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How to determine your own personalized last frost date from local weather station

There 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 can have its own microclimate so the date you find online or in a book could be many weeks off for your microclimate in neighborhood.

To be able to predict your last frost date accurately it takes some history, if you have lived in your current residence and happened been capturing your temperature data accurately by memory or paper you may the information to predict your last frost date.

For people that are relatively new to their residence, like myself, I felt there I had very little information to determine an accurate prediction of my last frost date.  This was until I remembered Weather Underground which logs results of schools or private individual that register their weather stations.

After a quick search I was able to find someone who had a weather station in my neighborhood.  After doing a custom query of the full range of the of the weather station, which in my case was a little over 4 years, I had all the information I needed.  By carefully adding a horizontal line at freezing using a graphics program (Microsoft Paint) I could easily see where the low temperature dipped below the freezing mark.

tempChart

From a quick glance at the data it appears that the first week of April is a pretty safe date for past few years.  But if you look the the irregular low temperatures we have had this month this might be harder than I thought.

Now of course your neighbor may a different microclimate than you, but should get you in a better ballpark than the generic number you find fro your city.  You could always spend $500-$1000 on your own weather station to get some better accuracy, but for me seems close enough to me.  Though the weather station would be a really cool gadget to have.

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.

Average last frost dates are only right half of the time

Our last frost date in my area should have been March 24th but given the frosty mornings this past week and the fact we had snow coming down most of today it looks like this year our last frost date will fall on the other side of the bell curve. Unfortunately, I was optimistic with some of the great weather we were having earlier and planted my tomato seeds for a much earlier move outside. The poor tomatoes are still growing in my PC grow box but starting to not look so good. As you can see from the picture below my tomato plants are in some dire need of some non-artificial light. I am afraid I might end up buying my tomato plants at my local nursery (yet again) this year. Vegetable gardening is definitely one of those hobbies which it does not pay to be a too optimistic.