School is back in, the trees are changing color, and bugs are (attempting) to move inside: fall is unavoidably here. Some of you may still be weeks away from killing frost, but we’ve already had a couple of light ones here in Alberta.
Last winter I geeked out and did a bunch of reading on frost, thinking some theory might help my practical application this year. I had visions of early planting, and harvesting veggies and displaying flowers well into October. Not much of my vision materialized, as I still live in the real world, and I definitely haven’t reached the caliber of Niki Jabbour, but it’s cool to understand more about how weather works. And spreading sheets over the pumpkins with a flashlight after dark last week was totally worth buying them another couple of weeks’ growth.
Here’s a few things I’ve learned this year that may help you predict, and hopefully outwit, Jack Frost, and buy yourselves a little more time in the garden.
First off, find a weather website you like or buy one of those weather stations from the hardware store. The Weather Network actually has a Lawn and Garden forecast, including frost predictions and watering advice. A little info goes a long way.
Next, be prepared for frost. Have some old sheets or lengths of burlap ready to cover plants, as well as something to weigh them down against the wind. Small straw bales, a cold frame, or cloches will also do the insulating job. Have a spot in mind in the shed or garage to move containers of annuals to when you get a frost warning. Do a little research if you’re not sure which of your plants need frost protection. Bronze, hairy, or compact plants, as well as those closely spaced, will be naturally more protected, but don’t count on most annual flowers, squash, tomatoes, peppers, or corn to stand up to frost without being covered overnight. Carrots, beets, and most members of the cabbage family, as well as many other vegetables, don’t mind frost.
If you want to actually predict a frost, the first thing to do is look up. Clear, calm skies are a sign frost make strike, especially if afternoon temperatures start falling fast. Frost is less likely to occur under a cloudy sky, or when there is fog, as the day’s heat is trapped closer to the earth. This is part of why covering plants protects them–it traps some of the heat from the earth close to your plants.
Then assess the wind. If it’s strong, especially if it’s coming from the northwest, cover things up. Movements of large, cold air masses often bring on killing frost. But very still nights allow the coldest air to settle to the ground, also risking the temperature for your plants to hit zero. A light breeze will keep temperatures higher, unless that wind itself is below freezing.
Higher humidity decreases the risk of frost. This has to do with all the high school chemistry you’ve blocked out regarding warmer air being able to hold more water molecules. (See the next bit on dew point.) I’ve known people to water in annuals to protect them when a light frost threatens, though I’ve never tried it myself. The science backs them up: when the air is dry, evaporation sucks warmth out of the soil, making for chilly plants. By attempting to increase the moisture available, these gardeners “insulate” plants from the cooling effect of evaporation. Same goes for the old standby of covering things–it keeps the moisture close to your plants.
So here’s the real nitty-gritty of how frost actually appears–feel free to skip this paragraph if you’re not into scientific explanations. Basically, the dew point is the temperature at which the moisture vapor in the air will condense back into liquid, based on the given factors, most importantly, humidity. If the air temperature drops to the dew point above freezing, you get wet summer grass and diamond-scattered spiderwebs. If the dew point is below zero, and the temperature drops to it, the water vapor is changed directly into solid form–ice–and you see lovely feathery crystals on the edges of everything. Now, if the dew point is below zero, and the air temperature drops below zero but doesn’t reach the dew point, you won’t technically have frost, but tender plants will be damaged by the freezing temperatures. The other thing that can happen is the water vapor condenses at a dew point above zero, leaving dew, but the temperature continues to fall below zero, forming a coat of ice. So if you know the dew point and the overnight low, you can predict a frost.
If you’re really of a mind to change your relationship with frost, you may want think right down to the bones of your garden, your location and its physical features. Ever noticed that your neighbor can get white tipped lawn when you don’t? Higher altitude increases frost risk because the air is thinner and the average air temperature is lower. But low areas in the garden can be more susceptible to freezing because cold air is heavier than warm and tends to sink. Gentle slopes that expose the garden to the sun are more protected, open spaces plagued by wind are not. Houses, fences, and water bodies can be heat sinks and/or wind breaks that protect from cold air. Allowing places in the garden where wind can move, and hence, cold air escape, will also be protective. And that old saw about starting with the soil if you want better plants? It’s true in this case too: Fertile soil holds more moisture and passes it into the air more efficiently compared with sandy or deficient soil. And we know that humidity is good.
There. Don’t you feel smarter?