USDA Hardiness Zones

babywatson(7)April 21, 2005

Someone mentioned in a post on another forum that these have been updated recently. Does anyone know where I can find it? The one on their site looks just like the old one.

Zone 7 is 0-10F, but I can't remember the last time we went quite that low.

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creatrix(z7 VA)

I didn't notice any change for my area either. Maybe the changes were too subtle for us to pick up, or in another part of the country.

I like the new heat zone info that is coming along. With our hot summers, it makes a lot of difference as to what we can grow!

    Bookmark   April 21, 2005 at 4:11PM
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kimka(Zone 6B)

Despite what AHS posted as a draft map, at this time the zone map has not yet been updated. The actual new draft, which for the first time will be a web based map, is in technical review as we speak.

Since I am the person responsible for writing the web page on which the new edition of the map will be housed and for publicizing the map, I guarantee the MAG forum will hear about this first, with the rest of the appropriate garden web forums a close second. You won't even have to wait to hear it from Adrian Higgins.

    Bookmark   April 21, 2005 at 4:30PM
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Kim, do you know what any of the changes are? Is it true that parts of northern Virginia have been changed to 7B?

    Bookmark   April 21, 2005 at 6:12PM
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Mandyvilla(7a No.VA)

Last year the announcement said our area was going to be reclassified as a zone 8 (Northern VA).....Since I read that article, I haven't heard another thing. Guess it will come out in another year and be news, all over again....Bottom-line: Push the zones. ~Suz

    Bookmark   April 21, 2005 at 10:54PM
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Brent_In_NoVA(z7/6 VA)

I don't know about zone 8. When I visit South Carolina and Georgia I am not sure we are really even in zone 7. The USDA used to have the draft map on its site but must have pulled it down sometime last year. From what I understand the new map got rid of the A and B zones. The old map has part of Loudoun County in zone 7 and part in zone 6 (I always forget if A or B is colder). The new map has all of Northern Virginia and much more of Maryland in zone 7.

- Brent

    Bookmark   April 22, 2005 at 10:35AM
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kimka(Zone 6B)


The real new map will not be based on the same data set as AHS used on the draft they submitted and they did not use the same analysis. Chances are any zone changes you saw will not be happening.

Against my wishes, AHS published their draft (with a very tiny draft stamp) before it completed technical review with USDA. They also gave it to the media. But the technical review committee rejected their submission as probably not using a long enough data set, but they didn't even really get that far because AHS did not have their draft done in a GIS-GPS compatible format, which is a standard for this kind of map today.

I have not seen the new draft yet, but the technical review committee is already looking at it. I believe there will not be a lot of zone changes like in the AHS draft.

There is no set deadline, but I hope by summer the new version will be on the web.

    Bookmark   April 25, 2005 at 9:18AM
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Brent_In_NoVA(z7/6 VA)

Thanks for the info Kim. Even Paul James did a segment on the updated zones and a lot of "zone finders" on the web use data from the draft map. That is why I updated my zone listing to be the vague "z7/6 VA".

- Brent

    Bookmark   April 25, 2005 at 12:30PM
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rockman50(6b SEMASS)

I have always thought that the USDA Hardiness Map is extremely deficient and potentially misleading. A good example is how it ranks my area (south coastal Massachusetts). We are technically in zone 7, simply because our winter tempperatures almost never drop below zero (0 to +10 being the minimum temp for zone 7). That would suggest that we should be able to grow most of the same plants here as in the DC area. But this is not the case. Although our lowest temperature may not differ that much during the winter, we have a much higher frequency of cold and windy weather than your area. So duration of cold is just as important as the lowest temperature. So a place like Atlanta has a brief cold wave, and one morning the temperature hits +5. MOst plants will probably survive that. However, if the temp dropped to +5 every morning for 10 days in a row and the afternoon temperature stayed below 20F with a strong wind, that is another story. This is particularly important for growing broad leafed evergreens such as southern magnolia and cherry laurels. I have a southern magnolia in my back yard here and it is growing fairly well and blooming but it will never thrive like the ones in the DC area. SO if anyone has a way of factoring in duration of cold, I'd love to hear about it.

