Plant hardiness can be described as follows:
Assuming the right amounts of nutrients, light and water the survival of a plant in a particular location is determined by the lowest temperatures it will have to withstand. We therefore say that a plant that can withstand the temperature extremes in a particular place is hardy in that place.
Plants, of course, have adapted and evolved to suit their native conditions. Thus, the Canna plant survives the extremes of heat and humidity in its native areas of India, but will succumb to the first heavy frost in Scotland where I live.
Nevertheless, one of the key features of horticulture has always been the desire of gardeners to raise plants away from the natural habits. We love to test the limits, growing bananas in London for example, and mixing plants from all around the world to create the visual effect we are looking for.
The famous British gardener Beth Chatto popularised the phrase “right plant, right place” to remind us that the plants that will do well in our gardens are the ones that are properly suited to the conditions we have.
Luckily, we can often amend or leverage our conditions to better match our plants’ needs, for example by improving the drainage and fertility in our soil or planting sun-loving plants in the sunniest spots in the garden.
But what we can’t do is move our garden somewhere else, so we are stuck with climatic conditions that apply to our area – and this is why we need to understand plant hardiness and the hardiness zone that we are gardening in.
What makes a plant hardy?
The hardiness of a plant is determined by its cell structures.
A plant is killed or damaged by cold temperatures when it cannot prevent the cold disrupting and fatally damaging its cell contents.
A plant that is hardy at -5°C (23°F), for example, will be able to prevent ice crystals forming within its cells at and above that temperature.
It can do this because it has a permeable cell wall. This means that water can pass through the cell wall and out of the cell in frosty conditions. As a result, the concentration of fluids within the cell increases and this lowers the freezing point within the cell by several degrees. Ice crystals will still form but they will be outside the cell structure and therefore will not destroy the cell.
In a tender (non-hardy plant) the cell wall is less permeable. This means that water cannot escape from the cell and the freezing point is not lowered. Therefore the water within the cell content will freeze and rupture the cell walls, destroying the cell.
A plant is therefore more or less hardy according to how well it is able to empty its cells of water in freezing temperatures.
USDA plant hardiness zones
Plant hardiness zones enable us to understand if a plant will thrive (or indeed survive) in a particular location.
In the USA, the US Department of Agriculture (USDA) provides the standard hardiness zoning of the country. As you will see from the map below, there are 13 full zones at 10°F intervals, based on the average annual minimum temperature.
The USDA also has an interactive version of the map, which calibrates more precisely by including half zones at 5°F intervals.
How to use the USDA plant hardiness zones
Here is how you use plant hardiness information to ensure you are choosing the right plants for your garden:
- Identify the zone your garden is located in on the USDA plant hardiness zone map.
- Note the zone number and the minimum temperature associated with your location.
- Identify plants that are hardy down to that zone or minimum temperature. Use the plant information provided on the label of any plant you intend to buy or the information available online or in relevant publications to establish the hardiness of the plant. Typically, the information will be along the lines of “hardy down to Zone 5” or “hardy down to -20°F”.
USDA plant hardiness map
The USDA Plant Hardiness Zone Map, 2021. Kindly made available byAgricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/
RHS plant hardiness classification for the UK
The United Kingdom has less extreme ranges of temperature than the USA and a different classification system that has been devised by the Royal Horticultural Society (RHS).
Details of the UK plant hardiness classifications are set out in the table below. It is worth noting that these ratings were introduced in 2012 and superseded the earlier system.
Note also that the temperatures in the RHS system are absolute minimum temperatures, rather than average annual minimums as used in the USDA system.
