Last frost dates are not the same
Mar 14, 2009 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:
| Â | 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 |
Tags: garden seeds, outdoor plants
March 14th, 2009 at 3:20 PM
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.
March 14th, 2009 at 3:54 PM
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?
March 14th, 2009 at 7:23 PM
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.
March 15th, 2009 at 5:21 AM
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.
March 16th, 2009 at 8:53 PM
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.
March 17th, 2009 at 5:55 AM
[...] The Cheap Vegetable Gardener points out in his post “Last frost dates are not the same” [...]
April 5th, 2010 at 9:23 AM
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.
November 7th, 2010 at 8:49 AM
[...] 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 [...]
February 27th, 2011 at 2:15 AM
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!
April 25th, 2011 at 1:31 PM
[...] 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 [...]
March 12th, 2012 at 2:13 PM
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.