Gardening: Foiling the Frost

Editor’s Note: I’m reprinting this column from 11 May 2022 because it deals with frost and we’ve been dealing with frost lately, too.


What to do this week

The unseasonably cold weather lately doesn’t encourage planting, but soon the days will warm enough that gardeners will engage in their annual game of frost roulette. We all want to get a jump on the season but as we found last June, it can be chancy guessing at the last expected frost date, even for Environment Canada. A good rule of thumb is to wait to plant warm season annuals until the apples and lilacs are in bloom. As gardeners, we have had a lifetime of trying to learn from our mistakes – but nature has had thousands of years to figure things out. Trees will make a better guess at the right combination of light and warmth required to dodge the likelihood of a killing frost. They are not infallible, as last year’s frost on the first day of summer demonstrated, but they get it right more often than we do.

Photo by Adleraugenblick, CC BY-SA 3.0, via Wikimedia Commons


Still, it is tempting to get a head start, when the weather starts to feel warm and settled. If you want to play that game, you have to hone your powers of observation in order to beat the odds – or at least cover things up when a late frost threatens. For a start, get to know your property. If you are lucky enough to be on a large body of water like a lake or the ocean your season may be cooler overall, but your frost-free will usually be longer. Inland, the days will get hotter but the nights will cool off more precipitously.

Look around for low spots. Cold air sinks and these depressions are rightly called frost-pockets. A little elevation goes a long way to lift the danger away. Be sure draining cold air is not impeded by walls or hedges. Sometimes a south-facing wall can hold enough heat to afford some protection, but not if it borders a north slope and the cold air pools against it. That is what I have on the south side of my house, and the pole beans I plant there every year get frosted at the bottom first before the top leaves get hit. My garden field is also north east facing, and I do not plant the cold-sensitive squash family at the bottom of the hill, no matter how hot it gets on a summer day.

To predict frost, and when you might have to scramble to collect sheets and blankets to cover tender transplants, it is important to keep an eye on the sky. Cloudy weather can be cool but rarely will allow a hard frost. Drizzle and rain are also your friends. Some large commercial operations set up sprinkler systems when frost threatens and this is quite effective, if your water supply will allow it. I have done it myself and seen the plants outside the sprinkler zone turn to ice while the ones getting watered survived nicely. A wind will also be helpful, as air movement keeps frost pockets from developing. But if the wind dies down just after sunset and the air is clear, watch out, the temperature is going to dip.

It is the rate of change in temperature that is most important to watch. If the temperature drops comparatively little between sunset and bedtime at 11 pm, you are likely safe. It can be helpful to keep records, if you are methodically minded. Take the temperature at 7 pm and 10 pm each night. If the temperature drops only a couple of degrees in that time, frost is unlikely. If it drops more than 4 degrees, get out your collection of old sheets.

Photo by Nikolay Sizonenko, CC BY 3.0 via Wikimedia Commons


Another more accurate way to assess frost risk is to measure the temperature at dewpoint. Dew will form rapidly if heat from the ground radiates quickly into the air, and this in turn will be affected by how much humidity is in the air. Generally speaking, the greater the drop in temperature, the drier the air. Dewpoint temperature is taken most precisely with a sling psychrometer at 9 pm each night. This instrument has both a wet bulb and dry bulb thermometer and dewpoint is calculated using a chart and the difference in readings between them after swinging it in the air for five minutes. If the dewpoint is lower than 2 C, frost will occur.

As mentioned before, if you took a chance and planted things out just a little too soon, you can protect the plants by covering with sheets, canvas or floating row covers. Try to cover as wide an area as possible and try not to let the material touch the plants. It also helps to remove mulches. Bare untilled soil will absorb more heat during the day than mulch or sod. Better yet, try to be patient. Often, if the weather is cool, those heat-loving plants just sit and pout and get purple from cold stress. I don’t plant out the squash family until the beginning of July in Cape Breton and the coddling that extra time in the greenhouse gives them makes all the difference. Beans and potatoes can be planted on or before the government’s last frost date since they spend their first week below ground anyway. But tomatoes, peppers and basil are strictly told to cool their heels until those apple blossoms say it’s time.



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.