northern hemisphere) and wind falling down the slope, you're creating areas that are more prone to frost. As I'm in zone 5, I've already got more frost than I know what to do with.

Does "controlling frost pockets" simply mean pushing them into these low, north facing zones to hold off on frost in the ideal areas longer? Is it just a deferral against a flat pasture which would all frost simultaneously? Or am I missing some key benefit to this, some oversight about how these frost pocket zones can be put to use?

Thanks!
--J.
 
Ian Rule
Posts: 84
Location: Nevada County, CA
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In my north face garden, Ive found it helpful to think of cold air like molasses that falls downward through the night... each 10' high terrace level averages 5-7 degree colder than the one above, with the house at the bottom. Oops! However, just giving it a low lying pathway to ooze downward should keep the worst of it off. Sortof like a Snow Cave/Cold-air sink


You will likely freeze in a snowcave without giving the cold air a low place to go. Same with creek basins/rivers... camp next to them at your own risk :p
 
Jesse Philips
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Location: Ontario, Canada
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Hey, thanks for that! So is the temperature at the lowest point the same temperature it would have been if it was flat, and everything elevated is just warmer? We aren't creating even-colder areas, just places for the already-cold air to move into, warming the rest?
 
Ian Rule
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Location: Nevada County, CA
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I wish I could be more helpful than "it depends".... Im also in the mountains, so I shouldnt speak about to field life. Wind, temperature and humidity fluctuations can all alter what happens at any given time, I assume that like 'solar income' theres generally X amount of brutally cold air that arrives nightly depending on cloud/tree cover and wind - give it a place to go or itll find a place to go. If the sun doesnt hit the cold sink, it could remain frozen all day - like my bottom terrace ~. ~

Perhaps to assist this you could pair it with some sun scoops/rock walls/hugelkultur? Ive also used hugels to redirect cold air with ~great success.
If its all totally flat, maybe a beefy rain garden? Lush and moist in the summer, frozen in the winter....theoretically!
 
Matthew Lewis
Posts: 39
Location: Canada
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With our property which has quite a few different hills and elevation changes as well as a low Creek area I was going to observe where the frost pockets occured for a year or so before coming up with the design. The intention being to enhance the warmer areas with earthworks and the colder areas will be planted with hardier species.

With a relatively flat area I'm not really sure what the design process would be like. I'm curious myself what would happen. Would creating southish facing mini slopes like hugels warm the entire area or just concentrate the cold air between rows? I'm interested if anyone has an answer.
 
Glenn Herbert
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Location: Upstate NY, zone 5
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Texture like hugels would give south-facing slopes more direct radiation from the low winter sun, and in my experience such places will have the snow melt off much faster, letting the dark earth absorb more heat than flat ground nearby. The north slopes and anything shaded by them would be colder, but I think the south slopes would be ready to plant distinctly sooner in spring.
 
Taryn Hesse
Posts: 41
Location: Rainy Cold Temperate Harz Mountains Germany 450m South Facing River Valley
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Hi

Frost moves slowly downhill. blocking it will alter its direction or trap it. it does flow down natural waterways as an outlet but because its warmer there the moisture doesn´t crystalize as often and is harder to see. having flowing\ standing water will reduce frost formation by keeping the air moisture warmer and therefore not crystalizing onto plants in fall. If you have berms or hedges or fences or walls that are not straight you can direct frost. if you make a "U" shape with the open end uphill you will collect frost in a frost pocket. if it is an "n" shape with the open end down hill you can drain frost away. if there is a depression it will fill with frost. Draining the frost away means that moisture will have to travel further downhill to cool and form crystals. Depending on the temperatures (air temp, soil temp, and any water bodies, tree cover radiates heat, etc) you can create microclimates in spring and fall mostly. when its cold enough, it gets to a piont where the air moisture, or dew, crystalizes in a short time and these measures have little effect because it has no time to drain away. shade also prevents frost from melting as quickly and prevents damage to some plants by blocking sunlight. I use frost pockets to plant plants which get tricked into flowering too early from an unusually warm week in spring like apricots. The ground temperatures in these places take weeks (2 or so depending on the weather) longer to warm up and the plant isn´t  woken from dormancy as early in spring. the other shape "n" is more or less a sun trap and the frost flows around it making it warmer in spring and fall and with a canopy cover and a pool of water it is like extra frost proofing. frost pockets collect dew and suntraps are dryer in summer for the same reasons because frost is just frozen dew. so, if you find any dew collection pionts you have also found a frost collection piont as well and  can then create any necessary drainage barriers or traps using walls, hedges, earthworks, water or shade.

here is a bit more detail with diagrams but there are HEAPS of sources online.
http://www.fao.org/docrep/008/y7223e/y7223e0c.htm
 
Matthew Lewis
Posts: 39
Location: Canada
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Thanks for the responses Glen and Taryn! A lot of good information between those last two posts.
 
Chad Anderson
Posts: 5
Location: Dawson Creek, BC, Zone 2a
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The molasses metaphor is a good one. I live on a steep east-facing hill (100 foot drop over a 660-foot long 5 acre lot), sheltered from strong prevailing winds from the west (the other side of the hill is British Columbia's first wind farm.) If it's not too windy on a cool fall night, we can actually watch the uphill neighbour's wood smoke roll down our hill a few feet above the ground. Last year, we had about 6 or 7 more weeks of frost-free growing season than family about 270 feet lower, a couple miles away and nearer the bottom of the valley.

But even a few inches makes a difference as the cold air rolls down the hill. I noted severe frost damage on tomato branches that fell over the side of 10-inch raised beds, several weeks before the tops of the plants finally got killed.

This year, I'm going to try to create some of those night-time "frost fences" to channel cold air away from some garden beds. The trick on an east-facing hill, is to design them as daytime sun scoops, and night-time frost fences. Basically, I'm trying to create hot, still air all day long for some heat-loving pumpkins, melons, etc., and trap that heat there as much as possible, in thermal mass and undisturbed air, overnight.

I've just ordered some cheap min/max fridge thermometers to do some experiments with some microclimates around the yard in the spring, to guess at fall conditions with similar daylight hours, when the evil frost returns to shut us down.
 
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