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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Guard against early frosts


A cat wanders through a Spokane Valley garden past tomatoes covered with a Remay floating row cover. 
 (Brian Plonka / The Spokesman-Review)
Pat Munts The Spokesman-Review

Several weeks ago I asked if anyone experienced a frost in late July or early August. I heard from several people north of Spokane that at least the tops of some their sensitive plants had been hit.

That’s the reality of Inland Northwest gardening.

For the rest of us, frost may not be far off. Around Spokane and Coeur d’Alene, the first average frost date is Sept. 15. We’ve been spoiled the past few years with long autumn seasons. Our luck is going to run out someday.

Protecting plants from frost is a matter of retaining one to five degrees of heat for a few hours before sunrise. That’s all. Most of the first frosts in the region tend to just touch the freezing mark, then bounce back.

Hard, killing frosts usually come a few weeks later. Those intervening weeks between frosts can be just enough to ripen the last of the tomatoes, squash and peppers.

There are a couple of different kinds of frosts.

An advective frost can occur when a cold front moves into an area, causing temperatures to drop below freezing across a broad area. The cold storm that came through at the end of August was an example of this.

They can be extreme, destroying crops quickly.

Radiation frosts are the second type. They occur on calm, clear nights that lack any cloud cover to hold in heat. Usually these frosts will fall just a few degrees below critical levels, making it very easy to thwart them.

Radiation frosts are the ones we usually have to watch for.

The location of your garden will have a major impact on if and when a frost will hit. Gardens in low spots will be affected first. My garden is about 10 feet above a small valley that has had frosts while the garden above it wasn’t touched. Gardens on tops of higher hills will also be affected first.

Keeping heat close to your plants will help stave off cold. Water holds a lot of heat, so keep your garden well watered when frosts are likely. Fill 5-gallon buckets of water and place them among tender plants to soak up heat during the day and share it with the plants at night.

Place large dark blocks of wood near plants to do the same thing.

New developments in weather technology can take some of the uncertainty out of knowing when a frost is arriving in your garden. Temperature sensors are available that can be placed in your garden and then either hard-wired or served by a wireless connection to a monitor in the house.

The monitor can be set to sound an alarm if the temperature drops close to freezing. There are several models available, some well under $100.

If frosts threaten, get your frost covers on late in the afternoon or early evening to trap all the heat you can. See the accompanying list for the most efficient types of cover. Be sure to anchor them down so that the wind doesn’t blow them off.