Hills, valleys, depressions, pavement, buildings, trees, shrubs all combine to make different microclimates

in a garden. Most importantly, do you know which direction is which? In our climate and at our latitude there is A VERY marked difference between cool north exposures and hot southern exposures.  Is it sunny? Is it shady? More likely is it somewhere in between? Take a close look at the sun patterns in your garden. Watch closely how sunlight is affected by trees and buildings and make sure you pay attention year round. Remember that cold air sinks. In many cases the lowest part of a garden with exposure to the sky is where you will find the coldest temperatures. Pay attention to where frost or snow stay the longest in your garden. Areas that are elevated  or with a southern exposure are often the warmest parts of

a garden. Those areas, and areas under eaves or decks require special attention to irrigation, even during the winter. Is a part of your garden exposed to strong winds? Taking stock of specific conditions will also

ensure success.

 

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

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.

Again, 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.

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.

 

 

For plants that are tender to cold here are several hints to aid in success. MOST IMPORTANTLY, tender plants (Zn8b and above) MUST BE WELL ESTABLISHED before they go into their first winter. A good example of this is hardy Fuchsias. Even the cold hardiest Fuchsias will not make it through an average winter if they have not become established. It is best to satisfy the ultimate requirements of each plant, with soil, water and placement to achieve the best growth during the season. Mulch may be used to protect the crown of a plant before winter. We recommend keeping around a supply of mulch that is dry before the onset of cold weather. Watch the forecasts closely....a great resource is your local National Weather Service website which will post forecasts up to 10 days out. In autumn, pile leaves around the base of tender plants. Before a predicted freeze spread mulch

over the top of these leaves. For woody plants do not let the mulch pile up against the trunk of the plant. Remove mulch when truly warmer weather arrives. St. Patrick’s Day is a good time to do this. At this point it is critical to protect emerging plants from slugs. Often they will annhilate the first emerging shoots.

If a truly arctic event is predicted. A period of temperatures with lows in the teens or lower and highs below freezing more protection may be required. If colder than normal weather threatens here is a simple and easy method of protection. Evergreen boughs. Before a freeze gently lay evergreen boughs over and around tender plants. This method allows a certain amount of air circulation, but protects plants from wind and bright sunlight. It appears to offer just the perfect amount of protection. One source of boughs is a christmas tree. We cut the boughs off of our own Christmas tree and keep them around as a  nearby source in the event of a post-Christmas “event”. Otherwise, there is really no shortage of evergreen boughs in our area. If you have to beg. Beg.

 

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 nearly constant temperature between 48º and 60ºF throughout the year. When air that has been modified by mild sea temperatues moves inland it brings with it a cushion of humidified air that moves inland over the surface. This humidifed 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 modifed 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 Plants.  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.

 

 

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

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 ultmate 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.

 

 

 

 

 

 

 

 

 

 

In the immediate Portland area the urban heat island effect plus the local Columbia River Gorge combine to create an even longer growing season. Stable Autumn air masses are often accompanied by an easterly breeze drifting out of the Gorge.  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. These 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. In Portland, this lack of cold can actually be deterimental to gardens. Lack of frosts and hardening to cold can be replaced suddenly by subfreezing arctic air that also spills out of the Gorge shocking plants that have yet to adapt.

 

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 landmasses. 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.

 

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.