Hardiness refers to a plant’s ability to withstand cold weather and, importantly, whether it can survive outdoors during the winter months in your garden. This is a key consideration when choosing plants, as not all species are suited to the same climate conditions, and selecting the right level of hardiness can make the difference between a plant thriving or failing. To help gardeners make informed choices, the Royal Horticultural Society (RHS) has developed a widely used hardiness rating system.
These ratings group plants according to the lowest temperatures they can tolerate, giving a clear guide to how they will perform in different parts of the UK. Some plants may only cope with light, short-lived frosts, while others are capable of surviving prolonged periods of freezing temperatures and harsh winter conditions. Understanding these differences allows you to position plants appropriately, provide protection when needed, or choose alternatives better suited to your garden.
The RHS hardiness scale ranges from H7 (the hardiest plants, able to survive temperatures below -20°C) through to H1a (the least hardy, which must be kept above 15°C and are typically grown indoors or in heated environments). This range covers everything from alpine and hardy perennials to tropical and houseplants, offering guidance for a wide variety of planting situations.
Some plant descriptions may not use RHS hardiness ratings, so it is helpful to recognise other commonly used terms.
- “Tender” plants (H1a-H2) are very sensitive to cold and usually need winter protection or indoor conditions.
- “Half-hardy” plants (H3) can tolerate some cool temperatures but may not survive a typical UK winter without protection.
- “Fully hardy” plants (H4-H7) are well suited to outdoor growing year-round and can cope with frost and cold conditions, making them reliable choices for most gardens.
H1a Heated Greenhouse – Tropical
Temperature: > 15°C
Needs to be grown as a house plant or under glass all year.
H1b Heated Greenhouse – Subtropical
Temperature: 10 to 15°C
Can be grown outside in the summer in hotter, sunny and sheltered locations (such as city centre areas), but generally performs best as a house plant or under glass all year round.
H1c Heated Greenhouse – Warm Temperature
Temperature: 5 to 10°C
Can be grown outside in the summer throughout most of the UK while day-time temperatures are high enough to promote growth. (Most bedding plants, tomatoes and cucumbers).
H2 Tender – Cool or Frost-Free Greenhouse
Temperature: 1 to 5°C
Tolerant of low temperatures but will not survive being frozen. Requires glasshouse conditions except in frost-free inner-city areas or coastal extremities. Can be grown outside once risk of frost is over. (Most succulents, many subtropical plants, annual bedding plants, many spring-sown vegetables).
H3 Half Hardy – Unheated Greenhouse/Mild Winter
Temperature: -5 to 1°C
Hardy in coastal and relatively mild parts of the UK except in severe winters and at risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good microclimate. Likely to be damaged or killed in cold winters, particularly with no snow cover or if pot grown. Can often survive with some artificial protection in winter. (Many cordylines, tree ferns, some shrubs and perennials like dahlias).
H4 Hardy – Average Winter
Temperature: -5 to -10°C
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. Some normally hardy plants may not survive long wet winters in heavy or poorly drained soil. Plants in pots are more vulnerable to harsh winters, particularly evergreens and many bulbs. (Many herbaceous and woody plants, winter brassicas, leeks).
H5 Hardy – Cold Winter
Temperature: -10 to -15°C
Hardy in most places throughout the UK even in severe winters. May not withstand open/exposed sites or central/northern locations. Many evergreens will suffer foliage damage, and plants in pots will be at increased risk. (Many herbaceous and woody plants, some brassicas, leeks).
H6 Hardy – Very Cold Winter
Temperature: -15 to -20°C
Hardy in all of UK and northern Europe. Many plants grown in containers will be damaged unless given protection. (Herbaceous and woody plants from continental climates).
H7 Very hardy
Temperature: -20°C >
Hardy in the severest European continental climates including exposed upland locations in the UK. (Herbaceous and woody plants from continental climates).
Continue your Gardening Journey
Need a little extra advice? Visit your local Tates of Sussex Garden Centre today, where our friendly knowledgeable, team will be happy to help you choose the right plants and offer expert guidance.
