Climate is to weather as a wardrobe is to clothes

The Pacific Northwest is very far to the north- the same latitiude as Minnesota and Massachusetts and

yet we experience much milder winters. Why is this? In two words the Pacific Ocean. Weather in the

northern hemisphere moves from west to east courtesy of the Jet Stream. That means that from autumn to spring the most common source of air is from over the ocean. Saltwater is a great insulator of heat.  It is both slow to absorb heat and slow to release it. At our latitude the Pacific Ocean remains at a narrow range of temperatures between 48º and 60ºF throughout the year. When air that has been modified by mild sea temperatures moves inland it brings with it a cushion of humidified air that moves inland over the surface. This humidified air retains the same characteristics as the ocean, it holds heat and loses it slowly.  As it moves inland it loses heat in the form of precipitation and cools slightly. As a result we enjoy and incredibly long period of the year with temperatures between 35º and 55ºF. For much of the year our climate can be summarized as the temperature of the ocean, cooled slightly by the land. In technical terms this is called modified marine air- it is the source of the majority of our weather during the year. We know it as nearly continual storms that dump rain on us from Autumn to Spring . As you know weather can vary quite a bit from day to day, this is just a broad description of the most important influence on our climate.

Occasionally in winter this east to west weather transit is interrupted and the jet stream veers from its course over the Pacific and is pushed up into Canada and then sent roaring straight from the north or northeast over us. The results are intrusions of cold arctic air and our coldest weather. Most of the time we are protected from this air by the Rocky Mountains and the Cascades and it moves east into the Great Plains.  West of the Cascades there are two gaps in the mountains that allow this cold air to intrude further. In northern Washington and Southern British Columbia the Frasier River bisects the North Cascades and allows this cold continental air to funnel from the northeast directly over Bellingham and into the Puget Sound.  In northern Oregon, the Columbia River Gorge scours a sea level gap through the Cascades and is another source of cold continental air from the east which arrives in the Portland Metro areas and moves south into the Willmamette Valley. The result is that both of these locations can be more directly influenced and can have dramatic local effects. These blasts of cold can last as little as a day or up to a week. If this cold air arrives in autumn or spring or after weeks of unusually mild temperatures plants are much more vulnerable to damage. In time, the westerly Jet reasserts itself and this cold weather is shifted to the east and is replaced again by warm Pacific air. In summer the jet stream is naturally deflected to the north and with it the parade of storms also goes north,giving us  drier weather and decreasing the influence of the Pacific Ocean. Occasionally hot dry air from the southwestern United States moves north and gives us our hottest weather of the year.

Local influences affect the weather.

Ultimate winter lows are not the only determining factor in a plants adaptation to climate. Length of cold temperatures and when they occur can be just as important. For most of Western Oregon and Washington

our growing season runs between April 15th and October 15th. That is the average period when we do

not experience a hard freeze (below 28ºF). This broad time frame includes rural areas which often experience colder temperatures earlier and later in the season.  Locations that are higher in elevation, above 1,000 feet, will often experience a shorter growing season as well. Higher locations can also expect more precipitation including snow. In those areas attention to heavy wet snows is just one more detail to take into account when choosing plants. Urbanized areas, areas near the Columbia River Gorge and areas adjacent to the modifying effects of salt water such as Puget Sound will often have a much longer growing season. Concrete and pavement in urbanized areas absorbs heat during the day and releases it slowly back into the atmosphere at night  This is known as the Urban Heat Island Effect. A striking example of this is found on south facing walls during the hot summer months, heat absorbed during the day will be released at night, those walls are often still warm to the touch hours after sundown. For plants to achieve their ultimate hardiness, it is best if there is a period of steadily colder weather.  Frosts followed by light freezes forcing them into dormancy. This seldom occurs, but when it does and the coldest weather occurs in mid-winter, plants that you thought were tender can survive surprisingly low temperatures.

Puget Sound inland maritime

In the Puget Sound proximity to salt water has another profound effect on the local climate. Just as the Pacific Ocean modifies our overall climate, Puget Sound does it on a local level. Saltwater acts as an insulator on a local level. It is slower to absorb both heat and cold. In the winter cold dry air is humidified above the salt water and acts as a cushion of warmer air that insulates adjacent land masses. This effect decreases rapidly as you move away from the Sound. The result in those areas are temperatures that are dramatically lower during cold snaps in just a few short miles. During the summer salt water is slower to absorb heat as well and acts as a natural air conditioner reducing summer time heat.  In those locations special attention must be paid to heat loving plants.  South facing walls and areas protected from summertime afternoon winds from the north are best.

