I have this question in the back of my head since a while and I want to be more clearly communicating with people with these guidelines as to what zone we are in. I tend to follow the extreme cold events as the guideline and not the zone map below.
So some maps like linked below suggest where I am (east of cologne) is a zone 7b, While from the same website it shows the lowest recorded temps being much lower at roughly -26c (-14f)
This winter we got down to only -12c or a bit more, the coldest in locals memory (20 years roughly) was 3 years ago at -22c.
So do most people just follow the zone maps or go with coldest recorded temps for how they state their zone in the forum. I notice similar discrepancies with US maps and local extreme minimums recorded.
The minimum temps seems to be the main factor people reference as do I but I am aware of the growing degree days(length of summer), duration of freezes and rainfall as other important factors.
I think you are over-thinking it, as asked. (And I’m going elaborate by over-analysing it.)
To answer: yes, I just identify by the zone as listed. It is compiled from a running tabulation of the extreme lows, averaged over time. And yes, I purchase most perrenials after using that as a starting point.
On the maps, I experience basically one of two things. The small, cheapo versions (think back of seed packets) tend to show me as a solid zone seven. They are unlikely to have been consistently updated and are likely demonstrating the old-style zones based on years starting before I was even born.
Or, the more academic/knowledge centric maps are newer versions, including hal-zone, and sometimes using slightly different batches of years. Thirty years is a wide window, so not too worried there anyway.
More significant is that I live in the countryside outside of a small town. I don’t even know where the weather station tracking the official recorded temps for my area is, but it’s not likely near my house, which, even as a single acre plot, has several micro-zones. Before I bought the house, I lived in a college town where the official weather station was on top of a fast-food place downtown (surrounded by concrete, car exhaust, and vented grills and grease pits). To say it represented the weather where people were actually growing things outdoors has the predictable but-howevers. Much of the official map will be based off equally inaccurate locations with an arbitrary line smoothing the half-zone boundaries. But the relatively consistant inconsistencies are still partly based on growth patterns for the zones as presented. They are accurate enough for most established plants commonly grown.
Most folks will already kindof know what is going to grow in their areas, so they’re just checking varieties to get one they are confident in. Most mass growers are already applying a variety of compensations that let them fudge by a zone, or two - if the cost benefits are there.
So, I’m cusp of Zone 6B/7A. Where it is sunny, I can generally assume 7, and where it is not, I can not. But I know the ground will help me deal with most trees/shrubs on the cusp if I baby them until they get deep roots and don’t skimp on the mulch. Unfortunately, spring temps may take out blossoms on a lot of things that will survive just fine without making babies.
For annuals. I’m in the south-east, in the mountains. Spring comes in on a pogo-stick, punching holes in all my plans. I look at the projected temps and plant accordingly, and I buy things that can ripen between the monthly norms relevant to the plant’s needs. And I use seeds for things like tomatoes that will not care that you don’t plant it until it is over 50° because it had already gotten reset by a week in a truck someplace where it was stuck at 36°.
As a last note: Mean, median, and norm are all different things. When we think of average, we tend to get a bit confused. The USDA is an average of extremes, which places it in a different category all together. Extremes often vary widely year to year. Average = mean. This is not the same thing as median. Median is halfway between the highest and lowest, but with several manipulations available. Average approaches that in data sets that are generally consistent, but temperature does not tend to be such a data set in the lifespan of a single human. Norm is the number that appears most often and has the best chance of predicting standard deviations in behavior.
TL;DR?: Yes, use the standard zone notations for saying where you are and as starting points for evaluating new plants and how much effort they may be to establish, but base the rest of your care, and some final decisions, on what the location you plan to plant it experiences. A tree that lives 100 years is more likely to experience the extremes than something you can only expect 5 years out of.
To assume the record cold never again gets that cold…is short sighted.
Someday it’ll get close if it doesn’t exceed those temps.
But, if you’re a gambler, you might succeed using zone 7 or 8 plants for a decade (possibly even a century) before getting killed by cold someday. (Or, that someday might hppen next year!) So, are you a risk taker?
Alright I have changed my Profile description to a 7a from a 5a.
I appreciate the lengthy and concise opinions. Taking the extremes into account for the long term trees and risking it with shorter term plants or fig like trees that can handle dying back and regrowing after the cold is solid KISS advice.
