Years ago, I read something in a Dave Wilson catalogue that changed my understanding forever: cold air is like water. It sinks.
Since heat rises, this makes sense. (Cold air molecules are more tightly packed than the molecules in warm air. This is what makes cold air heavier. ) These physical facts make a lot of difference to your plants.
If you live on a slope, like everybody in my area does, you can imagine the cold air running down it just as water does. If there are depressions in the ground–bowls or valleys–the cold air will pool there. If there’s a steep downslope, the cold air will keep on flowing until it finds a place to rest.
When you understand how this air flow works, it’s easier to take advantage of it–or to mitigate it, if it’s a problem. Cold air can back up behind houses and fences, the way water gets backed up in a dam. (Check the bottoms of fences, hedges, and walls for frost; if you find it, that’s a sign that this may be happening in your garden.)
All this can mean the difference between frost and no frost, which can mean the difference between a dead plant and a live one, or a bearing plant and a fruitless one. In my area, almonds (and often peaches and plums) don’t bear, because their flowers are way too early for the climate. They bloom, frost hits, boom.
Some people recommend planting these marginal early-blooming trees on a north slope: that way their sap stays cold and sluggish for awhile longer, and they bloom later, maybe late enough not to get frostbit. We live in hope.
If you allow for ‘drainage’, you can warm up those areas: an opening along the barrier will act as a kind of sluice, letting the cold air drain down the slope, and keeping the planting area by the barrier marginally warmer.
Even people who live in flat areas generally have some contours in the ground; dips and streambeds surrounded by trees; a slight grade to the ground behind the house. This allows cold air to back up in the same way. (Since it’s been a long time since I’ve been in a flat place, maybe someone else can answer this question: does a flatter gradient mean fewer degrees of frost, or not?) Just look at your ground and imagine water flowing over it, or a big rainfall. Where would the puddles and pools be? These are your cold spots.
Besides “draining” your cold spots, you can also just avoid them when you plant something that thrives on heat, or really should be growing in a warmer climate. I’ll have to test this, but it seems possible to me that it also works the other way around: if you have a plant that really prefers a cooler climate than the one you’ve got, doesn’t it make sense to try putting it in the cool spots?
Anyway, back to mitigating cold–besides draining, you can do it in the time-honored way, by wrapping plants in burlap and bundling them up with leaves, pine needles, or straw. Or, you can do it the modern way-with a spray that protects leaves from sunburn, or with hot caps, water walls, frost blankets, and other coverings.
Another possiblity is using passive solar heat by putting up a west-facing stone or concrete wall. The wall will absorb the sun in the day, and release it gradually at night. As long as the wall is “drained”, it will keep plants next to it just a little warmer . Warning: if you are in the shade (or in a cloudy climate) it doesn’t matter what direction the walls face; they will always be cold and clammy. There’s nothing quite as chilling as really chilled stone.
If you’ve played with this idea in your garden, I’d be interested to know how it worked for you. I don’t care if it was a success; I just want to know what you learned.
ancient Dave Wilson catalogue, since buried in the archives (read: piles of printed matter)
and from my local nursery: Weiss Brothers Master Nursery newsletter, Nov/Dec 2008