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Zones?

Posted by DenFen 8 (My Page) on
Wed, Oct 16, 13 at 12:10

I am curious how the zones are determined? Via this site I am zone 8, but based on Better Homes and Garden book I am zone 7.

Do these zone coincide? Or they totally different?

Thank you!


Follow-Up Postings:

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RE: Zones?

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Wed, Oct 16, 13 at 13:26

Where do you live? If you're in Portland or Seattle you are in USDA 8.

I'm near Seattle but automated zone finders and other sources try to put me in USDA 7, which is definitely incorrect.

A few people posting here list their Sunset climate zone without realizing the zones others are using are the USDA hardiness zones. The other two systems you may run into (mostly elsewhere) are the Canadian hardiness zones and the AHS heat zones.

Nursery catalogs and web sites, other sources may simply give hardiness zones without identifying them as being those of the USDA. Look for maps or tables indicating what their zones correlate to in those instances.


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RE: Zones?

First, it is important to remember that hardiness zones are guidelines only. Every garden has its own set of microclimates that can alter the "official" zone designation. Also, I have found some sources for hardiness zones are less accurate than others -- I believe the ones that track via zip code are more accurate than just the maps :-)

I pulled up the BHG zone map and the one I see lists most of western Washington as zone 8, which is typically considered to be the correct range. The higher the elevation and the closer one gets to either the Cascade or Olympic foothills, the likelihood of a declining zone listing increases - areas like the Issaquah plateau and Fall City are usually listed as a 7. The closer to the Sound and other large bodies of water, the higher the zone is due to the moderating influence of the water - some protected pockets of the Sound are upped to zone 8b or even 9 (but I would never rely on that as the gospel :-)). And there seems to be a rather unique south Sound pocket in the Olympia area that tends to the lower end, closer to a zone 7.

The basis for the zone listings are based on average winter low temperatures over a period of years. I think the most recent revision expanded that period to 30 years. And it is important to keep in mind these are average winter lows -- every so often this area will experience an Arctic front or cold spell that drops far enough below the average to zap many borderline zone 8 plants pretty well.


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RE: Zones?

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Wed, Oct 16, 13 at 15:44

Zip code based ones put me in USDA 7.


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RE: Zones?

I am in Sutherlin Oregon which is at the top part of what is considered southern Oregon. So when I put in my zip here it says zone 8. But if I put my zip in BHG it's zone 7. Maybe I'm on the cusp. Our elevation is 640, we do get snow dustings in the winter. Summer highs/lows are around 95/50, winter is about 40/22 (22 on the worst night). My zip is 97479.

Thank you guys so much for your information. I am so new to this area and learning what I can grow and what I can't.


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RE: Zones?

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Wed, Oct 16, 13 at 23:43

To be in USDA 7 your average lows have to be 0F to 10F. So there is a likelihood of sometimes seeing temperatures below 0F. If instead your very coldest winters are somewhere above 0F you are probably in USDA 8.


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RE: Zones?

Current USDA hardiness zone is based on the average lowest annual temperature for the years 1976 to 2005, a span of 30 years inclusive.

You have likely had temperatures colder than 22 during that time span. Perhaps a local newspaper lists daily records and the year they occurred.


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RE: Zones?

The National Gardening Association says I am 7b. The USDA says I'm 8b, BHG says 7, this site says 8 lol.

So I guess what I need to do is find what my average lows are and use that to determine what I can grow and what not. We do have so many micro climates here in Oregon. Because of this I did build a greenhouse. The zoning question is mainly for the landscaping I need to do outside. So once I find the plants that will grow, it will then be broken down as to what is deer resistant and what won't take over the yard.

I so appreciate you all taking the time to give me information! I've not been on a site yet that has been as helpful as the people here :)


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RE: Zones?

  • Posted by bboy USDA 8 Sunset 5 WA (My Page) on
    Thu, Oct 17, 13 at 14:12

For what you are talking about doing at this point Sunset may be adequate - their climate zones are better than the USDA hardiness zones that most people refer to in discussions. Their plant encyclopedia (the traditionally very popular, printed Sunset Western Garden Book still appears on shelves, including at CostCo in season) has thousands of entries with Sunset zones indicated for each.

Here is a link that might be useful: Sunset Plant Finder


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RE: Zones?

the new usda interactive map is good. the level of detail is a bit better than average. at least around Portland, i find it pretty accurate. the 9a rating for downtown follows my observations of phormiums and cordylines.

Here is a link that might be useful: usda map


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RE: Zones?

Interesting zoom on Portland. Since there are only two official weather stations with an extended history (KGW downtown and the airport station) the hardiness per zip code for the city either has to be an estimate or it relies on wunderground amateur stations.


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RE: Zones?

USDA Winter Hardiness zoning has only use in choosing shrubs, trees, perennial plants. That is all.
When it comes to summer and fall gardening has little or no meaning. Let me give you some examples:

Most PNW( Portland, all the way to Seattle) are rated as zone 8.
Big parts of Georgia and Texas are also considered zone 8.
But when it come to gardening the South and the PNW are two totally different climates.
Now they have come up with another concept, called HEAT ZONE. most of the PNW (usda zone 8) are in HEAT ZONE I or 2 . Heat zone one means : RARELY SUMMER HIGH GO HIGHER THAN 84F. if it happens like 10 days the it is heat zone 2
Although we have a long FROST FREE growing season, but it is too cold one month from each end(spring and fall) . So it cuts our growing season by almost 2 months. Then the rest of months are nor very warm either.


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