How accurate is YOUR hardiness zone?

highalttransplant(z 5 Western CO)January 15, 2013

Arbor Day has my location listed as a zone 6.
The USDA Dept. of Ag. has me as a 6a

The zones are based on the following temperature scale:

Zone 1: below -46 C (below -50 F)
Zone 2: -46 to -40 C (-50 to -40 F)
Zone 3: -40 to -34 C (-40 to -30 F)
Zone 4: -34 to -29 C (-30 to -20 F)
Zone 5: -29 to -23 C (-20 to -10 F)
Zone 6: -23 to -18 C (-10 to 0 F)
Zone 7: -18 to -12 C (0 to 10 F)
Zone 8: -12 to -7 C (10 to 20 F)
Zone 9: -7 to -1 C (20 to 30 F)
Zone 10: -1 to 4 C (30 to 40 F)
Zone 11: above 4 C (above 40 F)

Last winter was very dry, and warmer than normal for our area, so maybe that zone designation was close for that year. This year is more typical, lows between 10 - 15 below zero for at least 2 - 3 weeks straight. This would be a solid 5, or at least a 5b anyway. The snow that fell the week of Christmas is still there!

Just wondering if I am the only one in this area that disagrees with the designation for their location on the newer hardiness zone charts?

I've included a link in case you're not sure what zone they have you in now.


Here is a link that might be useful: USDA Plant Hardiness Chart

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I'm in 5b which seems accurate. Lowest lows -10 to -15F which we seem to get maybe only a few days per winter. Some winters we might not get below -5F.

I've come across some very hardy rosemary. Was checking it today after the -3F the other night and it still looks fairly green. Local nursery was selling Arp rosemary with a tag claiming to be hardy to -18F.

    Bookmark   January 15, 2013 at 9:41PM
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highalttransplant(z 5 Western CO)

I've only managed to overwinter rosemary indoors a couple of times. It dies when left outside, but I've always grown it in a container. Maybe I'll look for the Arp rosemary this season, and plant it in the ground.

Thanks for the info!

    Bookmark   January 15, 2013 at 10:40PM
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treebarb Z5 Denver

I think the zones are meant as a basic guideline and are as accurate as they can be given our recent crazy weather swings. I try to opt for zone 4 plants, a zone colder than my 5b just to be on the safe side.

Last year I planted a zone 6 Pseudotsuga menziesii 'Graceful Grace', a weeping douglas fir. It's in a south facing, wind protected microclimate and doing well. I accidentally got a zone 7 Cedrus deodara 'Prostrate Beauty' in spring of 12 and planted it in the same area but I think it's not going to survive our recent cold spell. I didn't check the zone tag on the plant before I bought it, foolishly thinking a nursery would only sell plants hardy to our area. Plant and learn for me, I guess!

Good to know on the rosemary.


    Bookmark   January 16, 2013 at 8:11AM
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Bonnie, I have almost given up on hardiness zones. Maybe you remember me "going on" about them a few years ago.

I was willing to follow the Arbor Day's lead and accept the zone 6 designation for my corner of the world. Then, we had -18F for a winter low. Last year, it was only down to 9F once. That is 9 above zero!

I have now lived here for 45 years. Never - never - have we had a 9 above zero as the coldest for the winter! It did NOT translate to a warm spring. Wow! we had record rainfall in March and the entire spring was very sub-normal for warmth! There were "below normal" records set!

Okay, winter is winter and spring is part of the growing season. But, if I were to buy a perennial that should be okay with a zone 6 winter, will I be happy to see it killed in its 4th winter when the temperature drops to -15F? What about living for 8 or 10 years and -20F killing it?

I would be really rolling the dice to believe this is now a zone 6 even tho' we may get thru this winter with no sub-zero temperature again! This has happened before and not just last year but about once every 10 years, or so. Obviously, we have had warmer than normal winters lately but I've still got about 45 years of memory.


    Bookmark   January 16, 2013 at 12:23PM
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highalttransplant(z 5 Western CO)

I hear you, Steve!

It's not that I expect the chart to be perfect. I guess my issue was why they changed my zone, when it was accurate before. Like you said, Steve, if I purchased plants based on that zone 6 designation, I would be pretty disappointed when the plants didn't make it through the zone 5 years here.

I was just curious whether their choice of zone was accurate for most of the Rocky Mountain gardeners, and I was just the exception.


    Bookmark   January 16, 2013 at 4:01PM
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A lot of those zone designations vis-a-vis the plants that can survive such temps don't take into account the relative humidity, which is really, really dry when it gets -10F.

Durango hit -21 F yesterday. Chez us, we've been a balmy -8 F at the coldest, and it hasn't gotten above 25 F during the day for 10 or so days now.

    Bookmark   January 17, 2013 at 12:41PM
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Yes, then you have Colorado!

Or, any other place with such a high elevation. The thermometer seems to go wild on a 24 hour basis. There must be an enormous amount of stress on plant tissue exposed to that sort of thing thru the winter.

David is down there in the Southwest corner but I just went thru Denver's January daily temperatures: average of 25 degree swings between high & low temperatures. Included is one day with a high of 60F (!!) after a low of 22. Here, there won't be those kinds of 38 degree differences except, maybe, only 2 or 3 days each year!

By comparison, the foggy grey sameness of here where the average January daily high and low temperature difference is only 12 degrees.

No, the high hasn't risen above freezing for the last 7 days. It also hasn't fallen below 5F. Even when a warm January breeze blew thru, the daily difference was still only 12 degrees. And, gee, it was the 9th of March last year before the thermometer hit 60F. And, it only did that twice in March 2012.

