Oh, boy. I can start planting bananas, pomagranates, eucalyptus, palmettos, and no longer have to cover figs outdoors for the cold months!
(Yeah, right…I’ve not moved and I remember cars driving on a local lake, half of the southern magnolias getting frozen one year, and just 9 years ago half of the Leyland Cypress biting the dust. And minus 19 just 4 years ago. But yippy, they say I’m in zone 7.)
Those are the 2 recent winters that stand out for me, ignoring the 1960’s blizzards when I was a kid.
Looks like I moved from 7b to 8a in Edgewater MD
Most of southeastern and south central PA is at least 7a now from 6b. It’s true that we rarely go below zero more than a night or 2 during winters and never below -4 where I’m at in the last 10 years.
A while back I ran some number to see what kind of winters my area typically gets. While I am zone 8, there is a lot of variability, with some winters being down in the single digits (so a more typical “zone 7” type winter) and some not even breaking into the teens (a “zone 9” type winter).
Here are the last fifty years, broken into two periods.
While I like my zone 8, those occasional “zone 7” type winters with 0-10 F minimums are such a pain…
Also not quite right. All of those winters are “Zone 8 type winters,” because they are all reasonably likely to happen in an area rated for zone 8. The zone is just an average that there will be variation around.
One thing I noticed was the very high resolution in the new map compared to the old one, so I took a look at their explanation of the mapmaking methodology. This explains the fine-grained detail in between weather stations:
The map was produced from a digital computer grid, with each cell measuring about a half mile on a side. PRISM estimated the mean annual extreme minimum temperature for each grid cell (or pixel on the map) by examining data from nearby stations; determining how the temperature changed with elevation; and accounting for possible coastal effects, temperature inversions, and the type of topography (ridge top, hill slope, or valley bottom).
Right, hence why I used quotation marks.
Unfortunately, we don’t have another system of naming for minimum winter temperature brackets other than the familiar USDA zones. I don’t think it’s too much an abuse of terminology to call a winter a “zone x” type winter if it falls within the norm of that zone.
Ultimately, spoken language just isn’t terribly well-equipped to deal with statistical notions. But to my mind a bit of imprecision is fine so long as it is properly demarcated. Hence “zone” type.
Yeah, well, it’s all based on thermometer readings, innit? NOT!
A complex algorithm was used for this edition of the PHZM to enable more accurate interpolation between weather reporting stations.
The USDA Plant Hardiness map is based on some sort of technique involving homogenization of temperature records. Such techniques are infamous for overstating rural temps and understating urban temps, so BEWARE! Vast rural areas may experience cooler temps than predicted, reducing hardiness of plants in those areas.
From the point of view of climate skeptics a lot of warming is due to population increases and corresponding expansion of urban areas. Here’s a recent map from UAH showing expected warming since the late 19th century but really calculated merely from population growth (with seasonal and latitude adjustments to match satellite records).
The hi-res version captures development along highway corridors in the American West. It’s reasonable that plant hardiness is improved in some areas, just not across the board.
Let’s not litigate what’s more properly a political discussion here.
I say this as someone who firmly believes that the maxim “when you mix science and politics, ya end up with just politics” applies to any political faction or party, and so am deeply skeptical of any political statement “based on science.” But more to the point, there’s a related maxim: when you mix gardening and politics, ya end up with just politics.
Let’s stick to gardening.
IMO there’s something else not factored into hardiness zones. That’s the variability of weather during winter. Areas like the central plains take more winter damage than equal zones with more consistent winter temperatures. Fo instance figs take less damage in Z6b in NJ than in Z7/8 in my area.
yep
They’re not homogenizing, they’re interpolating between known points. The old maps pretty much only used airport data and did a more-or-less straight line interpolation between them, with some weighting by distance, which lead to the effect you mention. The newer methodology uses more accurate interpolation methods, and includes factors to better account for known phenomena such as urban heat islands, frost pockets, maritime influence, altitude, etc.
Yep. doesn’t tell you anything about the duration or timing of the cold weather. Z6 Kansas had much milder, shorter winters than Z6 Mass. But it could still get as cold once or twice during the winter.
We had two instances recently here in what’s supposed to be 8a. In 2020 we had 36 hours straight below 20F in late October when my pecan was still in full leaf. That caused severe damage to the 60 yr old tree.
Two years earlier we were 17F in mid October. It never got below 20F the rest of the winter. That won’t happen near either coast.
Still 6b here in NE Kentucky. Guess I won’t have to change my moniker…
BTW we had a -4 Christmas Eve last year, coldest it’s been here since we had -14 in 2015. But overall the winter didn’t seem that cold overall.
We didn’t have our first hard freeze this year until Oct 23rd, about two weeks later than usual. Coldest it’s been so far this fall is about 25 a few weeks ago.
This is very true.
While a 30 year average low is useful, it’d be nice if there were a more comprehensive system that included things like 30 year minimum, total heat and cold units, a measure of durations of cold event and their relative severity (which ideally would incorporate windspeed and humidity for some kind of “cold index” figure), and of course frost-free growing days. And really, you’d want to see some indication of frequency and severity of late and early frosts–or phrased another way, the frequency and severity of unseasonably warm weather outside of the frost-free window.
I realize that such a system would be tricky to put together, and you’d end up with zones like “8a_norm/8b_median/6b_min/low duration index/900 cold units/? heat units/230FFD/high unseasonality.” But hey, that would contain a lot of really useful information.
Mapping that would be awful though… So many dimensions to project onto a 2D map.
I see no change for my state, certainly not in the areas near me.
It’s about right for a reasonable estimation of the lowest temperatures of winter here, even if it’s just a night or two. 20°-25°. We don’t get a full day below freezing here but maybe every 10 years or so, so we get by with a lot of plants that are in fact killed during those prolonged freezes.
Of course every so many years we hit the teens just like we have years that don’t make it below 25°
Hardiness maps seem reasonable to me as long as the reader/gardener understands in any given winter it won’t depict what you experience. It’s just a guide on what you have a reasonable chance of growing without a freeze that kills it.
Now chill accumulation maps are a much, much bigger shot in the dark…
That’s the core of the issue right there.
The sunset zones maps try to distill all those factors into one map. I think it’s a more accurate assessment of what will work overall, especially west of the Rockies, but they never quite took off. It seems to be easier to wrap your head around and compare a single quantitative difference (hardiness zone) than the categorical differences of the sunset zones. It’s also harder to test your plants as to what sunset zones it will work in.
I’ve tried to figure out zone 3/4 differences in “winter hardiness” between the NE US and the upper midwest for a long time. What you’ve stated about duration and timing is all I’ve been able to figure out. -25 is cold wherever you are, but when it’s -25 for literally weeks on end it’s got to impact plants differently than when it’s -25 a few days each winter. Also, when it’s -25 (I just picked that temp arbitrarily) in mid November without snow cover, it’s quite a bit different than when it’s -25 in late January with a nice blanket of snow.
