Hardiness zones

What is a plant hardiness zones (frost resistance zone) / gardening zone (growing zone) in plants

When you are going to grow a particular variety of plants, determining the correct gardening zone is not only a matter of survival the plants, but also its crop yield. For example, if you live in zone 6, you should choose plants from zone 6 or even colder zones: 1, 2, 3, 4, or 5. If you live in zone 3, you’d choose plants from zones 1, 2 and 3.

A plant hardiness zone (or frost resistance zone) meaning a geographic area defined to encompass a certain range of climatic conditions relevant to plant growth and survival. When you buy seedlings or seeds, a frost resistance zone is indicated for them according to one of the generally accepted systems. The original and most widely used system, developed by the United States Department of Agriculture (USDA) as a rough guide for landscaping and gardening, defines 14 zones by annual extreme minimum temperature. It has been adapted by and to other countries in various forms.

The USDA zones scale do not incorporate any information about:

• duration of cold temperatures;

• frost dates;

• frequency of snow cover;

• summer temperatures;

• sun intensity insolation;

thus sites which may have the same mean winter minima on the few coldest nights and be in the same garden zone, but have markedly different climates.

There are many other climate parameters that a farmer, gardener, or landscaper may need to take into account as well, such as humidity, precipitation, storms, rainy-dry cycles or monsoons, and site considerations such as soil type, soil drainage and water retention, water table, tilt towards or away from the sun, natural or manmade protection from excessive sun, snow, frost, and wind, etc.

Some fruiting requires «chilling period» - minimum period of cold weather with temperatures between 0°C (32°F) and 7°C (45°F) after which a fruit-bearing tree will blossom. Some bulbs have chilling requirements to bloom, and some seeds have chilling requirements to sprout.

The annual extreme minimum temperature is a useful indicator, but ultimately only one factor among many for plant growth and survival. Consequently, in regions of the same hardiness zone, there may be completely different conditions for growing plants

Therefore, in the UK, the RHS hardiness rating used to determine growing conditions and UK plant frost resistance zone.

Australia use it's own hardiness zones index, Canada uses both American and it's own zone index.

Note that winter hardiness zones mainly have a sense for perennial plants, since annuals do not face the winter. But annual plants may not grow out and not give a crop in the event of a short and/or cold summer, which is characteristic to climatic zones with low winter temperatures.

EU hardiness zones

Europe hardiness zones fit into the US hardiness system, 4a - 12a. The only Scandinavia and Baltic Sea Region zones are adjusted because the effect of the warm North Atlantic Current is even more pronounced here than it is in Britain and Ireland. Save for a very small spot near Karasjok, Norway, which is in zone 2, nowhere in the Arctic part of Scandinavia gets below zone 3. All these coastal locations have one thing in common, though, which are cool, damp summers, with temperatures rarely exceeding 20 °C (68 °F). This shows the importance of taking heat zones into account for better understanding of what may or may not grow.

Britain and Ireland UK RHS hardiness zones (growing zones)

From a broader viewpoint, the UK RHS (Royal Horticultural Society) plant hardiness rating only corresponds to American zones 6a to 13a (due to the Gulf stream).

The RHS’s classification offers the British gardener a more in-depth perspective on plant hardiness. It separates the four US zones into nine ranks (the numbering is reversed with respect to the indexes of the US Department of Agriculture). H1a represents the most tender of specimens, the tropical exotics. This is followed by H1b and H1c, then 2 etc. H7 comes last and identifies plants fully hardy for the UK.

Anything in the H1a zone will need constant protection indoors as it will only survive temperatures above 15°C.

United States hardiness zones (USDA scale)

The USDA system was originally developed to aid gardeners and landscapers in the United States. The USDA plant hardiness zones map was issued with 5-degree distinctions dividing each zone into new "a" and "b" subdivisions, based on 1976–2012 weather data, using a longer period of data to smooth out year-to-year weather fluctuations:

As the USDA system is based entirely on average annual extreme minimum temperature in an area, it is limited in its ability to describe the climatic conditions a gardener may have to account for in a particular area: there are many other factors that determine whether or not a given plant can survive in a given zone - factors such as frost dates and frequency of snow cover can vary widely between regions.

