September 22 brought the first day of Autumn – and we had seen the first frost already. Not really a killing frost, for most of our garden plants have enough sugars within their cells to resist beyond the initial 32 degrees. The zucchinis and cucumbers seem to be the first to show frost damage . . and those plants contain a lot of water.
In the eighties, we kept and protected a copy of the 1941 yearbook of agriculture – Climate and Man. It was valuable back then – it had the nation’s climatic summaries for spots within each county – in Lincoln County, the records existed for Fortine, Libby and Upper Yaak. There were 32 years of daily weather records for Fortine. The average date for the first killing frost was September 8, and the book showed an average 102 day growing season, with 17.43 inches of annual precipitation. I had thought that Fortine’s inclusion was due to Winton Weydemeyer’s volunteer efforts in weather monitoring – but as I look at the data going back to 1909 (when Winton was 6 years old) I realize that someone else started the job that Winton continued. Still, our climate records are a legacy of Winton’s quiet efforts.
Climate and Man is not so valuable anymore – https://morningchores.com/frost-dates/ – provides links to finding your frost dates by the Hardiness Zone method, and by NOAA records. So I type in 59918 (Fortine’s Zip) and find:
Your results:
Fortine, MT (59918)
Nearest Station: FORTINE 1 N, MT
Last Frost Date: June 4
First Frost Date: September 9
You have at least 96 freeze-free days in a year.
Intriguing – Fortine’s growing season was 6 days longer before World War II . . . but that is probably more indicative of projecting a relatively small (39 years) partial duration series than indicating global (or even local) cooling and climate change.
The link also provides some probability information:
To show the dates above, we use probability level of 50% and frost temperature of 32°F. If you want to start your garden earlier in the spring with a higher risk of frost, or later with a lower risk, use the following table:
Late Frost Date
°F 90% 50% 10% 36° Jun 2 Jun 28 Jul 24 32° May 14 Jun 4 Jun 26 28° Apr 28 May 17 Jun 6
Early Frost Date
| °F | 10% | 50% | 90% |
| 36° | Aug 11 | Aug 24 | Sep 7 |
| 32° | Aug 28 | Sep 9 | Sep 21 |
| 28° | Sep 1 | Sep 18 | Oct 5 |
Probability level (90%, 50%, 10%) is the chance of the temperature to go below the threshold after the last frost date or before the first frost date. Using a lower probability means you have lower risk of unexpected frost damage but shorter gardening days in a year.
The importance of the 28 degree frost is that it is generally the killing frost – the sugars and other organics in our garden plants mean that different species have different critical temperatures. Texas A&M Extension Cold Tolerance in Vegetables | Archives | Aggie Horticulture provides this explanation about the temperatures that kill our garden crops:
“This is very difficult to do and be accurate since cold tolerance depends on preconditioning. For instance, if broccoli has been growing in warm conditions and temperatures drop below 22 degrees F., it will probably be killed. If these same broccoli plants had experienced cool weather, they would probably survive the sudden cold.
In general, a frost (31-33 degrees F.) will kill beans, cantaloupe, corn, cucumbers, eggplant, okra, peas, pepper, potatoes, sweet potatoes, squash, tomatoes, and watermelon.
Colder temperatures (26-31 degrees F.) may burn foliage but will not kill broccoli, cabbage, cauliflower, chard, lettuce, mustard, onion, radish, and turnip.
The real cold weather champs are beets, Brussels sprouts, carrots, collards, kale, parsley, and spinach.”
Spring was cold and wet this year, and the killing frost took a long time to reach my garden. Your results may differ.
Trego's Mountain Ear 