I use the US Department of Agriculture “Plant Hardiness Zone Maps (PHZMs)” because I haven’t been measuring temperatures in my location long enough.
Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future. Gardeners should keep that in mind when selecting plants, especially if they choose to “push” their hardiness zone by growing plants not rated for their zone. In addition, although this edition of the USDA PHZM is drawn in the most detailed scale to date, there might still be microclimates that are too small to show up on the map.
Microclimates, which are fine-scale climate variations, can be small heat islands—such as those caused by blacktop and concrete—or cool spots caused by small hills and valleys. Individual gardens also may have very localized microclimates. Your entire yard could be somewhat warmer or cooler than the surrounding area because it is sheltered or exposed. You also could have pockets within your garden that are warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first. No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.
Many species of plants gradually acquire cold hardiness in the fall when they experience shorter days and cooler temperatures. This hardiness is normally lost gradually in late winter as temperatures warm and days become longer. A bout of extremely cold weather early in the fall may injure plants even though the temperatures may not reach the average lowest temperature for your zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants as well. Such factors are not taken into account in the USDA PHZM.
All PHZMs are just guides. They are based on the average lowest temperatures, not the lowest ever.
A complex algorithm was used for this edition of the USDA Plant Hardiness Zone Map (PHZM) to enable more accurate interpolation between weather reporting stations. This new method takes into account factors such as elevation changes and proximity to bodies of water, which enabled mapping of more accurate zones.
… zones along the Canadian border in the Northern Plains initially appeared slightly too warm to several members of the review team who are experts in this region. It was found that there were very few weather reporting stations along the border in the United States in that area. Data from Canadian reporting stations were added, and the zones in that region are now more accurately represented. In another example, a reviewer noted that areas along the relatively mild New Jersey coastline that were distant from observing stations appeared to be too cold. This was remedied by increasing the PRISM algorithm’s sensitivity to coastal proximity, resulting in a mild coastal strip that is more consistently delineated up and down along the shoreline.
The zones in this edition were calculated based on 1976-2005 temperature data.
For the most recent decades, the daily record highs have become more frequent than the daily record lows. The year of the daily record high is, on average, more recent than the year of the daily record low.
So for hardiness zone gamblers, the recent odds have become more in your favor.
The largest average temperature deviation in recent years is the increased daily minimum high temperature. It has deviated more than the daily maximum high and minimum low.
(meaning: milder nights)
Alright I have changed my Profile description to a 7a from a 5a.
This strikes me as a strange approach. First, USDA zones are defined based on average annual lows. Just as your all time low is less than your average annual low, the all time low for a true Zone 5a will be much lower than its average annual low. The claimed zones for plants typically take this difference into account. While you can do anything you want, you’ll be giving people a false impression of your climate to list yourself as 5a.
While there is a chance you will lose a tree if you were to completely trust the map and plant assuming you are in a Zone 7a, it would take an extreme winter for this to happen. Plants don’t immediately die if the low for their rated zone is reached. To avoid disappointment, it might be smart to consider yourself a full zone lower when thinking about what to plant for the long term, but realize that by doing this you’ll be giving up on a lot of things that you’d probably be able to grow with high likelihood.
Second, and much smaller, the usual definition for 5a is that the expected annual low is -15F (-26.1C). If your all-time recorded low is -14F (-26C), you aren’t quite there yet. I’m in what is officially a 5a on the maps, and our low so far this winter is -24F (-31C). We probably hit this low every 5 years or so, and our Zone 5 trees seem to be doing fine so far. It’s often sudden drops of temperature that kill trees, not absolute lows. It is possible or even probable that we’ll lose some trees if we ever dip to a once in a century low, but this is a chance we’ve decided to take so that we can enjoy some hardy peaches until then!
Thanks, this reassures me that most of my plants are safe. It does lead to me considering to plant warmer loving chestnuts. Otherwise I have little space left on about 10 acres of land, I have a hard time squeezing in some bare rooted rootstock and another delivery this spring of 10 plants. It is hard to stop buying plants though.
But anyhow I am cautious with cold temps as I planted, the main goal of this clarification is for communication in these forums. I see that most other German users are listed at zone 7 or 8 etc. So now I figured I am the only one using my extreme lows as the guideline for the zone listing and have updated it. I still plant 95% of my trees to be hardy to zone 5.