zone .. simply put.. is minimum winter temp ... and the issue is how long it stays at that min temp ...

i am not looking it up... but if z5 is 20 below.. and z6 is 10 below.. whats the difference???

the difference is ground freeze.. consistent snow cover... proper mulching.. etc ....

e.g. kansas city is z5 ... but i dont THINK the ground freezes ... and i dont think they have as constant a snow cover ... just winging it here.. dont quote me ... so winter sun.. can create all kinds of havoc with soil temps.. and plant loss ..

as compare to.. your alleged z6 .. with 2 feet of snow all winter.. no sun.. no breaking mid-winter dormancy ... no wind impact.. etc ...

ok i saw your link ... let me tell you straight out.. i am not zone 6 in lenewee county MI ... i smell something fishy with this map

you want to waste some money.. start bringing in z6 plants.. i will bet they will be short lived.. just waiting for that one z5 or 4 winter ... unless you like to play the zone pushing game.. then all the power to ya ...

i should probably have deleted this.. lol ... just take it all with a grain of salt is what i am trying to say ...

ken

It must the global warming. Maybe we'll all get promoted.
Duane

Most plants that are zone 6 are zone 5 viable anyway in my limited experience. Although I have a coworker who swears that cannas do better in downriver detroit because its "warmer" there than the suburbs north of the city.

The reason I use both is because I'm convinced parts of my garden are 6a while a couple spots are 5b. Z6 plants will survive here, especially if they're grown from seed via winter sowing. I'm pushing the envelope with Lobelia cardinalis 'Fan Scarlet' this year and hope I planted it in a Z6 spot. GardenWeb says I'm Z6 while other sources put me squarely in Z5. My son lives 14 miles south of me and he's Z5 so he's a little miffed that my ZIP code comes up a Z6. I told him it's because I'm closer to the Connecticut River than he is so that puts me nearer a termperate zone.

Temperature-wise, Z5b makes more sense because it's not unusual for us to see the temp drop in February down to minus 12. Last year we saw one night where it dropped to -19F but that's very unusual.

I find it fishy that there are no links to the organization that did this map. They don't tell you what data set they used--how many reporting stations how far apart, most reliable data set from NWS or their reliable data set (which has more holes in it) or data from somewhere else. How did they interporlate between reporting stations? I wouldn't trust this map as an authority.

As it happens, I am part of the team putting together the new USDA Plant Hardiness Zone Map, which will be released in the near future. Some of the best experts in the business have told me using weather data from 1940 to 1970 is a little questionable because there are lots of holes in the data sets. There is always a loud discussion over how many years of data to use to smooth out year to year fluctuations in weather (cold winter vs, mild winter) vs. the fact that perennials experience weather, not climate.

When the new USDA Plant Hardiness Zone Map is published, I will make sure to post it to Garden web sites.

That aside, no matter what the maps say, you know your garden better than anyone else. I'm in zone 7, but my house is virtually surrounded by Rock Creek Park (the largest urban green space in the country) and I'm a good half zone colder than friends who live just a few miles away but near a lot more black top and concrete.

I remember when the Arbor Day map first came out doing some investigation into their data sets. I was not impressed. They said where they were getting their information. That organization had a web site where you could order their data. The data itself wasn't available, but what data they possessed was listed. Given the Arbor Day criteria for data selection, and the data sets listed, there were 3 sites in my county that qualified, and 3 in the next county north. Four of those six were right next to the river, so would have gotten temperature moderation from a large body of water. Those four were also in sites that have seen a great deal of development in the last 20 years, so that would also create an artificial sense of warming.

All in all, it seems that around here at least, the first USDA map is the most accurate.

Lobelia fan scarlet has returned reliably for me in multiple locations (atleast for 3 seasons). I might be wrong but I thing it is shortlived though and should be allowed to seed.

I'm in the metro-Detroit area, and I'm zone 6. Venture a little further west (say, 15 miles or so) and now we're in zone 5 territory.

Keep in mind your own yard's microclimate - that's what really makes a difference.

Haven't we been on the verge of getting an updated USDA zone map for at least the last decade?

"Some of the best experts in the business have told me using weather data from 1940 to 1970 is a little questionable because there are lots of holes in the data sets."

If the new map is using 1940-1970 data at all, it will again place most of us in zones that are colder than what current reality tells us.
The current USDA map has me in zone 5, but in the decade I've lived here there have been at best two winters with minimum lows colder than minus-10F. We've had a couple winters with minimums above zero, and the rest primarily in the zero to minus-5F range. That's squarely in zone 6 by the current definition (remember, zones are based on average minimum winter low, not outlier temperatures on either end of the scale - if you get a freak winter low of minus-21F in Lexington, Kentucky (which happened to us one year when I lived there) it does not mean Lexington is in zone 5).

