Hardiness and Heat Zones

Throughout this post, we will explore the concept of plant hardiness and its corresponding hardiness zones—a vital variable in the realm of cultivation 

Plant hardiness and hardiness zones 

A plant must be able to survive the lowest temperature during winter in the place that they are growing in order to survive for the next growing season, and every plant has a threshold temperature below which they will die.  The ability of a plant to survive extreme cold temperature is referred to as its hardiness. A very hardy plant is able to tolerate extreme lows while a non hardy plant cannot tolerate cold temperature at all.  

Temperature is well recorded across the world and hardiness zone maps have been created that show what areas experience what temperature ranges. There are a number of different hardiness systems available, but the one I refer to in this book, and the one most commonly referred to, is the USDA Plant Hardiness Scale.  The hardiness scale ranges from 1a to 13b, 1 being arctic conditions and 13 being tropical.

USDA Plant Hardiness Scale 

Observations of which plants can survive in low temperatures based on wild plants, and on trial and error experimentation by humans growing plants in various locations, is also well documented and as such, every plant has been assigned a hardiness rating that is provided in all good plant catalogs, books and internet databases. 

For example Siberian pea tree - Caragana arborescens is given a 2b-7b rating that tells us that the plant will grow in hardiness zone 2b - 7b, which means it’s okay in areas that experience winter low temperature of -45 (F), but that it will not survive in Zone 1a - 2a where the temperature falls below -45 (F). It also tells us that the plant will grow in Zone 2b, 3, 4, 5, 6 and 7 but that it will not grow well in zones 8 to 13, probably due to it being too hot or humid for the plant in these zones.  

World Plant Hardiness Zone Map USDA Zones 1-13 

The hardiness rating is not set in stone and it’s fine to experiment with plants a few zones outside of your hardiness zone, especially with plants grown from seed as genetic diversity often leads to individual plants with higher thresholds of cold tolerance. I have had a reasonable degree of success doing this for a number of species. Even when I’m buying plants I’ll try plants in higher zones at least twice before I’m convinced they won’t make it. 

Local conditions such as altitude and topography should also be considered. For example, on most hardiness zone maps it shows our location 6b/7a, however our particular gardens are 580 meters above sea level and can experience very strong winds that result in winter lows of -13 F (-25 C), so we are effectively 5b. Micro-climatic factors within a garden can also provide differing conditions for plants. Planting areas by a south-facing wall will retain higher temperatures due to sun exposure and thermal mass while a low lying area along a north-facing wall will be colder and vulnerable to deep freeze. It’s also worth noting that urban and suburban areas are usually a few degrees warmer than rural areas.

Heat Island Group, Lawrence Berkeley National Laboratory 

Heat zones 

Cold isn't the only factor determining whether plants will survive and thrive. Heat also has an impact on plants. Less obvious than the damage caused by cold, heat damage can nonetheless be detrimental to plants. Damage includes flower buds withering, leaf drop, chlorotic leaves, stunted root growth and a general weakening of the plant which lowers its defenses and makes it more attractive to insect pests and microbial parasites. Plant death from heat can be slow and lingering. 

The AHS have created a Plant Heat Zone Map that divides America up into various zones based on the average number of days each year that a given region experiences ‘heat days  – temperatures over 86 F (30 C). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). Thousands of garden plants have now been coded for heat tolerance, with more to come in the near future. Currently heat zone maps are only available in the US.  

AHS Plant Heat Zone Map

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