A grasp of Zones in Portland
Everybody who gardens has at least a rudimentary grasp on the USDA climate zones. At our nursery I will often hear someone shout out that we are Zone 7 and yet the USDA map clearly shows Portland in Zone 8b. Which is correct? Well, both technically. I know that is confusing but I’ll explain the USDA Zones what they mean, how to interpret them, how I interpret them when choosing and describing plants for garden success.
Context and zones:
0º to 5ºF Zone 7a
5º to 10ºF Zone 7b- Portland’s coldest winter of the past 30 years. (9ºF February 5, 1989)
10º to 15ºF Zone 8a
15º to 20ºF Zone 8b
20º to 25ºF Zone 9a
25º to 30ºF- Zone 9b Portland’s mildest winter of the past 30 years. (26ºF, 2002)
Kissed by the Pacific
We are extremely lucky to garden in Western Oregon, its a benign climate with certain factors built in that leads to success with an amazing palette of plants. What makes this possible, abundant winter rainfall, dry summers, sufficient winter cold and abundant summer heat- all of these factors mimic climates from around the world. Perhaps our greatest attribute is lack of real winter cold. Arctic intrusions are brief and the moderating effects of the huge Pacific Ocean impacts our climate more than 80% of the time- we are literally kissed by the warm winds of the Pacific. Just as important the Cascade crest blocks arctic air from frequently intruding to the west and the east trending jet stream sweeps the coldest air predominantly to the lee of the mountains. All of this and we are at the same latitude as Minneapolis but escape any real arctic nightmares.
Get your weather geek on
This may be geeky but its incumbent upon the gardener to have a certain grasp of the details of their climate. Zones, I see you zoning out – stick with me, I will attempt a summary of our climate as a refresher and I’ll show you what USDA zones really mean as well as the finer points. For instance I garden in the city of Portland. Officially, this is USDA Zone 8b. That means that official reporting stations in the city average their coldest overnight lows over the past 30 years and come up with an AVERAGE annual low between 15º to 20ºF (-9ºC to -7ºC). To be more precise our average annual winter lowest temperature is 19.7ºF(-7.5ºC) at the Portland Airport. Wait a minute you say, I have been colder than 19ºF in the last 30 years. Yep, its an average and averages are made up of extremes. In the past 30 years we have been as cold as 9ºF and have had winters where the ultimate low was a balmy 26ºF. So, how do I use the USDA climate zone to garden?
In the zone
At Xera we use the USDA zone designations in a different way. When we grow a plant we take into account the coldest temperatures in the past 10 years. (11ºF). All plants have a lifespan and for instance, if you were to plant a shrub or a tree with a very long lifespan you would be in danger if you relied on the average lowest temperature- in a ten year period you would expect colder temperatures, which could lead to severe damage to death. So we use the framework of the USDA zones- the 5ºF increments but we adapt that to our climate. For example our arctic events are very short lived and have several components that I factor in. During our coldest events which occur every three years or so we not only experience cold overnight lows- but true arctic air makes subfreezing high temperatures occur as well. This adds stress to a plant and has to be factored in to the equation.
Australia offers up an example
One of the best examples of specific climate adaptation comes from many Australian plants. Alpine plants from Australia can be grown very successfully in our climate. And ultimate lows there are comparable to ours. But, Australia, though it can get cold, has no real source of unmodified continental arctic air as we do. There elevation plays a much more important role in the coldest temperatures. And even that is misleading as their very highest elevations pale in comparison to ours. Remember that cold air sinks and it is in mountain valleys in the Australian Alps that you find the ultimate coldest temperatures. And the coldest ever recorded is just -9ºF for the WHOLE continent. Compare that to Portland’s ultimate lowest -3ºF (February 2, 1950) and even the coldest ever recorded in Oregon which was an insane -54ºF in the Bear Valley at Seneca in Eastern Oregon. In Australia very cold overnight lows are often followed by fairly mild- above freezing high temperatures. This wide diurnal stretch allows plants to recover daily from some pretty bitter night time temperatures. As I said when arctic conditions occur here we can expect some very cold high temperatures- in the 20’s for at least several days. Thats the difference between us and Alpine regions of Australia. So a plant there might be zoned as 8a and survive 12ºF but daytime highs mitigate the exposure. Here it may survive 12ºF lows but succumb to daytime highs that don’t budge above freezing. It is one of the reasons alpine Aussie Acacias should work here but don’t. This large diurnal temperature pattern is also common in inland California as well as parts of the southwest.
Freeze the hell out of that plant
The trick then is to find plants that can handle our 10 year coldest temperatures as well as the inevitable subfreezing highs that accompany an arctic outbreak. And- each freeze is different (duration, depth, wind, snow cover) So how to come up with a hardiness rating? Observation over many years, provenence (a plants specific native origin- including elevation and latitude) and testing plants. For Xera that means we’ve killed a lot of prospective plants in order that they do not die an untimely death in your garden. And even all of that does not mean that a plant is full proof. As you know wind can have a MUCH greater element of stress to a plant. Our arctic events almost always are accompanied by arctic ouflow from the Columbia Gorge. Portland has a unique orientation to the powerful winds that roar from the much, much potentially colder source of air that is the Columbia Basin. In my best estimation of gardening here for 35 years I can say that observation of wind- precisely SUB-freezing wind, leads to a profound effect on cold hardiness. For example, a temperatures of 28ºF and a persistent 25 mph wind can cause the same affects as a calm 18ºF. Thats 10ºF of cold adaptation that must be factored in as well. This is how plants can die even though we haven’t technically achieved their coldest tolerance (in calm air) and they are rated for much lower.