Most gardeners typically know what the the lowest temperatures are in their region and so will know where their conditions fit into the system (climatic data is also available from the Met Office). The important point therefore is to select plants that suit the growing environment you have.
|RHS Rating||Temperature Ranges °C (°F)||RHS Category||Description for plants in this category||USDA Ratings Equivalent|
|Tropical (heated greenhouse)||Grow under glass year round.||13|
|H1b||10 to 15 |
(50 to 59)
|Sub-tropical (heated greenhouse)||Can be grown outside in summer in warm, sunny or sheltered locations (e.g. city centre).||12|
|H1c||5 to 10 |
(41 to 50)
|Warm temperate (heated greenhouse)||Will grow outside in summer in most areas whilst temperatures are warm (e.g. tomatoes, cucumber, bedding plants).||11|
|H2||1 to 5 |
(34 to 41)
|Tender (frost free greenhouse)||Grow outside when risk of frost is over. Will tolerate cold but not freezing (e.g Spring Veg, Cannas, Bananas etc.).||10b|
|H3||-5 to 1 |
(23 to 34)
|Half-hardy (unheated greenhouse or mild winter)||Hardy in coastal or milder areas of UK or sheltered micro-climates. Susceptible to sudden frosts. Will not survive a cold winter (e.g. mediterranean plants).||9b/10a|
|H4||-10 to -15 (14 to 23)||Hardy (average winter)||Hardy in most UK areas, except northerly areas or those at altitude or susceptible to frost pockets (valleys). Hardiness effected in poorly drained soils. Plants may suffer foliage and stem damage from frost ( e.g. many woody and herbaceous plants).||8b/9a|
|H5||-15 to -10|
(5 to 14)
|Hardy (cold winter||Hardy in severe winters in most places in UK (except very exposed and some northern areas.) Evergreen foliage may be damaged, (e.g. woody and herbaceous plants).||7b/8a|
|H6||-20 to -15|
(-4 to 5)
|Hardy – very cold winter||Hardy across almost all of UK and Europe (e.g. cold climate woody and herbaceous plants).||6b/7a|
|H7||< – 20|
|Very hardy||Hardy in all locations in UK and Europe including open and exposed sites.(e.g. cold climate woody and herbaceous plants).||6a to 1|
Plant hardiness in practice
We can get a sense from hardiness zones and from the available plant information as to whether a plant may be hardy in a particular place. But whether in fact a plant will survive in practice is also determined by various characteristics of the actual planting position.
Here are some of the factors that will affect plant hardiness in a garden:
- Shelter and exposure: Plants exposed to cold winds will be more susceptible to cold damage than plants that are sheltered.
- Drainage: The underground parts of some tender plants, such as dahlias, may survive several degrees of frost if well mulched and if the drainage around their roots is good. Conversely, they will will perish if they are left waterlogged in a cold spell for any prolonged period.
- Walls: Walls can provide shelter, but they can also absorb heat during the day which they then radiate out at night. This can keep plants nearby at temperatures materially above those in the rest of the garden.
- Frost pockets: Cold air is heavier than warm air, so it sinks. This means that at night cold air collects in valleys and hollows, making those areas significantly colder that further up the slopes. Paradoxically, although walls can absorb and give off heat, if on a slope, they can also trap cold air that would otherwise drain to a lower position. So, they may create a frost pocket of their own.
- Aspect: In the northern hemisphere, a south facing aspect provides maximum exposure to the sun’s heat, so plants in these areas will be warmed more quickly by any morning sun on a cold day.
Martin Cole has been an avid gardener for more than 20 years and loves to talk and write about gardening. In 2006 he was a finalist in the BBC Gardener of the Year competition. He is a member of the National dahlia Society.
He previously lived in London and Sydney, Australia, where he took a diploma course in Horticultural studies and is now based in North Berwick in Scotland. He founded GardeningStepbyStep.com in 2012. The website is aimed at everybody who has been bitten by the gardening bug and wants to know more.
Gardening Step by Step has been cited by Thompson and Morgan, the UK’s largest mail order plant retailer, as a website that publishes expert gardening content.
Check out my comprehensive step by step guide, with plain language explanations and ultra-useful images and illustrations. This is for you if you love dahlias and want to the best out of the dahlias you grow.