Those Inadequate Hardiness Zones

[digitS']

Average Annual Minimum Temperature. Yes. So you buy a tree with a tag and expect it to live 10+ years. During the 11th winter, a day happens with a temperature significantly below average, maybe not even a record. The plant dies.

Okay. The growing season weather and for our annual gardens, that usually doesn't extend into winter minimum conditions. Maybe one could count winter as a part of the growing season for Zone 9.

Okay, where is this Zone 9 located?

• Aberdeen, Washington
• Tucson, Arizona
• Lake Charles, Louisiana
• Orlando, Florida
• Charleston, South Carolina

🤔 That's from one side of the continent to the other and across some really different terrains 😵 !

Steve

 

[Ridgerunner]

Steve, life is complicated. There is more to growing a tree than just average minimum temperature. Length of daylight, chill days, rainfall, soil types (clay or sand), how well the specific area drains, is it in a frost sink, north or south slope. They are trying to make it simple and use one variable to cover everything. It's an important variable and a good guideline but sometimes it just doesn't cover enough.

 

[Zeedman]

The plant hardiness zones are intended to allow gardeners to choose perennials which will (most likely) survive Winter in any given location. To me, they are best used to eliminate possibilities, when considering a list of cultivars. It can be frustrating, not to mention costly, to invest time & money into something, only to see it fail after several years... especially in the case of fruit trees & fruiting vines.

There have been several unofficial versions of "new" plant hardiness zones published online. IMO, those are best taken with a grain of salt, and not used to justify plantings which would inevitably prove to be unsuitable. Many in Wisconsin discovered that the hard way last year, when we had -30 temps, and many trees & grape vines that were marginal for our climate perished.

The mistake many make is to use the hardiness zones as an indicator of growing season... seed companies often perpetuate this misunderstanding. "Survival" does not necessarily equal success. As @Ridgerunner stated, and as @digitS' examples prove, the hardiness zones are not an indicator of climate. The Sunset climate zones are far better indicators, I wish the USDA would produce a nation-wide equivalent to those zones.

 

[digitS']

Climate is complicated and (plant) life is dependent on it.

This partly addresses the question @misfitmorgan had about struggles to grow winter squash to maturity. Another important guideline but too casual in some locations is days-to-maturity. I remember Lois Hole, the Edmonton gardening author, describing it as "in ideal temperatures."

I really wish that days-to-maturity were designated instead by Growing Degree Days, as is true with corn varieties and some seed companies. ( https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/pastdata/degree_days/grodgre1.txt )

We were lucky enough to squeeze out both Devils Lake and Fargo this year at the nearest weather station listed. Still, yo-yo temperatures in June made life difficult for warm season varieties.

This location is considerably higher in elevation than those North Dakota communities and just about the same 48° North latitude. The winters don't compare because of the severity on the northern Plains but it's something of a toss up as to which location Spokane~Fargo~Devils Lake has the most growing season warmth.

But then ... the differences between night and day ... lows and highs. An 80°f day followed by a 40° night. You have something like a 65 day eggplant or a 90 day squash vine almost going into a state of dormancy. It could be worse and I often look at Colorado weather and shudder. I'm not sure that I could have a garden in many of those Rocky Mountain communities!

Then, I look at zone 9b Eureka California, near where I had some of my first gardens (and near the waters of the Pacific), and see that Fargo has nearly double the Growing Degree Days ... yes, @Ridgerunner , life is complicated.

Steve

 

[Pulsegleaner]

Or the reason I am leery of leaving my little Yuzu trees out for the winter, even though they are SUPPOSED to be good down to -5F. It would just need ONE bad week of super cold to undo my years of growing work.

I also recently ordered (though have yet to receive) a pair of Himalayan Mountain Ashes. I ASSUME they can take the cold here (since I would assume the Himalayas get pretty cold in the winter) and they SAID it was good to zone 6 (which I am) but there WAS that question mark.......

 

[Dirtmechanic]

Looking for predictible results has led me to simply tightening my USDA limit up one level. Plants that are marginal require more work, and if they are planted outside they do not flourish as well as they could. I would like to know more about the sunset modeling for the Southeast.

 

[digitS']

I had no idea that there was a hardy citrus tree, @Pulsegleaner ! Several years ago, I was given an orange tree by a neighbor. Promptly killed it by leaving the tree in the greenhouse into freezing weather. Dad had one that sat right beside him year after year at the dining room table - sliding glass doors.

I agree with @Zeedman that the Sunset "unique" zone map would be much better. But ... what data are used to put it together and are those people outside of Sunset allowed to look at that? And "unique" - if the nursery and seed industry, etc. choose not to use it, doesn't that take nearly all of the usefulness away from those zone maps?

Sunset is in the business of selling magazines and books. A quick search of their website provided me with no useful links to a U.S. map. A few years ago, I didn't have that problem but their online advice is to consult their books ... okay.

I did find a picture from the open pages of a Sunset book, @Dirtmechanic : LINK.

Steve

 

[Ridgerunner]

I use "days to harvest" to compare varieties, not to tell me when I'll be able to harvest. The clearest example for that is my cool weather stuff. Unless it gets enough sunlight and especially warmth it doesn't grow. It doesn't die, pretty much goes dormant as you mentioned. How much greens can you eat if they don't grow, just live?

I won't be planting any trees down here, no room. In Arkansas I was on the border of zones 6 and 7, depending in whose map you looked at. Any fruit or nut trees I got had to be good for zones 5, 6, 7, and 8 or I would not even look at them. Some of them did better than others.

 

[flowerbug]

Or the reason I am leery of leaving my little Yuzu trees out for the winter, even though they are SUPPOSED to be good down to -5F. It would just need ONE bad week of super cold to undo my years of growing work.

I also recently ordered (though have yet to receive) a pair of Himalayan Mountain Ashes. I ASSUME they can take the cold here (since I would assume the Himalayas get pretty cold in the winter) and they SAID it was good to zone 6 (which I am) but there WAS that question mark.......

-5F, we get below that at times, and the wind chills can be much worse. already this winter we've been in negative wind chills. eek...

 

[Pulsegleaner]

I had no idea that there was a hardy citrus tree, @Pulsegleaner ! Several years ago, I was given an orange tree by a neighbor. Promptly killed it by leaving the tree in the greenhouse into freezing weather. Dad had one that sat right beside him year after year at the dining room table - sliding glass doors.

Steve

There are actually quite a few of them. There is this Asian citrus fruit called a papeda. Fruits that are created by crosses with it tend to pick up a LOT of cold resistance (Pure papeda has the most of all, it can take down to -15c but the fruit is inedible.) The Yuzu is one but others are the kabosu the sudachi, the Ichang and the Taiching (those last two may be the same thing with different spellings)

Trifoliate orange is also super cold resistant but again the fruit is not considered good to eat (usually, there are supposed to be crosses with better tasting fruit.)