Dull sameness . . . i hope the plants appreciate it. . .


    Bookmark   January 17, 2013 at 6:22PM
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highalttransplant(z 5 Western CO)

Denver had a 60 degree high in January??? Wow, I forgot how different this side of the state is. My temps are usually similar to David's. We've had 10 days of lows below zero this month, the highest low was 8 degrees, and the highest high, was 43, with mostly highs in the teens and 20's.

    Bookmark   January 17, 2013 at 7:58PM
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Skybird - z5, Denver, Colorado

LOL! It was 58 on the "warm" side of my backyard today--thermometer is in the shade! Tomorrow will be over 60 for sure. The day before it got COLD last week it was over 60 on the warm side and I was out with short sleeves--and getting kind of hot when I was in the sun! Have I said before how much I LOVE Denver!?? The next day our high was at midnite---and it just kept getting colder all day! Had two nites at 0 here where I am, but other parts of the Metro area were below 0! Now it's back--officially--into the 50's, but "officially" is a little bit tricky here in Denver because the "official" comes from DIA which is halfway to Kansas! The actual Metro area is usually warmer than it is out at the airport.

I don't pay much attention to the "new" zones 'cause any zones are just somebody's guess based on the averages for some given period of time. Before they ever came up with "a" and "b" I was just plain z5--and that's how I still think! I generally go with z5 stuff when I'm thinking of new stuff, but I will gladly go with z6 stuff if it's something I want. And if I manage to grow some perennial I wanted for a few years and then it dies---well, at least I had it for a few years! But actually (knock on wood!) I haven't yet had an "established" plant die over winter. If I put something in late, that first winter is when I tend to worry about them--but even then I haven't had anything die yet. On the other hand, every summer I have a couple plants abruptly keel over on me---for no apparent reason whatsoever! That's always frustrating because I usually have absolutely no idea what happened! But it's the same as if something died over winter--at least I had them for a while, and if it's something I REALLY wanted, I just try again! I've had some things that were recommended for the "warm side" of z6 make it, and some not! Sometimes I think it has little to do with absolute coldness. Many of the "new introductions" are still at least somewhat "unstable" since everybody is trying to get their stuff on the market as soon as possible, and in a lot of cases with the "new things" I think they haven't really been growing them long enough to have a realistic idea of how much cold they can take--they come up with a zone recommendation and get it on the market--so combine a maybe not very vigorous plant with a somewhat unknown cold tolerance and who knows if it's gonna survive!

The standard zone map you linked above, HiAlt, doesn't let you get in close enough to know for sure what "they" think your zone is, so I still recommend using the "interactive" USDA Agricultural Research Service zone map I've linked a couple times before around here, where you can zoom right in to "street level" of your neighborhood and then click on the different zones on the top and left side to see what zones they say are where--by clicking them "on and off" it's easier to see just exactly where something starts and stops. They show me in zone 5b, but I still think "-20" when I'm looking at things--even tho I can't remember the last time it got that cold here--even at DIA! The interactive map shows Silt in 6a--but 5b sure isn't very far away! Especially in situations like yours--out in the mountains, I think it's best to pay attention to your own "private" neighborhood micro climate and plan based on what you and others around there have been able to grow--from what I've seen you do pretty well with your plants. And one other thing to consider is whether or not you tend to have snow cover during the coldest parts of the winter. If there's more than "a little bit" of snow, the air temperature is far less relevant than with bare soil. Snow insulates the soil to an almost perfect 32 degrees and the roots stay all nice and toasty! Just too many factors for the suits sitting in an office somewhere to take into consideration when they're deciding just exactly what zone each of us is in!

And the other problem with "zones" is that if you look up any particular plant on several different sites you're very likely to find that different places "think" the same plant belongs in several different zones! On occasion I've found recommendations for the same plant ranging from z4 to z6 or even z7, tho that big a spread is pretty unusual.

So I just start with my old z5 designation, and then decide if I'm gonna try it based pretty much on how expensive it is and how much I want it. If it's from seed I'll try pretty much anything! Virtually no money involved, and I get to have the fun of seeing it grow from seed--even if it doesn't make it later! If I were thinking of trees or expensive bushes I'd probably be a little more conservative.

When it comes to growing veggies, I think Digit's "growing days" system is probably a better way to go for trying to figure out what "might" work for you--but even that is just a bunch of averages, and averages come from "somewhere," and that includes the extremely cold and the extremely warm years!

For those of you looking for a "hardy" rosemary, yes, 'Arp' is (or at least was when I was still "in the industry") the most cold hardy one available--and that's one of the plants that comes with a wide range of "zone" recommendations! So if you're looking for one you can overwinter outside, 'Arp' is the one to try--I'd recommend picking one of your "nicest" warm micro-climate spots, definitely plant it in the ground, and plant it early in the year so it has the summer to establish a good root system. That's one I won't be trying! I think rosemary is one of the most disgusting things I've ever smelled! Sorry for the heresy, all you lovers of rosemary!

Let's all just be happy we're not a zone 3 up in Canada!


Here is a link that might be useful: USDA interactive zone map

    Bookmark   January 18, 2013 at 2:40AM
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Digit, if you think our normal 30 degree swings are a lot you should see the swings inside my coldframes. Not unusual to drop to 18F at night and soar to 95F during the day. I usually try to vent them to keep the temps from going over 80 but sometimes get too busy.

All things considered the plants seem to deal with it just fine.

    Bookmark   January 18, 2013 at 12:35PM
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