Last frost dates are not the same

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.

12 Responses to “Last frost dates are not the same”

  1. Daphne Says:

    I was going to say to check NOAA for real data on historical frost dates, but that is where you found your probability information. I’ve found so many people saying when my last frost date is, but in reality I’ve lived here for a while. I know when I can plant. Each person’s micro climate is different. My last frost is earlier than my neighbors at the bottom of the hill.


  2. Cinj Says:

    Funny how everyone thinks it’s so different. I figured mine was mid May, that’s about what your links said too. Nice to have all of those resources at your finger tips, isn’t it?


  3. The Cheap Vegetable Gardener Says:

    I seem to move too often to get some good history to get my frost dates figured out. I definately agree a few miles or even feet can make huge difference. One thought I did have is to check out wunderground.com to check out someone else’s history for the past few years.


  4. Chiot's Run Says:

    I can’t officially find one for my area, it’s very rural. I can only find cities 30 miles away. So I’m trying to figure it out myself. Calculating this is difficult as well because of microclimates. I know that my front gardens are probably a zone ahead of my back gardens because of their sloped south facing nature. I’ve had tropical plants overwinter in my front flowerbeds (and I’m a zone 5). This is a great reason to keep these stats for yourself. Then you can calculate it much better for yourself.


  5. Red Icculus Says:

    Farmer’s almanac be damned. I put them out in mid-april in the midwest. If I hear it is going to frost, the plastic sheeting comes out. I have had no problems thus far.


  6. The Start of a Wonderful Past time | Computing Central Says:

    […] The Cheap Vegetable Gardener points out in his post “Last frost dates are not the same” […]


  7. Ben Says:

    I guess it depends on what you are planting and how hardy that plant is. Tomatos and peppers, I wouldn’t transplant outside before the 5/14 date.

    I am trying to figure out how hardy the cosmos are that are starting to fill up enough space in my seedling area to want to transplant. Zinnia’s too…

    It seems the best authority I’ve seen for Seattle is the Seattle Tilth planting guide which puts transplant dates for these flowers in ‘late april’. I am just going to put them out when they get their 2nd set of true leaves and see what happens.


  8. How to make your own cheap weather station Says:

    […] results and have current and historical data to have some better info to better guess your first/last frost dates or when it is safe to bring out your tender seedlings you are growing […]


  9. Sara Says:

    I was just doing a search for frost dates when I found your page! I mainly wanted an approximate date that I could count back from for indoor planting, instead of a date that I can put plants out. I have found it generally isn’t too difficult to figure out if it is going to frost closer TO the actual time of transplant, but I have had issues with the indoor planting times I have been given in the past. And those silly percentages are, well, silly. I agree with your strategy. Thanks for the informative post! 🙂


  10. Where To Find Your Last Frost Date | One Green Generation Says:

    […] of these resources will have different dates (as the Cheap Vegetable Gardener points out), plus this is the average last frost date – which means sometimes it happens earlier and […]


  11. SMB Says:

    I also found your site by doing a search for frost dates, after I realized I’d gotten wildly different dates for Seattle’s last spring frost from different publications. Thanks for pulling all of this information together for us! I’ve bookmarked this site and will be back.


  12. How to determine your own personalized last frost date from local weather station - The Cheap Vegetable Gardener Says:

    […] was a discussion in comments on my previous average last frost post on how to determine an accurate last frost.  A great point was made by Daphne where every garden […]


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