    Bookmark   June 24, 2005 at 2:59PM
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kimka(Zone 6B)

The analysis system being used for the new edition of the map will be using coefficients for slope of hill, elevation, and several other factors. It will not use duration or number of times that the lowest temperature is reached.

But past USDA zone maps and the newest map carry disclaimers that say other factors must be taken into account such as moisture levels, wind breaks etc. when deciding what plants will survive in your specific garden--that simple cold is not enough. And of course the scale of any hardiness zone map is not going to take into account the mini microclimates in your garden such as low spots, sheltering walls, dry areas and so on. The new map when it is finished review and formating will most likely include a sentence like: no changes in this map, nothing should over rule your own common sense about what thrives in your garden.

I hope we will have the new map on the web this summer, but the review committee is taking a second look at a number of analysis choices such as the data set that will be used. One set is "highly accurate" but comes from about 1,000 reporting lication; another is "accurate" and from ~4,000 reporting locations. The review committee wants to know what the actual difference is between the highly accurate and accurate and how much difference does it make in zone boundaries depending on which set you use.

Rockman is right about other factors being very important. And the technical review committee is discussing how future maps might incorporate more factors than temperature such as moisture, wind, duration of cold, etc. But that would end up creating possibly dozens if not hundreds of zones and will plant breedeers do that kind of specific testing. If we don't have coding of plants by those zone, does it do any good to have the zones identified geographically.

As I've posted before, don't believe any drafts you've seen or heard refered to. I know none of them are the least bit close to what will be the new edition of the USDA Plant Hardiness Map--and I sit in on some of the techical review committee deliberations.

    Bookmark   June 27, 2005 at 11:04AM
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Brent_In_NoVA(z7/6 VA)

Again Kim, thanks a bunch for your informative posts.

I know details are not real important when it comes to the zone one point I was at a web site that had a very detailed map on the Virginia USDA zones. The site had something to do with farming. Does anybody know of a detailed USDA map of Virginia or the Mid-Atlantic?

I think that the current USDA map has my home in Sterling in zone 7a, but I am real close to the zone 6b boundary and south of me looks to be 6b. This is mostly just so I can say what zone I am in, even if it really does not matter.

- Brent

    Bookmark   June 29, 2005 at 10:36AM
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Brent_In_NoVA(z7/6 VA)

Kim: Any updates? Just curious.

- Brent

    Bookmark   July 18, 2005 at 5:22PM
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Antique_Geek(7b/8a (S. MD))

Rockman, I have a different perspective between DC and southern NE. I was from RI & now live near DC. I believe you CAN grow many of the plants that can grow in DC area. The big exception would be plants whose northern limit is in the DC area. Figs come to my mind as an example. (There is also an evergreen whose name escapes me.) I would also submet that micro-climate is a factor.

I wounder if this isn't a glass half full or half empty.

    Bookmark   July 19, 2005 at 8:48AM
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Antique_Geek(7b/8a (S. MD))

regarding my last post, after several hours of thought the second plant in question is ryptomeria japonica, I bleieve. A third example comes to mind which is Magnolia grandifloria - southern magnolia.

    Bookmark   July 19, 2005 at 1:32PM
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Okay-I'm a new comer, so bear with me. I live on Long Island New York. What is my hardiness zone? Do you know how small Long Island is on the hardiness map?

    Bookmark   July 20, 2005 at 2:12PM
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gardenpaws_VA(z7 NoVA)

You're probably in zone 7, possibly 6, as Long Island benefits from a very substantial 'water effect' in moderating winter low temperatures. This forum will be a good fit for you, but remember that summer temps for you may be milder than they are for the Baltimore-Washington-Norfolk belt, where many of the posters live. Usually, that's an advantage!

    Bookmark   July 21, 2005 at 12:05AM
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springcherry(6/7 Philly,PA)

I have seen a zone map, I forget where, that showed a blip in the middle of Long Island aroung Huntington. The blip was slightly colder than say ... Oyster Bay and the Rockaways and the South and North Forks. I was suprised since I figured if any part was cooler it would be the central morraine running lengthways (since farthest from the warming ocean/sound.)

Anyway I thought it was fascinating but have never seen that blip again or run into a description of its boundries or an explanation of why it exists.

