Last Frost Dates Are Not The Same - The Cheap Vegetable Gardener

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Last frost dates are not the same

16.8 years ago last frost date

It came to my attention it seemed every time I see my average last frost date it comes up as a different date.

Average Frost Date (Seattle, WA) Location
3/10 The Old Farmer’s Almanac
4/20 Victory Seed Company
3/22 Ed Hume’s Seeds
4/15 USDA Zones
5/14 USA Gardener
3/25 Clyde’s Garden Planner
3/24 Garden Web

Now the humorous part of this is I didn’t go out trying to find as many non-matching dates as I could, these came in order from my Google query for “last frost dates”.

Under conventional wisdom, this number should be simply an average of the last frost dates for the past 30-40 years to give you about a 50% chance of avoiding frost given past history.  So it seems really strange why these numbers vary so much.

Then I came across U.S. Climate Normals this site includes no only the dates but the probability of them being true.  So if you are a betting man/woman (or just impatient) you can press your luck and plant with variable odds.

For my area (Seattle, WA) I have the following options:

Probability Level
Threshold (°F) 90% 50% 10%
36 °F Mar 27 Apr 11 May 18
32 °F Feb 13 Mar 10 Apr 22
28 °F Jan 01 Feb 25 Mar 20
From the information above, only The Farmer’s Almanac had the number I was really expecting.  But given the information above and how late frosts have “bitten” me in the past I think I will give myself a couple extra weeks and plan on my last frost being Mar 24th to hopefully avoid and hard frosts for tender seedlings.

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Tags: garden seeds, outdoor plants

Tag » When Is The Last Frost In Seattle