USDA Updates Map


They say third time is charm…


Hey, you have to pick some period over which to summarize the data. Too short and you may as well be following a weather report, too long and you miss important trends. The 30 year average has proven to be a good compromise. As I stated elsewhere, no, it’s not 100% accurate because that’s impossible. However, it is useful, more accurate than previous versions, and helps show what the current state of affairs is. You just have to interpret it correctly and use it for what it is: an estimate and a guideline, not a gospel.


From Climate Normals 1991-2020

"Normals serve two purposes: a reference period for monitoring current weather and climate, and a good description of the expected climate at a location over the seasons. They provide a basis for determining whether today’s weather is warmer or colder, wetter or drier. They also can be used to plan for conditions beyond the time span of reliable weather forecasts.

A 30-year time period was chosen by the governing body of international meteorology in the 1930s, so the first normals were for 1901-1930, the longest period for which most countries had reliable climate records. International normals were called for in 1931-1960 and 1961-1990, but many countries updated normals more frequently, every 10 years, so as to keep them up to date. In 2015 this was made the WMO standard, so all countries will be creating normals for 1991-2020."


we get well below zero every winter so, from the lows in the last 5 years it feels incorrect to be zone 7! although I do know it’s not as cold or as many chill hours as it used to be. it may get ten below now or so, but not twenty, for example. but a Chicago hardy in the ground will die here, roots and all. yet it’s zone 7a, which doesn’t make sense to me

things that are zone 5, do fine, zone 6, some years kill those off even though it’s been 6b here for two successive USDA maps.

1 Like

If you look at the outline of where 7a appears on the new map, it’s pretty limited to the immediate Spokane heat island:

I tried to find NOAA stations within the heat island itself, but all of those have only been in existence for a few years now. The only two locations with an analyzable amount of data were the international airport (GEG), which has over a century of measurements, and Felts Field, which only has data since 1998 (and 1999 was the first complete year).

GEG is far enough out of the city that it is still in 6b according to the new map, and the data confirms that, but Felts Field is pretty thoroughly within the 7a range despite not yet having a full 30 years of measurements:


I’m inside the city, we have a corner lot in town. we were previously 6b, but everything claimed hardy at 6b dies in winter

I blame it on the average-low method, we get lower than the average by a considerable amount every single year but they use the average? cold temp. or the average low, which one warm year can throw off above what the norm is. and we get that one warm winter every five to ten years.

I’m curious what @NuttingBumpus and @SpokanePeach think of this, they’ve been growing here longer than I have – I moved here just about ten years ago from the Oregon coast and had a rough few years adjusting. (my poor outdoor fig collection from that time…):sparkles:


They don’t use the average each year, they take the coldest day each year and then average those numbers. According to the records at Felts Field, 2022 (-7°F) was the first time below zero since 2016 (-1°F), and the last time it hit -10°F was 2008, which is also the lowest temperature measured there since records began in 1999. The international airport has been a bit colder almost every year, and a lot colder on the coldest years, like 2008 and especially 2004. Here’s the lowest temperature each year since 1999:


Interesting to find in the map Swincher supplied (thank you!) you, Anji, live in a cooler/colder pocket than I, just two miles west of you.
Nearly everything I grow is hardy to zone 4. The red fleshed apples and black currants are even hardier. If a graft was put to a meager root, limiting growth of both root and scion, that often determined whether or not it would endure a killing early frost, which we experienced October '16 & '17. That experience taught me to nurture a tiny root for a year before committing to a graft on it, and to make note of the anomaly on a calendar for future reference.

Now you put the question, I grew up in this neighborhood and it has seemed to me most winters had more snow and longer on the ground than we find in this century. Winter of '68/'69, when I got my first driving license, was the benchmark for decades following. Before that, my eye level was closer to the ground & I spent more time outdoors to forget school and home, so I have distrusted my youthful impression.
Swincher is right: hardiness is based on average lowest low. It is unfortunate his chart (thank you for supplying it) isn’t marked the way I think, which is “winter of 2023/2024.” Perhaps it is set for January first of each year representing that winter, in which case each number shows the second half of that season.
('Course I’m laughing at myself…)


It just is based on calendar year because that formula was easier, but I could redo it the other way I’m pretty sure. The way it is now, each winter is split between two years, meaning that each year has halves of two winters.


