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A 'New Normal' for Minnesota's Last Spring Frost Date?

Photo: Julie Weisenhorn, UMN Extension
If you had to name the date, when is the average last frost date for Minnesota? Mid-May sometime, perhaps? One expert says the unofficial data shows it's a lot earlier than you might think!

According to Dr. Mark Seeley, University of Minnesota professor emeritus and meteorologist, we can expect new normals in the last spring frost date for Minnesota. In his March 29th program on MPR Weather Talk radio, Dr. Seeley said in the last 20 years, April 20th was the average date for the last frost! 

When I spoke with him by phone recently, Dr. Seeley said,  “Since 1999, we have only had two last spring frosts in May, the rest have all been in April. The latest was May 9, 2010 and the earliest was April 8, 2006.”

Data becomes official in 1-2 years

The years 2019-2020 still need to be added to the new historical data (1991-2020) that will come out in late 2020 or 2021, but “for now we can see that the new median spring last frost date is shifting much closer to April 20.” This is of great interest to gardeners, who are anxious to begin planting.

What are the chances for a May frost?

Current weather records 1991-2010 show a 10% probability of 32°F. as of May 10th. 
Although cool weather may still occur in late April and May, the chance of a frost is decreasing. Cool season vegetables such as lettuce, carrots, cabbage and kale grow well in cool temperatures, but wait until the nights are consistently above 50°F. to plant warm season vegetables such as tomatoes, squash, peppers, eggplant and beans.  

Master Gardener seed trial picks favor hardy veggies

When selecting vegetable varieties, take a look at what Extension Master Gardeners have trialed and selected.  To learn about this year's winners and those from previous years in Minnesota, click on Master Gardener Seed Trial Results.

Author: Mary H. Meyer, Extension Horticulturist and Professor

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Unknown said…
What geographical location or zone is this "new normal" ?
Edward Watson said…
All that... and no mention of the 'still existing' so-called 'previous' average date.

That would be useful in context.

Also, it was my thinking that 30 year periods determine the time span for establishing new normals. Since when has it been 20 years?
Mary H. Meyer said…
Thanks for your patience in a reply to your question. The zone for the new normal referred to in this article is Central Minnesota or USDA Zone 4a.
Mary H. Meyer said…
Thanks for your good comments. We should have included this information: According the NOAA info, this is the current average last spring frost date:NOAA 1981-2010 Day of Last Spring Freeze
May 16-31 Northern MN; May 1-15 Central MN; April 16-30 southern MN. See:

And 30 years has been the time period for NOAA's current info, as noted in the paragraph immediately above. Dr. Seeley indicated that he expected to see some new last spring frost based on 20 years, or 1991-2010, as new summaries become available. I do not know if this will mean every 20 years we will see new summaries in the future. I have heard Dr. Seeley refer to recent years being very different from historical data, and perhaps NOAA is wanting to show recent summaries as well as longer historical data. Dr. Seeley had compiled the information in the April 5 article on his own comparisons of recent historical last spring frost.
I hope this helps to answer your questions. Best regards.
Mary Meyer