Just one more of life's mysteries.

Well, at least for now.


    Bookmark   July 28, 2005 at 11:30AM
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kimka(Zone 6B)

I went to the actual USDA Plant Hardiness Zone Map which is 4 foot by 4 foot (and out of print as of a few months ago). At this scale, Long Island is pretty clear.

Square in the middle of Long Island is an oval of 6B. Everything else around it is colored as 7A, most likely an affect of water mitgating the temperatures. On a smaller scale map the center oval could easily be omitted. This is one of the reasons, we--USDA-ARS--are working on a try digital version that will clickable to small areas like just about county level.

I caution against making this little bit of a difference, which could be as little as 1 degree, a big deal. Areas in 7A can have zero degrees as the coldest average low temperature, while areas marked 6A can have minimum average temperatures only as colder as -1 degrees. A and B ranges in a zone are each 5 degree temperature bands.

At this level, mini-microclimates in your own yard created by walls, low spots, prevailing winds and windbreaks, could easily allow you to be a half zone different from say your next door neighbor, at least in some places in your yard.

    Bookmark   July 28, 2005 at 4:54PM
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chills71(Zone 6b Mi)

I have measured my coldest winter temperatures for the past 3 1/2 years (as long as I've lived in my house) and the coldest temperature I've measured is -3 F. One winter my coldest temperature was 2 F, the other it was -2 F. Each time the coldest temperatures were only recorded for 2-3 days, the exception being the 2 F temperature which was recorded for 5 days (once for 3 in a row, and once for two other separate occasions). All done manually (I've got to get a digital system) and faithfully.

Within a mile of my house are two fig trees (that I know of) one of which is unprotected and is at least 20 years old, the other is protected each year and is pretty close to 20 years old as well.

I guess I'm in 6b, but who knows..


    Bookmark   August 7, 2005 at 11:43PM
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Whew, you had me wondering for a minute, then I realized you live in Michigan. I was going to say I can't remember ever going that low around here (VA).

    Bookmark   August 14, 2005 at 8:41PM
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gardenpaws_VA(z7 NoVA)

no guarantees, babywatson - Annapolis MD normally is warmer than I am here in Herndon VA, but Annapolis has hit -7 F. in my lifetime. It sort of stuck in my head - I wasn't gardening there, but I had the dubious privilege of walking about a mile to church that Christmas Eve. We just knew it was cold, and found out the actual reading later. As KimKa says, the zone temps are average lows, not the lowest temperature ever recorded.
Chills sounds like he/she is right on the lake, to have such moderate temps that far north.

    Bookmark   August 15, 2005 at 12:21AM
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I've never understood the moderating effect water has on temperatures. I can understand why the coastal areas are moderated by benefit of the Gulf Stream (although why England and Ireland are warmer zones than the mid-Atlantic you got me--they're pretty far up) but in Michigan right near Canada with the Jet stream right there I would think the lakes might actually not make much of a difference, but I've never been there, couldn't say.

I do remember about fifteen years ago going down to 0 degrees for one day, but that's it. That was the winter we had all those ice storms, no not fifteen, I guess ten.

    Bookmark   August 19, 2005 at 6:34PM
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Chevygold85(z7 Eastern Shore MD)

Ok. I live on the Easternshore MD (Easton,MD) zone 7a as USDA Hardiness Zone Map indicated. The past 6 yrs the winter low have only dropped to 6.1*F in my area, shouldn't we be zone 7b? Also, I heard that the closer you live near the body of water the cooler it gets, right??? Our high temps is also as hott as DC metro area!! How did it happen?

    Bookmark   August 20, 2005 at 1:43AM
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kimka(Zone 6B)

Several facters play into how water and temperature interact. Large bodies of water take longer to heat up than the land and then can retain heat longer (after the whole body of water gets up there in temperature).

There is also temperature brought by the introduction of warmer water like the warm gulf stream current that ends up mitigating the Canadian winter in Yarmouth, Nova Scotia.

And if prevailing winds, which will reflect cooler or warmer water temperatures, blow from the water onto the land, it will have major impact on the land temperature.

    Bookmark   August 22, 2005 at 8:20AM
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