At least whatever the USDA comes up with is bound to be more useful than the heat zone map, which does not account for humidity and nighttime lows and is close to useless.

Good cultural practices (including providing excellent drainage) and careful siting of plants that are marginal or not supposed to survive in a given zone will always trump zone definitions.

If the new map is using 1940-1970 data at all, it will again place most of us in zones that are colder than what current reality tells us.

Maybe.

There have been two official USDA zone maps published. The first was put out @1970, and used roughly that data set. The second, still currently official one, came out @1990 or a bit earliest (it's hard to tell since the USDA does not copyright material so these do not have useful copyright dates) It used data from a shorter time span, and seems to have been done in response to a cooling climate, so many places were colder than described on the first map. For example, on the first map, the far southeastern corner of Pennsylvania, around Philadelphia, was zone 7. On the second map is was changed to zone 6. The first map had a long, very narrow tongue of zone 6 following up the Hudson River almost to Albany. The second map gets rid of it. Now we are being threatened with a third map that wants to move the zone lines about as far north of the first map lines as the second map moved them south.

The reality seems to be that there are a number of people, who, whether they realize it or not, never agreed with the second map. At least in areas that I'm familiar with, there seems to be a detectable pattern that people who were moved down a zone in the second map, agree with being moved back up a zone to where they originally had been, but people who were in the same zone in the first two maps, but moved up a zone in the third one think the third one is crazy.

The last time this discussion came up, I spent a lot of time looking for a copy of the first map online, without success. A lot of gardening books from the 1970's include it. The biggest noticable difference is the lack of a and b subzones. However, older books may have a copy of the Arnold Arboretum map. It's similar in concept, but uses different temperature ranges to define the zones.

Wait a minute - doesn't the zone indicate the *average* minimum temp? So a "freak" episode here or there (in either temp direction) isn't going to skew the data all that much, dependent on the length of time of the date set, of course.

Please correct me if I'm wrong about this...

miclino - thanks for posting the info about L. Fan Scarlet. I did indeed harvest seeds from the plant I bought this year and will winter sow the seeds for more plants. I'll site them in a few different beds here and there around the garden to see where they're happiest.

The new USDA map project began in 2007. The model has been finished for some time, but the lack of a budget for two years until almost the end of the federal budget year kept the map on hold. When the government is on a continuing resolution it is very hard to go out with a call for a big new contract. But we finally got the money to create the Internet interface and to host the map site with enough capacity for all of the people who will hit the map web site the first 6 months without crashing the serving.
The new USDA map will be based on an algorithm that interpolates between actual data points with weighted factors for nearness to bodies of water, slope of land, winds, changes in elevation and some other factors. In addition, we ground truthed the new map, having experts from different areas review what came out of the data and equation. Sometimes they said the zones didn't look like what they knew about an area. Then we checked the data for biases. The New England people said New Hampshire looked too cold. When we went back to the data we found out that all of the weather reporting stations in the data set were from up high in the White Mountains. When we went and got some additional weather reporting stations from lower elevations, it changed the pattern of the hardiness zones. Other places there were no biases in the data and the zone pattern stood.
One of the big problems with the three past USDA maps (1960, 1966 and 1990) was scale. Hard to see much detail when you squeeze the whole U.S. into 2 X 3 or so. The new map will be clickable down to a very fine scale and will be much more accurate.
One thing to remember is that a one-degree average change could move you a zone either way if your previous average temperature was at the edge of the 10 degree band.

But, really - the map should be used as a *guide*, not as the absolute truth. It's one's microclimate(s) that also plays an issue in plant survival (or lack thereof), as well as appropriate site location of said plant in addition to multiple other variables :0/

ken_adrian wrote:

zone .. simply put.. is minimum winter temp ... and the issue is how long it stays at that min temp

Ken, that is much too simple. Then again maybe this is the case for the United States but it is definitely not true for Canada or Europe.

And I quote from the "Agriculture Canada" website:

Agriculture Canada scientists created a plant hardiness map using Canadian plant survival data and a wider range of climatic variables, inincluding minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed.

Is it still the case that the US Dept of Agr still only considers minimum winter temps in arriving at the US plant hardiness zones? If so I am very surprised.

Ken, I'm in Kansas City, and the ground most definitely freezes here. Snow cover can be iffy, but that doesn't mean the ground isn't frozen. What it does mean is that the plants aren't insulated from the cold. Not to mention, it's very windy here. In order to survive this climate, the plants have to be able to survive cold Arctic air in the winter and hot, humid Gulf air from the south in the summer. Most plants can adapt to one or the other, but not necessarily both. It bites!

Deanna