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Freeze Data: Ritzville, Washington

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(Note that all data are the most recent available.)

What Is "Freeze"?

The freezing point of water is 32° Fahrenheit (or zero Centigrade): so much everyone knows. But the definition of a "freeze" for gardening purposes is not so simple; worse by far are the utterly (and literally) vital questions of what constitutes a "first-frost date" and a "last-frost date".

As all informed sources agree, there is no precise definition of a "killing frost" because different plants react to cold in different ways. One can find various approximations in the literature; here is one set commonly found:

In the charts below, there are probabilities given for several minimum temperatures: 36.5°, 32.5 °, 28.5 °, 24.5 °, and 20.5 °. While the purpose of this page is to give you information from which to draw your own conclusions, it is worth taking a moment to consider what a "growing season" is, so that those in other areas can apply the same methodology to their particular data.

Let us first look at using 32.5° as our standard for a "frost". What then? Well, as one source noted, "In the historical weather data, a frost occurred after the official last-frost date 10% of the time." That implies that official "last-frost" (and, presumably, "first-frost") dates are set by the 90% probability date, which makes sense: there is little value in using the absolute historical worst-ever dates, because those will be quite rare. When we look at the tables below, then, we are looking for the 10% probability dates, because that means that 90% of the time those will mark the first or last (as may be) frost dates. On that basis, we see 6/2 as the 90% probable date for last frost and 9/17 as the first frost; the interval, our "frost-free days" period, would thus be 106 days, which is not a lot.

If we change our standard for "frost" from 32.5° to 28.5°, on the ground that we are concerned with "light to moderate" freezes as defined above, the dates and count become (again from the tables) 5/16 to 11/1, which is 168 days. That is much nicer, but remember that it does not apply well to "frost-tender" plants, which is the sort we are usually most concerned with when looking for long growing seasons.

An interesting sidelight on the different bases we use as "freezing" is the growing-season "center" date. At 32.5°, the half-way date is 7/25; when we use 28.5°, that date shifts materially, becoming 8/8. In other words, the early and late frost dates are not symmetrical: the early date changes by 17 days, while the late date changes by 45 days. Thus, if we want to take chances with our plantings, we are better to take our risks on the end date rather than the start date. Putting that in practical terms, don't rush your early plantings.

If you review the vegetable-type choices set forth on this site, you will find few or (as best we recall without looking at each) none with a growing season over 106 days in length. It thus behooves us to be conservative and assume June 2nd as a "last-frost" date and September 17th as a "first-frost" date when planning our planting and transplanting dates. Indeed, combining the ideas that we are best to minimize risks at the front end and that 106 days is more than anything we are planting is going to need, we are perhaps even better off to take something like June 7th or so as the "last-freeze" date in our reckonings.

(Of course, what date to use for those cool-weather crops for which the general advice is to plant out "as soon as the ground can be worked" is another matter yet.)

Spring Freeze Probabilities

Probability of Overnight Low Temperature Less Than X, by Date
Temperature °F. Earliest 90% 80% 70% 60% 50% 40% 30% 20% 10% Latest
36.5 05/10 05/17 05/21 05/30 06/06 06/09 06/14 06/19 06/29 07/01 07/18
32.5 04/04 04/26 05/05 05/09 05/12 05/16 05/20 05/25 05/29 06/02 06/13
28.5 03/17 04/13 04/16 04/26 04/28 05/02 05/05 05/06 05/11 05/16 05/29
24.5 02/08 03/06 03/19 03/28 04/05 04/11 04/15 04/18 04/25 04/30 05/29
20.5 01/03 01/31 02/12 02/16 03/03 03/04 03/09 03/23 03/30 04/18 04/30

For example, on May 5th there is a 70% chance that the nightly low will be at or below 32.5 °F., but by June 2nd there is only a 10% chance of seeing an overnight low below that temperature. For temperatures in between those shown, one can interpolate by gosh and by golly, or check this graph of the full data.

Frost-free periods graph

Fall Freeze Probabilities

Probability of Overnight Low Temperature Less Than X, by Date
Temperature °F. Earliest 10% 20% 30% 40% 50% 60% 70% 80% 90% Latest
36.5 08/18 09/05 09/08 09/15 09/17 09/20 09/22 09/26 10/01 10/06 10/18
32.5 08/23 09/17 09/21 09/23 09/27 09/29 10/02 10/07 10/14 10/17 10/24
28.5 09/14 09/26 10/03 10/08 10/11 10/15 10/17 10/22 10/27 11/01 11/17
24.5 09/24 10/13 10/17 10/25 10/30 11/03 11/05 11/09 11/13 11/17 12/07
20.5 10/09 10/28 10/31 11/06 11/11 11/15 11/21 11/25 11/30 12/12 **/**
**/** = there has been a year when the minimum temperature didn't go below the threshold temperature during the July 31 to Dec. 31 period

For temperatures in between those shown, one can interpolate by gosh and by golly, or check this graph of the full data.

Frost-free periods graph

Freeze-Free Probabilities

Probable Length of "Frost-Free" Season For Various "Frost" Temperatures
Temperature °F. Shortest 90% 80% 70% 60% 50% 40% 30% 20% 10% Longest
36.5 40 71 81 90 99 105 108 115 123 133 155
32.5 84 115 120 128 133 135 139 145 149 166 183
28.5 120 140 152 158 164 170 175 178 182 190 229
24.5 149 171 184 197 203 209 212 222 230 249 262
20.5 183 206 231 239 243 251 261 279 284 306 ***
*** = there has been at least one year when a minimum temperature below the threshold was not recorded

For temperatures in between those shown, one can interpolate by gosh and by golly, or check this graph of the full data.

Frost-free periods graph

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