Anyone find that different "Plan Zone Maps" have different numbers listed for their Hardiness Zone? I am listed anywhere from 5 - 6 depending. Why is this? Even different USDA links list my zone different. Sometimes it does not matter on the plant but sometimes it does...UGH!

It could be some of the maps are older? With the climate changes the zones have been changed also.

That is probably the case with the USDA map. They updated it in recent years. Once something is on the internet it stays there so a lot of sites still have the older maps posted.

I think Seed and Jay are correct, Cat. My zone was 7b until a few years ago when it was changed to 8a. Some older zone maps still show the previous number.

Yes, I guess that makes sense. I have always planted for zone 5. Now some recent searches have me listed anywhere from 5a to 6. Lol.

So you can experiment with slightly warmer zoned plants.

The maps are just general information and not something you can bank on. You also need to adjust for your micro-climate which can be affected by your soil type, watering habits, reflected light, wind exposure, and so many other things.

My zone is supposed to be 8b.

Example of a micro-climates in my Savannah, Georgia yard.

I have some zone 10 plants. I brought most of these cold-sensitive plants into the house for the winter months, but as an experiment I left 3 plants outside during the winter. One of the plants survived the extremely cold winter we recently experienced, so that spot is the warmest micro-climate in my yard. It has rich soil, lots of overhead trees for cover, bright shade, and ample water.

The front of my house is exposed to more wind, gets less water, has very sandy soil, receives no protection from any overhead tall trees, and is more desert-like; good for Xeriscape gardening.

Funny thing is that I learned all this in a very odd way. About 15 years ago I went to a nursery to buy some new landscape plants for a house I had purchased. The nursery owner (who appeared to have no sense of humor) asked me where I lived. I said, 'Waycross, Georgia'. No, no, no. He wanted me to be more specific. I told him, 'Saint Mary's Drive'. No, not good enough. He needed me to be even more specific. When I told him the house number, he thought I was making a joke. It seems that I had purchased the housed that used to belong to him 50 years earlier. Once he got over the shock of the coincidence, he gladly explained micro-climates in great detail. And yes, I bought a car load of new plants. And he became my go-to guy for plant questions.

Well I knew what you meant by Micro-Climate but was more used to worrying about it for the seeds I have started and the cuttings I have done. Just guess I didn't really worry about it for the "Out Side" plants. Unless of course it was for some transplanted cuttings or something like that.

I agree with Greene - the USDA Hardiness Zone is a very general guideline and your micro-climate does vary a lot.

Also, the USDA Hardiness Zone only states the average coldest temperature reached in the last (??) 50 or 100 years for some weather station somewhat near you. I bet the WAY they do the average and the weather station's exposure make a big difference.

Statistics is a way of making fuzzy things sound much more precise than they are.

The thing that I think makes Zones almost useless is just the fact that they ARE an average. Even if there were no microclimates and every spot in a ZIP Code zone had IDENTICAL temperatures and wind, what does it really mean to me that HALF of the time, the temperature WILL go lower than the Zone's "limit" says? If that were the whole story, would I plan to grow things that I would EXPECT to die in half of all winters?

If the Zones WERE precise, and predicted a low temperature that was only exceeded on 10% or 5% of years, it might be a more useful guide. But it isn't even that.

Another drawback is that it records the very coldest temperature reached, even if it was only reached for 30 minutes one night that year. I don't think most plants are killed instantly by few minutes of temperature! Three cold nights and days in a row might do the deed.

A hard cold snap after a deep blanket of snow is not a big deal. Snow insulates. The same cold in a bare yard will freeze soil much deeper and kill lots more plants.

Second to last, cold still air is "meh" ... maybe it'll kill something and maybe it won't. Cold air howling around a plant at 30 MPH will freeze it and blow its shards away (so to speak).

Last, for most plants it matters HUGELY whether the cold snap comes after a gradual fall in temperatures over several weeks in mid-winter, or if the mercury drops like a rock in the middle of an otherwise mild fall. Plants that aren't ready for winter are probably 5 times as vulnerable to cold as fully vernalized plants that have made every adaptation to winter they know how to make.

Long story short, nature is almost always more complicated than scientific theories can easily express or predict. And anything that has been boiled down to just one little number has NOT captured the whole story.

At least you have a pretty good idea that plants "listed for" Zone 6 are more likely to survive in any given spot than a plant "listed for" Zone 5. If you almost never lose a Zone 5 plant, try some Zone 6s.

And you may find some genus of plants that "most people" can only grow down to Zone 7, but YOU have the mulch or shelter or drainage or TLC that THAT one genus needs to survive YOUR Zone 5.

Although gardening is all about experimenting, it isn't likely to become a science. There are too many variables, and science is only good at generalizations. Every garden and every gardener is unique, beyond science's ability to reduce to a number.

And that's GOOD!

Greene, loved your story!!

I'm not a fan of bumping zone numbers up at all because all it takes is 1 unusually harsh winter to wipe out some/most/all of the marginally hardy plants. Plants don't give a hoot about averages. And don't forget, these averages are based on less than 200 years of weather data in most areas. If one looks at the dates of record low temps, there's no trend upward, many of them happened in recent years. There's no reason to bump zones except a financial one.

There is a trend for warming temps in general though, which is said to be expected to produce more wild extremes, of both heat and cold. IDK if that's true, but if it is, relying on a bumped zone number will not have good results.

Agreed, the diff between z 5/6 may be the least distinct on the scale. Moving to Z7 is where, if the ground freezes at all, it's not deeply or for very long. Going below Z5 is approaching arctic conditions. Given a choice, I would stick with being in Z5 vs. Z6 because the snow cover is more reliable. In the exact same temps, a winter with much more snow cover would be much less likely to damage plants than one during which the ground is bare most of the time.

Yes Tiffany I am more likely to buy a Zone 5 plant. That's the zone I usually bought plants for and that has worked out pretty well so far. I have lost an occasional plant here and there but that could have also been some other reason and not just the weather.

Tiffany wrote "I'm not a fan of bumping zone numbers up at all because all it takes is 1 unusually harsh winter to wipe out some/most/all of the marginally hardy plants"

Absolutely! The updated USDA map was barely dry (figuratively speaking), which bumped my zone from 4b to 5, and we had the most brutal winter in many, many years. We have a mountain ash in our back yard that is native up here that was severely stressed by the cold -- well, our frost was down 8 or 9 feet! -- and may or may not bounce back next year. The maps are helpful as a guideline, but there are certainly no absolutes...

If plants don't know they're now in a warmer zone, maybe they should be told. Maybe we're supposed to show them the new maps?