i am a little surprised that bonneville dam is so warm. the ice storms will take out most borderline plants that could normally survive there i suppose.

also, i am glad you ran the figures on coast range towns in oregon. elkton being so warm is somewhat of a surprise for me. i was just driving through the alsea river valley by waldport, and the cordylines seemed to do well decently far up the river, indicating a relatively mild climate. sadly, the farther up the river, the properties stopped being landscaped, so i couldn't tell how far the mild weather penetrated. based on your elkton findings, i would expect the coastal influence be strong there as well.

I want to move to Port Townsend!

Thanks for the work Ian.

Some Oregon coastal temperatures were a surprise, seemed low.
Could you run the Tillamook numbers? It is a bit inland and usually posts a wider range of temperatures.

Bonneville is an anomaly, right on the unfrozen water and in the wind tunnel. Columbia Gorge ice storms occur during relatively warm weather, 29-33°. The ice prunes, rather than kills, plants.

Where they put the weather station in each instance affects readings. For instance, Seattle is represented by both the Montlake station at the UW (10-103F) and the Tukwila station at Sea-Tac Airport (0-100F). I've been having stuff burn or die here on Camano - where it apparently keeps dropping below 10F - that does not have the same problems in the other garden near Seattle.

Do they have the Port Townsend station in the downtown area, level with the water or somewhere up on the bluff above, where it would seem most of the town is?

Although mostly bleak otherwise the publicly visible landscape of Port Townsend has long featured landmark Monterey cypress trees, seemingly indicating a similar mildness to the lower outer coastal districts and other hypermaritime locations where large, long-persisting examples of this tree are seen.

Temps for Humboldt County (Eureka and Scotia) seem a bit high. I live in McKinleyville where the county airport is located and where the Coast Guard weather station is located and my lowest was 17. Granted, Eureka is always much warmer, but the reported temperatures are usually those here at the airport.

And thanks for including Northern California, we are usually treated like a bast*rd step-child by others in the PNW.

The coastal strip from Gold Beach south has a milder climate shared with California but not the rest of Oregon northward. Likewise much of northern California has a different climate. And a different flora starts in southwest Oregon, the natural range of the coast redwood for example ends on the Chetco River. Excluding California when talking about the Pacific Northwest would not be a slight but rather a recognition of actual differences.

Hitchcock/Cronquist, Flora of the Pacific Northwest ended their southward coverage in (as I remember it) Lane County, Oregon, because of the different, more southerly floristic elements that began to appear at that point.

Let's see if I can respond to all the replies at once without excluding any details I wanted to remember.

Tillamook: 17.37 = 8b

Here's the coldest temperature every winter for Tillamook going backwards in time from 1999-1969 (that's how I made the spreadsheet - sorry!):
10
26
21
22
23
22
17
24
7 21
8 21
23
14
16
9 22
14
21
15
15
21
16
17
22
11
4 15
20
24

The same for Eureka, CA:

23
30
27
30
29
31
31
32
22
27
27
32
30
31
32
31
31
30
32
29
27
30
32
30
31
31
21
29
31
33

The Columbia Gorge was a bit of a surprise to me as well. It makes sense though if you consider that the coldest nights in most places in the Northwest occur when after the arctic air has settled, and the wind dies off and the air is still. However the geography of the Columbia Gorge area dictates that the air pretty much never stops moving through it one way or the other. That doesn't really make it a better place for growing plants - I might rather be in, say, Salem (zone 8a) than near Bonneville Dam with cold winds constantly howling anytime arctic air comes along.

Then you have southwest Oregon between about Cottage Grove and Sexton Summit (Elkton, Roseburg, etc) - many people don't realize this area is much milder than either the Rogue or Willamette Valley. This also shows up on the Western Regional Climate Center site if one looks at record lows for various locations. With hotter summers than the Willamette Valley this area starts to look like a horticultural sweet spot.

As far as Oregon Coast temperatures being low, I agree - Newport was especially a surprise. I think #1 as I said the period I used was a relatively cold one in history, #2 if these weather stations are at airports then they're probably colder than most people's gardens.

I don't know exactly where the Port Townsend weather is recorded but I'm certain it's not at the airport, which is much colder. It's probably either downtown somewhere or at the Courthouse. The Port Angeles and Aberdeen sites are both at their respective airports. I was a bit surprised how mild Port Angeles was since the site is a few hundred feet up and not that close to the water. BTW, I found a large Monterey Cypress in an unexpected place in Sequim - on the north side of Bell Hill, southeast of town. I don't know of any other substantial ones in the area but there might be a couple in Port Angeles.

I'm rather interested in Northern California. Personally I think I wouldn't mind living in the Redding area though it might be too hot for certain other people in my family. That Shasta Dam site sure is remarkably mild for how far inland it is.

Port Townsend records at the fire station. It's right in the middle of the lower part of town.

