Plant Hardiness Zones: A Comprehensive Guide to Temperature and Gardening

For gardeners and landscapers, understanding the environment is the first step toward success. One of the most critical factors in determining whether a plant will thrive or perish is the hardiness zone. A hardiness zone is a geographic area defined by its average annual minimum temperature, a metric that directly impacts the survival of various plant species.

While different regions use various systems, the most widely recognized standard is the one developed by the United States Department of Agriculture (USDA). This system provides a rough guide for landscaping by categorizing regions into zones based on long-term average extreme minimum temperatures.

(See table below) — Temperature scale used to define USDA hardiness zones. These are annual extreme minima (an area is assigned to a zone by taking the lowest temperature recorded there in a given year). As shown, the USDA uses a GIS dataset averaged over 1976 to 2005 for its United States maps.

Key Facts

Hardiness zones are primarily defined by the average annual extreme minimum temperature.
The USDA scale is the most common system used in American gardening contexts.
A plant described as "hardy to zone 10" can withstand temperatures between −1.1 to 4.4 °C (30 to 40 °F).
Heat zones, unlike hardiness zones, are defined by annual high temperatures.
Climate change is causing hardiness zones to shift northward.

The USDA Hardiness Zone System

The USDA system divides regions into 13 distinct zones. Each zone is further subdivided into 'a' and 'b' categories to provide more granular temperature data. This system has been adapted by other countries, such as Canada, to suit their specific climatic needs.

It is important to distinguish "hardiness zones" from "climate zones" used in building and HVAC (Heating, Ventilation, and Air Conditioning) discussions. In the context of the International Energy Conservation Code, climate zones follow a different numbering logic, where Zone 1 is warm and Zone 8 is cold.

2023 update of the USDA Plant Hardiness Zone Map

USDA Temperature Definitions

USDA Hardiness Zone Temperature Ranges
Zone	Sub-zone	From (°F)	To (°F)	From (°C)	To (°C)
0	a	< −65	< −65	< −53.9	< −53.9
1	a	−60	−55	−51.1	−48.3
5	a	−20	−15	−28.9	−26.1
10	a	30	35	−1.1	1.7
13	b	> 65	> 65	> 18.3	> 18.3

Note: Table shows selected samples of the full USDA scale.

Global Variations and Alternative Systems

While the USDA scale is a global benchmark, other organizations have developed specialized rating schemes. For example, the UK Royal Horticultural Society and the US Sunset Western Garden Book utilize their own systems. Additionally, the American Horticultural Society (AHS) uses heat zones, which are determined by the average number of days per year when temperatures exceed 30 °C (86 °F).

Global Plant Hardiness Zones (approximate). Antarctica is not shown; most of it except the peninsula is zone 0 or 1,[citation needed] and is almost completely covered by the Antarctic ice sheet.

International Approaches

Different nations employ unique formulas to calculate suitability. Canada, for instance, uses a complex index involving monthly mean minimum temperatures, frost-free periods, and maximum snow depth. In Europe, hardiness is mapped across diverse climates, from the subarctic regions of Finland to the Mediterranean warmth of Italy and Spain.

Hardiness zones of Europe and surrounding regions 1 2 3 4 5 6 7 8 9 10 11

Hardiness in Europe and the UK

In the British Isles, hardiness zones are mapped to help gardeners select appropriate flora. For example, cities like London and Paris fall into similar USDA-scale brackets, while more northern locations like Edinburgh or Helsinki require much hardier species. In southern Europe, cities like Valencia or Rome offer much warmer environments, allowing for a wider variety of plant life.

Britain and Ireland's hardiness zones, USDA scale, 2006

In southern Sweden, such as Simrishamn, the climate allows for specific garden types that might not survive in the more northern reaches of Scandinavia.

A garden in Simrishamn, southern Sweden.

Climate Change and Shifting Zones

Hardiness maps are not static. The USDA released an updated map in November 2023, based on weather data from 1991–2020. This update reflects a continued warming trend, showing a northward movement of hardiness zones. Data from 2012 indicated that much of the US had already become a half-zone (approximately 2.8 °C or 5 °F) warmer in winter compared to the 1990 release. Research suggests that as climate change continues, these shifts will persist, requiring gardeners to adapt their plant selections over time.

Frequently Asked Questions

What is the difference between a hardiness zone and a heat zone?

A hardiness zone is defined by the average annual minimum temperature, which tells you how cold a plant must withstand. A heat zone is defined by annual high temperatures, specifically the number of days the temperature exceeds 30 °C (86 °F).

How do I use hardiness zones for gardening?

You should check the hardiness rating of a plant and compare it to your local zone. If a plant is rated for zone 8 and you live in zone 7, it may not survive the winter in your area.

Why are hardiness zones changing?

Hardiness zones are shifting due to climate change and warming trends. Recent USDA updates show that many areas are moving into warmer zones, meaning plants that previously struggled in certain regions may now survive.

Does the USDA scale apply to the whole world?

While the USDA scale is widely used as a reference, many countries like Canada and various European nations use their own specific calculations and indices to define their local hardiness zones.

What does "hardy to zone 10" mean?

It means the plant is capable of surviving minimum temperatures ranging from −1.1 to 4.4 °C (30 to 40 °F).