2b or Not 2b – The Story Behind Plant Hardiness Zones

Most gardeners are familiar with the Plant Hardiness Zone (PHZ) maps that are found in many seed catalogues and garden centers, and with the paired numbering system (2a, 2b, 3a, 3b) etc. found on plant labels.  But what do these numbers really mean? How did they come about? And are they really relevant for native plant gardeners? 

The earliest PHZs were delineated in the 1920s by the Arnold Arboretum at Harvard University. In the 1960s, the US Department of Agriculture (USDA) devised their own set of PHZs using different criteria resulting in two different maps. However, the Arnold Arboretum map remained the standard until 1990, when the USDA, in conjunction with the US National Arboretum, and using data from thousands of weather stations, created the maps we now see. These maps are based on the average minimum winter temperatures. Each zone is marked at 10°F intervals, with the division between a and b at the 5° interval (e.g. zone 6a has an average winter minimum temperature of -10 to -5°F, 6b is -5 to 0°F). The logic behind this system is that plants have a cold threshold they won’t survive beyond.  

USDA PHZ: https://planthardiness.ars.usda.gov 

Unfortunately, these zones are a bit too simplified because plants are affected by more than just minimum winter temperatures, especially the further south you go. In snowbelt areas, for example, temperatures may drop well below freezing, but a deep blanket of snow can keep the plants safe. Basing the plant hardiness zone on minimum temperature doesn’t always work. Regardless of what minimum winter temperatures the plants will succumb to, some plants also tolerate heat much better than others. So the American Horticultural Society (AHS) devised their Plant Heat Zones based on the number of days the temperature went above 30°C (86°F), but their methodology created some problems as well. For instance, some plants can take lots of heat in the daytime but need cooler nights to recover and so just counting the number of days the temperature is above 30°C doesn’t really work. Also, plants like hostas love the heat, but they need cool winter temperature in order to go dormant so that they can start to grow again in the spring.  

AHS Heat Zone Map: https://ucanr.edu/blogs/dirt//blogfiles/37486_original.jpg 

If you’ve ever looked at the Canadian PHZ map, you’ve perhaps noticed that it doesn’t quite align with the USDA map. This is because the Canadian map is likely more properly called a Plant Suitability Map (though it still goes by the Plant Hardiness Zone moniker).  

Canadian PHZ: http://planthardiness.gc.ca/?m=1 

Originally developed in the 1960s, it uses a wide range of variables, not just minimum and or maximum temperature. The variables are: monthly mean of the daily minimum temperatures (°C) of the coldest month; mean frost free period above 0°C in days; amount of rainfall from June to November, inclusive; monthly mean of the daily maximum temperatures (°C) of the warmest month; winter factor expressed in terms of (0°C – X₁)Rjan where Rjan represents the rainfall in January expressed in mm; mean maximum snow depth; and maximum wind gust in (km/hr) in 30 years; and it incorporates them into a complex mathematical formula. You can read more about the formula and how it works here.

The original formula is the result of statistical analysis of the survival of 174 different trees and shrubs from 108 locations across the country (McKenney and Campbell, 2002, Getting into the Zone – what does Canada’s new plant hardiness zones map really mean? Frontline Technical Note #103 – see link to the paper at the end of this article). The newest version of the map was generated using computer models and interpolating climate data generated at more than 3400 locations across the country.  

Which is the best (i.e. most accurate and reliable) system? Being Canadian, I’m probably biased. But it would seem that the more thorough and complex Canadian system is likely the more accurate. The challenge is getting everyone to agree on one system. Most garden centers use the USDA system, probably because it has been around for a long time and the plant labels they purchase (likely from US sources even here in Canada) use that system.  

Surprisingly, even though the American and Canadian calculations are very different, in the southern Great Lakes region the results are close enough to be compatible for most of the plants – although some sources I’ve seen suggest that the Canadian and American systems may differ by as much as one complete zone in some locations. Also, both the US and Canada have been widely mapped using the USDA PHZ criteria, but I’ve not found any maps yet where the US has been mapped using the Canadian system. So, it doesn’t look like we’ll have a unified system any time soon. 

USDA Plant Hardiness Zone Map (North America): http://www.perennials.com/content/plant-hardiness-zone-maps/

But even with all the variables used in the Canadian maps, there are so many things that will affect the plant’s survivability that just cannot be easily mapped. Extreme weather events, microclimates caused by built structures, local topography, even mulching and watering, can have a significant impact on plant survival. 

When it comes to native plants, though, a lot of research is still needed to verify the zones they will survive. Accurate range maps overlaid on PHZ maps will go a long way to answer these questions, but native range of so many of our native plants have not been mapped with any level of detail.  

Then there is the question of whether we should even worry about what areas our native plants are capable of growing in – if the idea is to keep native plants in their native range, the more important issue is to figure out where the plants actually grew before we started putting them in our gardens everywhere. This way our native plant gardening efforts will better serve the insects and animals the plants evolved with, and not just our personal aesthetic.  

Until next time, happy native plant gardening. 


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