For example, zone 8 covers coastal, high latitude, cool summer locations like Seattle and London, as well as lower latitude, subtropical hot summer climates like Charleston and Madrid. Farmers, gardeners, and landscapers in the former two must plan for entirely different growing conditions from those in the latter, in terms of length of hot weather and sun intensity. Coastal Ireland and central Florida are both zone 10, but have radically different climates 99% of the year.

As an extreme example, due to the Gulf stream most of the United Kingdom is in zones 8—9, while in the US, zones 8—9 include regions such as the subtropical coastal areas of the southeastern US and Mojave and Chihuahuan inland deserts, thus an American gardener in such an area may only have to plan for several nights of cold temperatures per year, while their British counterpart may have to plan for several months.

Canadian hardiness zones (PHZ)

In Canada, there are no areas warm enough to go beyond USDA zone 9, so Canada's hardiness zones range from 0 to 9 (0a/0b/1a/1b — 8a/8b/9a), white zero is the coldest and 9 is the warmest average conditions. An index, mapped to Canada's Plant Hardiness Zones, comes from a complex formula based on eight Climate Variables used to develop Canada:

Y = -67.62 + 1.734X₁ + 0.1868X₂ + 69.77X₃ + 1.256X₄ + 0.006119X₅ + 22.37X₆ - 0.01832X₇

where:

Y = estimated index of suitability

X₁ = monthly mean of the daily minimum temperatures (°C) of the coldest month

X₂ = mean frost free period above 0°C in days

X₃ = amount of rainfall (R) from June to November, inclusive, in terms of R/(R+a) where a=25.4 if R is in millimeters and a=1 if R is in inches

X₄ = monthly mean of the daily maximum temperatures (°C) of the warmest month

X₅ = winter factor expressed in terms of (0°C - X₁)Rjan where Rjan represents the rainfall in January expressed in mm

X₆ = mean maximum snow depth in terms of S/(S+a) where a=25.4 if S is in millimeters and a=1 if S is in inches

X₇ = maximum wind gust in (km/hr) in 30 years

For practical purposes, Canada has adopted the American hardiness zone classification system, but the Canadian government publishes both Canadian and USDA-style zone maps.

* General rule of thumb to use USDA zones in Canada is just to add one zone to the designated USDA zone. For example, USDA zone 4 is roughly comparable to zone 5 in Canada.

Australian hardiness zones (ANBG scale)

The Australian National Botanic Gardens (ANBG) have devised another system keeping with Australian climat conditions. The zones are defined by steps of 5 degrees Celsius, from -15 — -10 °C for zone 1 to 15—20 °C for zone 7:

They are numerically about 6 lower than the USDA system. For example, Australian zone 3 is roughly equivalent to USDA zone 9, see ANBG Plant Hardiness Zones for Australia for details.

All of Australia (excluding Macquarie Island) is covered by just over four US zones (7b — 11).

Zone 1 covers the alpine areas of south eastern Australia.

Zone 2 the tablelands of south east Queensland, New South Wales and Victoria, and the uplands of central Tasmania.

Zone 3 coverc much of the southern half of the continent, except for localities on or near the coast.

Many of the coast territories or on off-shore islands have often a zone or two higher than adjacent mainland territories because of the warming effects of the ocean in winter.

As a result of this warming effect zone 4, which covers a broad area from coastal Queensland across the continent to Shark Bay and Geraldton in the west, also includes Sydney and the north coast of NSW, the Mornington Peninsula, areas adjacent to Spencer Gulf and Adelaide, the south western coastal zone, along with a number of localities dotted all around the southern coast of the continent.

Zone 5 covers, some of the Queensland coast, Western Australia north of Shark Bay and across the Top End.

Zone 6 includes the Queensland coast north of Cairns, Cape York Peninsula and the coast of the Northern Territory.

Zone 7 is mainly restricted to islands off the north coast.