What is a Hardy Plant?
When we say a plant is ‘hardy’ we usually mean that it can stand up to low temperatures and survive.
- As a rough guide, we say that a hardy plant can survive a temperature of -15°C
- A plant described as frost hardy can survive a temperature of -5°C
- A plant described as half hardy can only stand temperatures as low as 0°C
- A plant described as tender will not survive temperatures below +5°C
Although the British Isles is quite a small geographical area, the range of winter temperatures we experience affects the plants we can grow.
Near western coastal areas the influence of the Gulf Stream means that some frost-hardy, and even half-hardy plants can survive the winter out of doors. This also applies to some urban areas.
In mountainous and exposed moorland areas, on the other hand, plants may have to survive temperatures lower than the ‘hardy plant’ temperature of -15°C.
Most of the British Isles experience winter temperatures no lower than around -11°C, which means that generally speaking hardy plants do well here. However, no ‘temperature rule’ can guarantee a plant’s survival.
In Britain we often talk about the weather because we have a very changeable climate. Temperatures here can change many degrees within a very short time and this can be difficult for plants to deal with. A plant can be frozen one day; basking in warm sunshine the next day and blasted by icy northerly gales and rain two days later: our plants have to be tough to withstand this changeability.
What can we do to help our plants?
Conditions within our gardens have a big influence on how well our plants deal with weather extremes and getting to know how our gardens work is important. What is the soil like? Where are the places that are in shade all day in winter? Where does a cold draught blow? Where do temperatures fluctuate a lot during the day? Understanding the unique microclimate of our gardens will help us position our plants to give them the best possible chance of survival.
Soil conditions are important. During the winter, plants are either dormant or growing only very slowly. When they are in this phase, their roots are not very active and suffer during long periods of cold and wet: in effect, they can drown. Plants will not be able, in heavy, moisture-retentive soil, to survive temperatures as low as they can in free-draining soil. Improving winter drainage will help our plants to survive low winter temperatures.
We can insulate the roots of our plants from penetrating frost by spreading a mulch of well-rotted, free-draining material around them. This keeps the soil around the roots at a more even temperature during fluctuating temperatures.
Cold winds can draw the moisture out of the leaves of plants that keep their leaves over the winter, as well as subjecting them to the effect of wind chill. Planting in sheltered places, among other plants, against walls and fences or with a ‘mini-shelter’ of fleece or twigs will help.
Plants that are in areas that enjoy the morning sun in winter can suffer from frost burn. If sun falls on leaves or flowers that are still below freezing, the sudden rise in temperature can destroy the plants’ cell structure. Avoid putting fleshy-leaved or early-spring-flowering plants in east-facing situations.
Getting the right plants
When you are buying plants, choose ones that have a good-sized root ball, especially if you are planting in the autumn.
If the plant still has leaves, see how ‘tough’ they look. Some plants have only recently been taken from under glass or polythene and they will need to be gradually hardened before being planted in the garden. Stand them somewhere sheltered for a while before planting out in the open.
If the plant has no leaves over the winter, it is ‘herbaceous’ and has died down naturally to a dormant crown to go through the winter.
Look to see what information the label gives you.
If the plant is described as ‘Hardy Perennial’ it should stand up to average low winter temperatures and come up each year for several years.
A ‘Hardy Biennial’ will build up a strong root and leaf system in its first year, survive the average winter and go on to flower, set seed and die off in its second year.
A ‘Hardy Annual’ will take twelve months to go from seed sowing to seed setting. Many hardy annuals are winter hardy which means that they stand over the winter without damage and flower in early summer.
The label may also include a code referring to the RHS Hardiness Rating.
- These ratings refer to UK growing conditions unless otherwise stated. Minimum temperature ranges are shown in brackets
- H1a - Under glass all year (>+15°C)
- H1b - Can be grown outside in the summer (+10 to +15°C)
- H1c - Can be grown outside in the summer (+5 to +10°C)
- H2 - Tolerant of low temperatures, but not surviving being frozen (+1 to +5°C)
- h2 - Hardy in coastal and relatively mild parts of the UK (-5 to +1°C)
- H4 - Hardy through most of the UK (-10 to -5°C)
- H5 - Hardy in most places throughout the UK even in severe winters (-15 to -10°C)
- H6 - Hardy in all of UK and northern Europe (-20 to -15°C)
- H7 - Hardy in the severest European continental climates (<-20°C)
If the plant comes with a name but no other information, it may be helpful to look it up in books or on the internet. Finding out where a plant comes from in nature can tell you a lot about how hardy it is.
Becoming a member of the Hardy Plant Society, including joining one of the local groups, will give you access to the wealth of knowledge and experience that members have and are happy to share with you.
This information is also available as a two sided brochure for you to print out and keep (or give to others):