Ideally you would like the temperature in Fall and early Winter to become steadily colder exposing plants and naturally hardening them off. I welcome light freezes in November and December that naturally prepare plants for possible colder temperatures later on. And almost all somewhat hardy plants can be affected by this- even surprising plants such as Phormium benefit from gradually cooling temperatures. Unfortunately, that is almost never the case. Arctic outbreaks can come rushing in following mild temperatures and this can wreak havoc on the garden. So, I also factor in that element as much as I can.
Achieve the ultimate
In order for a plant to achieve its ultimate tolerance of cold it must be grown in the most optimal conditions. Make certain that if a plant requires full (all day) sun that it receives it. An example of this is Callistemon (Bottlebrush) which can be significantly more tender to cold in shaded locations- it really requires exposure to harden off. Establishment is a very potent factor- the older and more established a plant the hardier it will be to cold temperatures. A bold example of this is Fringebush (Loropetalum) small new plants succumb to 20ºF and established large specimens roll through 0ºF with no problem. Its important that if a plant is borderline hardy Zone 9a or even Zone 8 that you plant it no later than May 15- that allows it to maximize establishment with the longest growing season possible. Agaves are another case in point. If they have put down a taproot in a long growing season they are many, many zones hardier to cold than when not. Greater summer heat also creates cold hardier woody plants as well and luckily we have just enough days above 90ºF for this to be a benefit.
Sodden and frozen
Our heavy winter rainfall can also have an adverse affect on cold hardiness. Sodden soils do several things to plants in combination with severe cold. Wet soils mean that a plant is full of water- thus restraining dormancy and when jack frost hits- it results in injury or death even at temperatures much warmer than a plants native haunts. Cold and dry- well drained conditions that many plants are adapted is a much better state of things. Many southwest natives such as woody Salvias, Agastaches, Yuccas, Cacti and Agaves can take incredible cold if they are in dry conditions. Even alpine South African succulents- ice plants (Delosperma) sail through winter in places like Denver- exposed to temperatures well below zero for many weeks but in dry conditions. In western Oregon Delosperma either freezes out at wet and 20ºF or is seriously compromised. These plants cannot tolerate the lack of oxygen in the soil that sodden frozen conditions create. If a plant calls for good drainage, amend the soil with non-organic material, plant it on a slope and do not cut back the frosted foliage until all danger of freezing conditions have passed- usually mid-April. The rainfall averages vary greatly over the city of Portland and this often shocks gardeners who find that from a low of 37.5” at PDX which is at sea level to more than 65” annually in the West Hills above 700’+. Thats a huge difference in just a few miles. In western Oregon its all about elevation. Not just because of snow levels but influencing overall precipitation accumulation as well- the higher you are the wetter you are.
Outlying regions are colder
I’ve been discussing Portland but its important to know that areas away from the city are quite a bit colder. Hillsboro, Vancouver, and Salem are generally a half to a whole zone colder. So look for plants that are hardier for those locations. The urban heat island is not only apparent in Portland it appears that it is having a much more profound effect on our gardens. In those locations take advantage of favored microclimates if you are unsure of a plants cold hardiness. That is south facing aspects, at the top of a slope or adjacent to asphalt or concrete. Also, many slightly tender plants benefit from the over head protection of a tree canopy.
Your own garden climate
So you can see how we can be considered both Zone 8b (annual) and Zone 7 (30 year stretch) and the many factors that play in to cold hardiness in plants. It is great to set up your own thermometer and get to know the intimate climate in your own garden. Microclimates can really vary between reporting sites and everything from amount of asphalt in your neighborhood to elevation can play an important part. If not the National Weather Service has a wide web of reporting stations and its not difficult to locate the closest to your garden on the observations page of their excellent website. Get to know your own zone and improve your gardening acumen. There are more complex factors than just a simple low temperature. -Paul
Portland/Vancouver Metro Weather Recording sites:
Portland weather: Fun Facts
Portland’s coldest temperature was -3ºF on February 2, 1950.
Portland’s warmest and only frost free winter was 1934/35
Portland’s average lowest temperature (at PDX ) is 19.7ºF USDA Zone 8b
Portland averages 2 days per year with subfreezing high temperatures
Portland averages 33 days a year below 32ºF (Ave. Freeze date Nov. 20 to March 19.)
Portland’s coldest high temperature ever 14ºF January 1968
Portland’s highest temperature ever is 107ºF in 1965 and twice in one week in August 1981.
Portland’s warmest overnight low was 74ºF on July 30, 2008.
Portland averages one day above 100ºF per year (101.3ºF) and 14 days above 90ºF
Portland’s most days in a year above 90ºF was 2015 with 29 days.
Portland averages 37.50” of rainfall at the airport 55”+ above the elevation of 500’ and higher
Portland averages 4.3” of snow per year in the last 30 years.
In the last 30 years PDX has been as low as 9ºF and as high as 106ºF (twice).