I've always had the impression that airports were warmer, not colder, than surrounding areas, but I'm not sure.
There's the concrete jungle effect of course. Then also Sea-Tac seems to always be warmer than other places in the area that I've checked. I live in Marysville and I have a direct line of site (5mi) to Paine Field in Everett, at a similar elevation & distance from the water, but I always seem to be 4-5 deg colder than them. Is it their acres of concrete?
- Tom

Tom, I think the answer is it depends on the airport, but usually, yes airports are colder. The main reason is that airports are invariably on flat land and frequently on valley bottoms which are frost pockets. Some of them (i.e. Sea-Tac) are on plateaus which aren't as cold as valley floors but still have inferior drainage to sides of hills and any kind of sloping land. Some people live on flat land but a lot of people live on slopes and hillsides especially closer to the water (it would be fun to study what percentage of residences are on slopes of, say, 5% or more).

But certain airports should be considered as exceptions because there is so much development around them - including Sea-Tac, Boeing, and Paine Field. All of them are in the middle of concrete jungles and thus probably aren't any colder than the surrounding areas. By contrast you have airports like Olympia and Spokane which are relatively undeveloped themselves, not surrounded by a concrete jungle and definitely much colder in winter (verified by locals, temperature readings and indicator plants) than the cities they represent.

I have some support for this idea. If you plot a running average of the lowest temperature every winter for Sea-Tac there is an abrupt rise during the 1950's-60's that is much greater than that of other weather stations in the region. This can only be attributable to the amount of development in and around the airport during that period. I provide the raw data for that here (although the article is actually about something else):
http://www.desertnorthwest.com/articles/enso_effects.html

That's interesting that Port Townsend records at the fire station - it's about 170' up and 1/3 mile inland, on top of the bluff; meaning that downtown itself, and places like Fort Worden, are probably even milder. Is there a list online somewhere that tells you where each weather station records? I'd love to see it.

Here's a quick survey of the coldest temperature during the December 2008 freeze in a few Port Townsend Locations:
Jefferson Co. Courthouse 19F
Madison St. and Foster 17F
Port Townsend Paper (right on the water) 22F
Woodland Dr. (near airport) 13F
Far Reaches Farm, Hastings Ave. 8F (says them)

Don't know about a list but seems like station latitude and longitude coordinates are usually given when you are reading a report for a particular area.

This zip code based search result also gave the station's elevation.

Here is a link that might be useful: 7-Day Forecast for Latitude 47.84°N and Longitude 122.33°W (Elev. 318 ft)

Thanks for the Tillamook data, it makes sense given the slightly inland location.

Sudden shifts in data are sometimes caused by change in recording apparatus or methods and the apparatus being moved.
Some datasets have algorithms applied to compensate for the equipment changes.

Great post Ian and no surprise that my neck of the woods, SE Olympia, is sadly not a zone 8 area. The airport is on a valley floor, but going up from the valley to Yelm Highway, Yelm itself, Tenino, Ranier, etc, are definitely 7ish. Thanks for this great information, David

What would be useful would be to post a listing of the single lowest temperature for each location during the 30 years, as that is what tells you how hardy a plant that you want to last for many years needs to be there. Doesn't matter if most of the time a location was mild enough to end up in the USDA 9 spread when 30 years of lows for every year were averaged to make that determination. All it takes is one night below the minimum for an otherwise Zone 9 suitable plant to be spoiled.

The USDA web site says to plant stuff rated one zone lower than you appear to be in, in order to avoid losses. Many people don't want to buy and plant things for their yard, only to see them go brown (or worse) some winter.

I see many Cotoneaster lacteus around here got cooked last month. Nursery sources like to say it is hardy to Zone 7. In 1990 the hedge of it across the street froze all the way down, starting over later from the root crowns.

So it sounds like your saying "zone 9 plants" are potentially unsuitable for zone 9.

Yeah I've considered doing a survey of record lows - not a bad idea at all. Maybe it can be one of my next projects. It would also be useful to put all that data on a map.

We have a huge Cotoneaster lacteus that the previous owners planted here. In December 2008 it had some dieback. After this freeze it appears to be completely defoliated.

Statements made like "We had a Zone _ winter this year" are meaningless because the USDA zones are based on temperature ranges, rather than single temperatures. Similarly, calling a plant a "Zone _ plant" with the idea that this refers to a particular temperature does not really fit the situation. It's more a conclusion based on observation rather than prediction. You go someplace that is widely agreed to be Zone 9, the plant is seen having grown there for a long time, in numbers then that makes it a Zone 9 plant. Going someplace like Port Townsend, where you don't see a lot of California plants at all and announcing that because it appears the temperature regime there fits the Zone 9 criteria a galaxy of "Zone 9 plants" will grow there just fine is quite another matter.

Well I agree, as far as that goes. I don't recall having made any such statements, though I know they're in common usage among some gardeners. It's only slightly less meaningless than "California plants."

Actually, "California Specials" as I have heard it used is, again, based on past experience showing that these kinds survive in California but don't last in most places up here. During the 1970s I became aware of a woodland garden south of the Fauntleroy ferry dock in Seattle that contained "California Specials" because the near-waterfront location had California-like minimum temperatures much of the time. One of the owners would bring trusses to rhododendron society meetings of kinds being grown outdoors there that were otherwise associated with northern California. I remember in particular him bringing an open, pristine truss of a red R. arboreum hybrid ('Cornubia', if I remember correctly) to a January meeting. That really brought the apparent softness of his gardening site conditions home.