A recent project that I've been working on has made me aware that there is some confusion about how to use hardiness zones.
First, let's talk a bit about what hardiness zones are. Each plant has inherent resistance to a minimum cold temperature in mid-winter. This, more than almost any other factor, determines which plants can grow in a given climate. For example, lemons are not hardy in the Owens Valley. Nor is papaya. The cold weather kills the plants. Can't grow them here. Period.
To get an idea of how cold each locale gets, we use a system of dividing the USA into a series of zones delineated, at 10 °F intervals, by the average minimum annual low temperature for a 30-year period. This map is derived from weather data and is available here. (And elsewhere.) If you're in Zone 7, the average minimum temperature in a year will be 0 °F. Zone 6 is -10°. Most of California is fairly mild due to coastal influences, but we're quite a bit colder so hardiness is important in the Eastern Sierra.
Because of the important role of hardiness in plant selection, it is the primary criterion for plant for most of the USA and Canada. Since we have lots of data on hardiness both in terms of understanding plant species AND the local climate we often find nurseries and catalogues group plants by their recommended zones. Here's an example: One of the nation's largest nurseries, Monrovia, has a plant selection tool for their product line. On the top row of options to filter the catalogue is "USDA Cold Hardiness." There are dozens of similar resources online for woody plants and perennials that you can find on Google.
Looking at my data, the thing that seems to confuse gardeners most is how to put to use the hardiness descriptions for plants. Most catalogues and reference materials will say something like "USDA Zones 4–8." And that's where the confusion creeps in.
I found this plant description I quickly found on the Internet as an example of what's usually encountered:
Marjoram (Origanum vulgare) Zones 4-8
Culinary and medicinal herb
Marjoram is sometimes used to treat minor ailments like cough and sore throat. It's also used in regional Italian and Greek cuisine. Marjoram is related to oregano.
This means the plant is grown successfully in USDA hardiness zones 4 through 8. Whether you grow it in Zone 4 or Zone 8, it is still a plant hardy in mid-winter to -30 degrees F. The reason there is a range is to guide our plant selection. This plant doesn't just grow in Zone 4. It also grows well in Zone 5, 6, 7 and 8! If plants only grew in one zone, then Lone Pine would have entirely different plants than Bishop. But that's not what we see. Why not 9 and above? Well, maybe it is typically too hot overall there in those warmer zones. There's no telling why, exactly, from this. It's a guide of where the plant is commonly grown.
So when you see a plant description that says "Zones 5 to 8" that means it's hardy to Zone 5 conditions (-20°) but it can be grown in Zones 5, 6, 7, and 8. It doesn't mean there is an average hardiness of Zone 6.5 or that we're not sure but it's somewhere between the two extremes.
Generally the safest bet is to chose plant materials that are suitable for both a colder and warmer zone than your location when a span is listed. So If you live in Zone 6, a plant hardy in Zones 4 — 9 is probably a safe bet in terms of cold and heat tolerance.* If a plant just said "Zone 4" and you planted it in Zone 6 it would still be hardy.
In most of California, resources like the Sunset zones are more useful, but here in the mountains Sunset zones can be overly conservative and we have great differences due to elevation in our two counties.
Instead of Zones it would be more useful to have a statement of "Hardy to -10°F" in plant descriptions, but historically we've used this system because it's easy to convey which plants work, and most states have a more homogeneous climate than California.
Now you should be well armed to choose plants for your landscape.
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*If you really like the idea of climate zones, there is also a Heat Zone Map for you to consider as well.