Climate Zones Gone Wild!
Climate zones have the unfortunate habit of confusing new gardeners, and some seasoned gardeners still don't really know what zone they are in or what it means (whether or not they would like to admit it). This is why I have written the following piece, employing as a literary tool the garden writer's best friend, cynicism.
Two primary climate zone systems find themselves in popular use in the United States today.
The USDA system is widely used across the country. It is based strictly on one single factor: a multi-year average of the coldest temperature recorded every winter at a given location. It does not take into account any other factors that effect plant performance such as summer heat and precipitation quantities and patterns. It does not even matter what your overall winter average temperatures are. All that matters is your single coldest temperature every year, averaged over a certain time period.
This average is then assigned a zone, according to the following scale:
The zones are then divided into 'a' and 'b' sections, with 'a' being the colder half and 'b' being the warmer half, as follows:
Sounds simple enough, right? And yes, it could be simple - the problem is that there is no public resource whereby one can easily find the necessary historical weather data to determine the average of their coldest temperature every winter - a statistic that has no other popular use besides the calculation of USDA zones.
It's fortunate, then, that the USDA has done us the service of putting all this data on a map for the whole nation. As Mike Myers would say, 'NOT.' In the Pacific Northwest, we know that you can drive 3 miles and be in a different climate. So is it really possible to create a map accurate enough to accurately put this data on a map? The answer is, as they say in Spanish, 'no.'
Furthermore, in the production of the 1990 Plant Hardiness Zone Map, which is still popularly used (though newer editions are available), the USDA goofed by selecting a rather short and anomalously cold time period to collect their data. No one really knows why, but it doesn't help provide anyone with a more accurate picture of their climate.
The moral of the story is, not only is the USDA map inevitably inaccurate, it is also a tool with very limited usefulness. I'm not saying it's completely worthless - and it's certainly more useful in the Eastern United States, where the topography is smooth enough not to interfere with accurately mapping the zones, and where other factors affecting plant performance (summer heat, rainfall) are more equal over a much longer distance than in the West. But out here in the West, it isn't a particularly useful system no matter how you slice it. Still, that doesn't stop some people!
That's why we have the Sunset System, developed by Sunset Magazine for their Western Garden Book. The original Sunset System is designed specifically for the West and takes into account such factors as precipitation, summer heat, and even plant performance, besides just temperature, in assigning zone designations. The map is quite accurate - not perfect, but as good as can be expected given the diverse topography and abrupt climate changes from one place to another in the west. The map is good enough that most people have little difficulty figuring out their Sunset zone.
So the Sunset system is pretty good. The only real beef I have with them is that there are not enough zones. It might have been appropriate for them to make three times as many zones for the necessary specificity to cover the whole West. Newer editions of Sunset's Western Garden Book have subdivided some of the zones - I guess that is a step in the right direction.
So, what's so confusing? Let's review.
First, the two zone systems. Someone can say 'I'm in zone 5' without mentioning which zone system they are using. That is the first thing to get cleared up for any discussion about climate zones to be meaningful. Of course, if someone says 'I'm in zone 17' that pretty much excludes the USDA system as their zones only go from 1 to 11.
Second, there is the impossibility of having an accurate map with the USDA system, as described above.
Third, there is the difficulty of finding the needed historical weather data to know one's USDA zone exactly, also as described above.
Fourth, the USDA zone system is based on a single factor, one that is indeed extremely limiting when it comes to the practical part, where we endeavor to assign zone ratings to actual plants. Let's consider for a moment the palm Phoenix canariensis. This plant is hardy enough to become a tree in zone 8a in Las Cruces, New Mexico. But it's not able to do so in zone 8a in the Pacific Northwest. In fact, it isn't even hardy enough to become a tree in mildest zone 9a microclimates in the Pacific Northwest. So, it is hardy to zone 9b? Or hardy to zone 8a? No reference can really say without being regionally specific.
But can't we simply use some scale adjust the zones downward for regions with wet winters and cool summers? Not really, because there are plenty of plants that can flourish long-term in Seattle that would freeze in an ordinary winter in Las Cruces. And who's to say that's Seattle's zones should be adjusted down? Maybe we don't like that in Seattle and we think Las Cruces' zone should be adjusted up, if anything. (I jest to make a point.) Furthermore, that would mean deviating from the USDA's premise that their climate zone system is based on one single factor, as described above.
Fifth, somewhere along the way, people (including many book authors and nurserymen) started assuming wrongly that USDA hardiness zones can be assigned based on the lowest temperature a plant will tolerate. For example, a plant tag might read: "Cordyline australis, Hardy to 15°F/USDA zone 8." But the truth is, nothing that is going to die below 15°F is going to survive very long in zone 8. The USDA zones are based on averages, and zone 8 is defined by areas where the average coldest temperature every winter over a period of years falls between 10°F and 20°F. But not every winter is the same: in a colder than usual winter pretty much all zone 8 will fall below 10°F at some point. Goodbye Cordyline australis.
Sixth, a consequence of that is that people have begun to refer to plants as "zone X plant". Phoenix canariensis is a zone 9 plant. No, it's not, it won't even grow in zone 9 in the Pacific Northwest. No, it's a zone 8 plant because it grows in zone 8 in New Mexico. There's no purpose in defining a climate based on what will grow in it. If I can grow cacti does that mean I live in a desert?
This particular problem may actually have been instigated by the USDA itself in their description of how to use the map. They cite examples of plants one might expect to encounter in particular zones, which isn't bad in itself, except that it might lead people to assume a climate zone can't be present if that particular plant isn't there to prove it (which is like saying, 'if I can't see any Delphinium, I must not be in a flower garden'). For example, Phoenix canariensis is on their list of plants one might be expected to find in Zone 9. Does that mean Florence, Oregon can't be zone 9 since there are no mature Phoenix canariensis in town? Actually, it is zone 9, but every once in a while it gets just cold enough there to kill Phoenix canariensis. The same could be said for the pockets of zone 9 in western Washington and British Columbia.
Are we confused yet? I tried. Anyways, my conclusions about climate zones are as follows.
#1. The USDA system is fraught with peril. There's not much point in anyone using it unless you understand exactly how it is meant to work and what its limitations are. That's unfortunate, since it is meant to be user-friendly. But the truth is, it's not, for all the reasons listed above.
#2. The Sunset system is better. Sort of. At least it makes an effort to be more useful.
#3. Plants can't read books or maps. Know your garden, your absolute minimum temperature, and other factors pertinent to plant performance. Use some discretion and common sense, but when in doubt, just try it and see what happens. Who knows, maybe it will perform better than you expect.
In Britain, plants were traditionally categorized as 'hardy', 'half-hardy', 'hardy in sheltered gardens', and 'tender'. Perhaps a more regionally specific approach with these simplistic categories would be more beneficial than the one-size-fits-all approach the USDA attempts to use to cover the whole country.
For a well researched and informative article about the USDA Climate Zone System, elaborating further on some of my points, check out Plant Hardiness and Mapping Out a Strategy (offsite, courtesy of Tony Avent and Plant Delights Nursery).