Last frost dates are not the same The Cheap Vegetable Gardener

E-mail Comment Del.icio.us Digg Reddit Technorati Furl

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:

	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.

This entry was posted on Saturday, March 14th, 2009 at 2:39 PM and is filed under last frost date. You can follow any responses to this entry through the RSS 2.0 feed. You can also leave a response.

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

Daphne Says:

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.
Cinj Says:

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?
The Cheap Vegetable Gardener Says:

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.
Chiot's Run Says:

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.
Red Icculus Says:

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

March 17th, 2009 at 5:55 AM
[...] The Cheap Vegetable Gardener points out in his post “Last frost dates are not the same” [...]