The weather forecast this weekend is screaming snow and ice. Whilst for some that brings happiness and plans for winter walks are being made, others see the news of snow and start to panic about the survival of their plants and their ability to cope with these extreme weather conditions unaided. Fear not, not all plants are pansies! Some plants have adapted to such extreme weather and built up their own protection, this is known as ‘hardiness’.
Hardiness relates to a plant’s ability to survive outside during winter. Initially, when choosing a plant species, one must ask themselves “will this species endure the conditions in my area?” The main conditions to be considered are wet and cold temperatures, which the UK is of course familiar with! Certain plants will take to a variety of cold conditions as some will be able to take a light frost, whereas others can withstand freezing temperatures for a considerably long time.
The origin of a plant can impact its ability to survive in harsh conditions. Plants native to the UK have adapted to unusual environments and external factors, such as harsh climates, whilst other plants can thrive in a diverse range of locations and are able to adjust to different environments. A great number of these native plants are deciduous and have therefore adapted over thousands of years to attain features that ensure their survival. Deciduous plants shed their foliage during the winter season, whereas their alternative evergreens keep their leaves and so are exposed to the effects of freezing. Evergreens however shouldn’t be disregarded as their thick structures can, more often than not, cope with cold temperatures. Especially when planted to maximise their exposure to the sun or sheltered to protect them from winds.
See RHS‘s hardiness scale…
|Rating||Temperature ranges ºC (ºF)||Category||Definition|
|H1a||warmer than 15 (>59)||Heated glasshouse – tropical||Needs to be grown as a house plant or under glass all year round.|
|H1b||10 to 15
(50 to 59)
|Heated glasshouse – subtropical||Can be grown outdoors in summer in sunny and sheltered locations but generally performs best as a house plant or under glass all year round.|
|H1c||5 to 10
(41 to 50)
|Heated glasshouse – warm temperate||Can be grown outdoors in summer throughout most of the UK while daytime temperatures are high enough to promote growth.|
|H2||1 to 5 (34 to 41)||Tender – cool or frost-free glasshouse||Tolerant of low temperatures but will not survive being frozen. Except in frost-free inner-city areas or coastal extremities requires glasshouse conditions in winter, but can be grown outdoors once risk of frost is over|
|H3||-5 to 1
(23 to 34)
|Half-hardy – unheated glasshouse / mild winter||Hardy in coastal / mild areas except in hard winters and at risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good microclimate. Can survive with artificial winter protection.|
|H4||-10 to -5
(14 to 23)
|Hardy – average winter||Hardy through most of the UK apart from inland valleys, at altitude and central / northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Plants in pots are more vulnerable.|
|H5||-15 to -10
(5 to 14)
|Hardy – cold winter||Hardy through most of the UK even in severe winters. May not withstand open or exposed sites or central / northerly locations. Many evergreens suffer foliage damage and plants in pots will be at increased risk.|
|H6||-20 to -15
(-4 to 5)
|Hardy – very cold winter||Hardy throughout the UK and northern Europe. Many plants grown in containers will be damaged unless given protection.|
|H7||colder than -20 (< -4)||Very hardy||Hardy in the severest European continental climates including exposed upland locations in the UK.|
Gardeners who have stayed in one location over a number of years become familiar with their conditions and have maintained gardens that thrive throughout the year. For those who are new to a location, we advise becoming familiar with your seasonal changes by noting down signs of first and last frost, along with frost frequency, rainfall and wind direction. Or, why not get to know your local garden enthusiasts (easily spotted at your local allotment or found tending to their front gardens on a daily basis!) and ask for their findings to compare to the RHS rating descriptions.
A number of gardeners have the tendency to look past the initial stage of research and opt for their favourite species with no regard to ‘hardiness’. Before choosing a plant for your garden, ensure you gather all the information you need in regards to the species and your area specific conditions. This will stand you in good stead and allow you to watch your plant establish through the thick and thin of the unreliable UK weather.