Willamette Valley the rainforest illusion

In the Willamette Valley summer heat is more pronounced. We expect about 16 days to be above 90º

and at least one day to be above 100ºF.  In July and August our warmest average high temperature peaks

out  83ºF. Statistical averages are important to get a picture of the overall climate. More importantly,

for the gardener understanding the patterns of our summer weather can greatly assist in when and how

to irrigate. Our summers rarely average the same high temperatures consistently. Normally we go through

a pattern that occurs beginning each June and lasts  into September alternating between a period of mild weather to very hot weather quickly and then back to mild weather. These fast transitions mean that close attention should be paid to forecasts. Gardeners know that a container that looks perfectly fine on a day when the high is 78º can melt rapidly if the temperatures shoots up to 94º the next. day.  During these heat waves water-loving plants may require daily irrigation- which is most appropriately done in the very early morning. During the hottest periods (multiple days above 97ºF) you may even have to water more than once a day. That is when you realize that it is all well and good that a perennial (Astilbes, Rodgersias etc. ) may be hardy to -30ºF but they have no hope in hell surviving on their own in our climate without massive intervention or a naturally occurring bog.

Zones temperature and hardiness:

USDA zones are based on 30 year temperature records kept at official weather reporting stations. Of course the weather can vary widely between those reporting stations so it is important to pay attention to weather in your own garden. Most importantly it is best to know that our climate is similar to others but when it comes right down to it , it is our own. Therefore, we don’t like to classify it as anything other than what it is. If you live in western Washington, its western Washington’s climate. In  western Oregon, guess what? You got it.

We have a winter wet, summer dry climate. Winters are cool and wet, the amount of time we spend between 35º and 55ºF- IT CAN BE MONTHS! Spring, quite often is showery and cool and summer is dry and pleasant, intermittently hot, before fall which is quite often showery and wet. Arctic air can intrude and last for a day or up to 10 days. It is usually gone pretty quickly and gives us our coldest weather in quick dramatic shots. Cold hardiness, the adaptation of plants to take cold has many determining factors. It is important to meet a plants ultimate cultural requirements for it to achieve its full hardiness to cold. Even when those requirements are met- topsy turvy weather can cause even more havoc. Take into account a plants adaptation to soils, moisture, siting and we think its best to look at each plant on a case by case basis.

In western Oregon and Washington our winters fall into these zones.

Zn10a  35º to 30ºF

Zn9b   30º  to 25ºF

Zn9a    25º to 20ºF

Zn8b    20º to 15ºF

Zn8a    15º to 10ºF

Zn7b    10º to  5ºF

Zn7a      5º to  0ºF

Zn6b      0º to -5ºF

Zn6a   -5º to -10ºF

Zn5b  -10º to -15ºF

Zn5a   -15º to -20ºF

Zn4b   -20º to -25ºF

Planting based on zones

We list the specific zone for plants not just a as a guide to over all cold hardiness but as a guide that also tells you when it is best to plant. We like to follow this rule:  If a plant is hardy to zone 7 (below 10ºF) or lower it may be planted in any season of the year. If it is designated as zone 8 (10º) or above it is most safely planted in the spring before June 15th. This allows the plant to become well established before its first winter in the ground. Take special care to water spring and summer plantings regularly through the dry summer.

Portland and the Gorge effect.

The Columbia River Gorge has a profound effect on the weather in Portland. These effects can be confusing and it is to the gardeners benefit to understand how this feature influences our weather. The Cascades create a barrier that blocks drier colder air from the interior of the continent from moving to the west. The Columbia River Gorge is a sea level path that cuts between the mountains and under certain conditions air is drawn through this gap. When higher pressure sets up east of the mountains and lower pressure develops to the south and west- air flows from high pressure to low pressure through the Gorge  and spills out into the Portland Metro area. We know it as the east wind, a reversal of the more predominant  westerly flow from  the Pacific Ocean. The effect of this wind varies with the seasons and most importantly with the temperature of the air mass east of the mountains.  In Portland one profound effect is to lengthen the growing season by inhibiting frosts in Autumn.

Stable Autumn air masses are often accompanied by an easterly breeze drifting out of the Gorge. During this time of the year the air mass east of the Cascades is not appreciably colder than the west. The result is that this air movement keeps the atmosphere mixed up and interrupts the evacuation of heat from the surface. Adjacent areas away from this effect will experience no wind. On those still nights radiational frosts are much more common. This explains why the growing season in Portland is up to six weeks longer than nearby Hillsboro and Salem. Often, if the air a east of the Cascades remains mild this flow can extend well into winter  preventing frosts quite late into the year.  In those locations away from the influence of this flow radiational frosts can actually be beneficial to gardens, nudging cold hardy plants to go safely dormant and hardening them off for colder weather that comes later. Away from the Gorge fog is another result of this stable weather. It can linger day and night for weeks in the central and southern Willamette Valley.  In Portland, the east wind prevents fog and it is dry and sunny.

Climate Zones

Temperature and patterns of precipitation have a profound influence on our gardens. It allows us in the PNW to grow an extraordinary range of plants. The more you know about the climate a larger diversity of plants will succeed in your garden

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