I have never gotten the right zone for me from an internet zone finder. The main thing about where you are is cold air blasting down the Fraser River. See Sunset Western Garden Book (2007 Sunset Publishing, Menlo Park).

Printed maps tend to lump most of western WA into zone 8 mostly due to the small scale the maps are created at. Olympia's zone 7 stands out as a bright orange thumbprint in a sea of zone 8 green. I've never looked at Bellingham on the maps but the time I've spent there, I can well believe most of Bellingham being zone 7. That wind off the Frasier River in the winter is something else.

In any case you should decide for yourself what zone your garden is in. Does it behave as a zone 8 or as a 7? We can't tell you that although many will insist that all of western WA is zone 8. It could easily take more than the 3 yrs you've been there to decide. Sunset's zone maps, as mentioned above, are more detailed than USDA in any case.

I believe the USDA hardiness zones are based on average minimum temperatures, and by that standard we're zone 8 in Bellingham. However, that doesn't take into account the length and frequency of low temperatures, which can be far greater here than in Seattle.

I think the main thing is that we often get profound drops in temperature, into the teens, after spells of mild weather. When you add in the fact that summer temperatures are lower and wood doesn't ripen as well as even in Seattle, it means that a lot of borderline evergreens won't last long here without special care. Most perennials, on the other hand, do just fine here, even ones that Sunset says aren't quite hardy in their zone 4, like Nerine and Alstromeria.

USDA is average annual minimums for a specific 15 year period. People think they are in USDA 7 because they have occasional temperatures between 0 and 10F - that is not what Zone 7 is based on.

Sunset is the total climate, including summer temperatures. It says in their zone descriptions a place can have a record low of 21F and still be in Zone 4.

If you compare maps Sunset 5 lines up with USDA 8b and Sunset 4 corresponds to USDA 8a.

Zones are just generalized guidelines anyway and of limited usefulness. To say that the same plants will thrive in zone 8 Texas as zone 8 Washington is obviously nonsense. Sunset, though more accurate, is way off on many plant hardiness ratings, too. The bottom line is, yes, there are many plants that aren't hardy in Bellingham that are hardy in Seattle, or at least get by long enough to be worth growing there.

Sunset doesn't give hardiness ratings, per se. Their zone assignments are Sunset Climate Zones, they don't routinely zone a plant for an area where the summer is too hot. Because the USDA zones are for hardiness only, the AHS Heat Zone map was developed.

The next step would be to develop USDA climate zones, like Sunset has done all along. Sunset mapped the midwest and east with climate zones years ago, when they produced the Sunset National Garden Book.

A whiz-bang state-of-the-art new USDA system was supposed to be coming out some years ago, there were even web pages up talking about how it was going to be done. Supposedly the Bush administration quashed it, because it would show the country was warming.

I have just moved here from zone 9b/10a and when I asked a Seattle master gardener about our zone in Bellevue, he said that I should think of it as more a 7 than an 8 because it's cooler east of Lake Washington.

But then I asked if I am at the top of a hill, would that factor in and was told I'd be warmer because hot air rises??!

I am not sure but I guess I'll go for the zone 7b maybe for now while I'm choosing plants, especially because my plants will all be in containers.

If anyone has any insight on these above comments, pls advise!

The USDA hardiness zones represent much bigger leaps than something like going from Seattle to the east side of Lake Washington or up a hill in Bellevue.

I've never lived anywhere with so many micro-climates! I'm originally from Los Angeles and 10 seemed standard for large areas even with temperature shifts. I've found that the "Bellingham" temperatures you get with the Weather Channel and other on-line sources are usually taken at the Bellingham airport, and where I am off Mt. Baker Hwy. is almost always a few degrees hotter or colder than that which certainly makes a huge difference in bloom times. And a friend who is a spectacular gardener living in Ferndale (where those Frazier River winds REALLY roar down with a vengeance) just told me that she sticks with plants that are hardy to USDA 5!
I guess the best thing to do is to talk with local nurseries (VERY local) and don't buy anything that seems borderline if you're not prepared for it to die.

California has many more climates than we do up here. Have a look at the Sunset Western Garden Book.

That sounds like a good plan but Ferndale isn't that cold. I have friends that overwinter Dahlias in the ground there. They can grow just about anything we can here. I'd say if you want to be conservative then stick to zone 7 plants but really there are plenty of zone 8 plants that will do fine. Checking with garden centers and neighbors is the best bet, like you said.

So if I'm in zone 5 for Sunset climate zone, how do I compare or match that up with USDA zone, which like you said , is not as specific as Sunset's zones?

Maybe there's no way to narrow USDA zone down?

If you compare maps these USDA and Sunset zones do happen to line up, that is not always the case with others - they are two different kinds of zones, one representing total climate and the other merely average annual minimum temperatures for a specific 15 year period.

Sunset Climate Zone 4 = USDA Hardiness Zone 8a

Sunset Climate Zone 5 = USDA Hardiness Zone 8b

that is very helpful!