Gardening Tips: Frost Warning?

Editor’s Note: This column first appeared on 26 April 2017


What to do this week

Optimistic beginner gardeners are often tempted to plant after a few days of great weather in May. I think it is important to let you know how to reasonably predict the risk of frost, just in case you find yourself in that situation. We shouldn’t have plants outside for another month or more, but learning how to predict frost now will have you well prepared.

Frost. (Photo by Thomas Bresson [CC BY 3.0 (], via Wikimedia Commons)

Frost. (Photo by Thomas Bresson, CC BY 3.0, via Wikimedia Commons)

Generally speaking, the condition that makes frost likely is clear, still air. Clear means no humidity. Still means no wind. If there is even a little breeze, it mixes the warmer air that is radiating from the earth with the colder air above, so the frost doesn’t have time to settle. If it is very windy, then you might be getting a cold front, but let’s not think about that. Even if you have had wind all day, but the wind dies down in the evening and the temperature drops quickly, you can pretty well guarantee that you are going to get frost that night.

The other condition to watch for is low humidity. Low humidity actually increases the likelihood of frost. If you have cloud cover, or it is a bit drizzly, you are pretty safe.

Location is also something to think about. If your garden is up on a hill it will be better off than a garden at the bottom of a hill. Cold air sinks on those still nights. It is heavier. So if your garden is at the bottom of a hill, you have to watch it more closely. Also, if you have a hedge or a barrier of some kind running across the middle of a hill, it will trap the cool air as it is settling, and create a frost pocket there. I like to plant along the contour of a hill, but I leave spaces so that the air can drain and I don’t get frost pockets.

If you are scientifically minded, you can check the dew point reading. This is a combination measurement of humidity and temperature. You can look this up on the Environment Canada website, but you won’t necessarily be getting an accurate forecast for your particular garden. If you want to measure your own you have to find a sling psychrometer. This is like a thermometer that is wrapped in a humid fabric. You swing it around in the air. The humidity in the fabric causes the thermometer to give you a different reading compared to a dry thermometer. The wet thermometer reading is the dew point. It is this reading that is the key to predicting frost. If your result is below 2 degrees Celsius, frost is very likely.

Another thing that can influence the likelihood of frost is whether or not you are inland, or if you have a large body of water close by. Of course you can’t really move your house, but it is good to know so that you can take action. I have two farms in the family. Mine, inland in Skye Glen, and Rosie’s on the North Shore. I have a much warmer growing season in Skye Glen. I have higher temperatures in the daytime in summer, but I have a much shorter growing season because I don’t have that tempering influence of a body of water. Rosie has a much longer growing season, by almost a month on either end, but it does tend to be on average 5 degrees cooler than the farm in Skye Glen. So, it is all about knowing your particular garden, and being able to accurately predict whether or not you have to get those old sheets out to cover the seedlings for the night.

Next week I am going to get into planting trees and shrubs. It is important to get them planted before they burst out with life, and that is around now.

Featured photo: Frost by Thomas Bresson, CC BY 3.0, via Wikimedia Commons




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Market gardener, farmer, workshop leader, seed-saver, political candidate and mother, Michelle Smith has spent over 30 years coping with the challenges of our bioregion and in the process has built a store of practical and technical knowledge. The Inverness resident has served on the board of Seeds of Diversity Canada and represented Alternative Producers with the Federation of Agriculture but can do nothing about her hair. She is pictured with a head of Club Wheat, a seed that shares her approach to hairdressing.



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