The destabilization of the jet stream / polar vortex probably has something to do with the average warming but cold plunges right? Cc @weatherandtrees


I did the breakdown by winters, just for the international airport (GEG), though. Trying to do these formulas on my phone in google sheets is a challenge! It’s a shame they don’t have all the full features of the desktop version in the mobile app. But here’s what it looks like through 2021/2022 (I hadn’t even requested the data for the second half of the 2022/2023 winter, so can’t include that):

winter of minimum (GEG)
1950/1951 -3
1951/1952 -10
1952/1953 9
1953/1954 -9
1954/1955 -3
1955/1956 -12
1956/1957 -18
1957/1958 17
1958/1959 -5
1959/1960 -3
1960/1961 6
1961/1962 -13
1962/1963 -7
1963/1964 -5
1964/1965 -20
1965/1966 -1
1966/1967 7
1967/1968 1
1968/1969 -25
1969/1970 -5
1970/1971 -2
1971/1972 -12
1972/1973 -11
1973/1974 -12
1974/1975 -10
1975/1976 4
1976/1977 -2
1977/1978 -11
1978/1979 -22
1979/1980 -6
1980/1981 2
1981/1982 -15
1982/1983 -2
1983/1984 -18
1984/1985 -12
1985/1986 -21
1986/1987 1
1987/1988 -5
1988/1989 -11
1989/1990 2
1990/1991 -16
1991/1992 10
1992/1993 -5
1993/1994 -5
1994/1995 2
1995/1996 -24
1996/1997 1
1997/1998 -2
1998/1999 -4
1999/2000 9
2000/2001 -2
2001/2002 8
2002/2003 7
2003/2004 -22
2004/2005 -6
2005/2006 -3
2006/2007 -10
2007/2008 -1
2008/2009 -18
2009/2010 1
2010/2011 -10
2011/2012 6
2012/2013 4
2013/2014 -5
2014/2015 8
2015/2016 1
2016/2017 -7
2017/2018 4
2018/2019 -1
2019/2020 13
2020/2021 9
2021/2022 1

Use of “winter” lows instead of calendar-year lows may cause some annual lows to be missed.
Are the above numbers based on Astronomical or on Meteorological winter?

It would be interesting to compare the averages of annual lows versus seasonal lows.
And in crunching the numbers, has there ever been an annual low outside of winter dates?

Portland was in the teens once in November, might have been an annual low.


I just divided each year on July 1, so any date before July 1 is treated as being the preceding winter, and all dates on or after July 1 are placed in the following winter. So it includes spring, fall, or even summer freezes, if there are any in the dataset.


Swincher: thanks. You included two winters before my time! I remember that day in January '69 to -25F. I got frostbite & have worn caps in cold weather ever since.


Thanks for posting. Looks like we moved to 8A from 7B

Seems like we were 7A about 15 years ago, so we have moved a full zone.


I moved to Spokane in 1980. That streak of cold winters in the early '80’s are indelibly etched in my mind. Week long periods of lows near 20 below… I just thought that was normal for Spokane. Looking at the data now, I can see it was a little anomalous to have a cluster of years grouped together with such low lows. Winters like that seem highly unusual this century (I am thankful for that).

The 2003/2004 winter resulted in no peaches from the only peach tree I had at that time (a Redhaven). All winters since 2003/2004 have yielded peaches. Clearly Spokane is warmer in the past 20 years.

Lately, it seems like we have experienced some very abrupt temperature changes as we transition from Fall to Winter. I suspect this may have been hard on some plants.


Ever look at the USDA zone map of Europe?

Ever notice most of Europe matches common USA zones? Parts of the UK are warmer then Southern Georgia!

But I realize the spread of say chill hours could vary. But it’s true; at times we get cool foggy months like coastal UK.

1 Like

Most of the UK is more like the comparable zones in the PNW, with a strong maritime influence that minimizes extremes in either direction.


I like it. That means Isle of Wight Howgate Wonder and Irish Appletown Wonder have more then a fair chance to do ok “round hear”…lol…If they can take the summer heat. {cue up shade cloth and part shade planting.}