What does Frost Hardy Mean and why does it matter?

Gardening books, magazines and plant labels refer to plants as "Hardy" or "Frost hardy"  with a "H" rating, which until recently, was a fairly basic indication of the degree of cold and frost the plant or shrub would withstand during the winter. It is important to know because if the plant is not sufficiently hardy for UK winters, it will die over winter, or will need winter protection. It is time consuming to grow less hardy plants, which need to be moved under glass, or sheltered and/or wrapped up for the winter. It is also costly if they do not survive the winter. It is easy to spot a lovely-looking plant in the garden centre only to find it is not fully hardy, which means the plant's ability to survive an English winter is limited.

Confusingly similar plants and plants in the same group can have different hardy ratings. For example, some varieties of Lavender are more hardy than others, as are different types of Salvia. This means when buying a plant you need to check the label for information not just about the group, or genus, but also the variety. 

 The original basic system of hardiness classification was to demonstrate the level of cold and frost the plant could withstand and was in widespread use until relatively recently. It was a simple *** rating. 

The previous system was fine up to a point, but had several drawbacks. It was broadly based on the USA zones, which didn't correspond with the UK climate. Also in practise, the previous classification was not sufficiently detailed for the wide variety of growing conditions in the UK.  The conditions experienced in more exposed Northerly, or wetter gardens, meant in effect the plant was in reality less hardy than its classification; equally, most gardens have some more sheltered areas and micro climates.

Illustrated above are the three extremes of plant hardiness: the Orchid  (left) originally from tropical areas is very tender and really only an indoor plant, Sweet Peas (image center) are hardy annuals and will take a degree of frost, and Heathers (image right) are very hardy, as tough as old boots growing as they do at high altitudes in the North and Scotland. 

It isn't just the cold which takes its toll on plants, other factors such as rainfall, chilly winds, high levels of water. Pittosporum tenuifolium which initially survived for several years in my garden, in a sheltered spot, was classified as *** hardy under the old plant hardiness rating system, perished in a bad winter, because my garden was no just cold but wet as well. Under the  2013 scheme, Pittosporum would be classified as H3 showing a more tender nature. In a more protected spot or in a different garden the Pittosporum may have survived.  

Lavender, more particularly the English Lavender Angustifolia, is generally hardy but really dislikes the wet. English lavender may be fine in most winters if it is in the right spot, which is dry and well drained soil, but it loathes having its roots in the wet soil, which will kill it more quickly than the cold. The French lavender, Stoechas is borderline hardy. The the current system introduced in 2013 takes  into account more variables.

The hardiness classification system is there to inform the buyer.  More detailed information has refined the classification. For example, on the question of lavender,  Lavender Angustifolia 'Hidcote' has been moved to H5 so the buyer knows if choosing a Lavender for a more exposed site to pick H5 not H4. The same applies when choosing shrubs, allowing a buyer to choose a shrub which has higher hardiness rating. All the RHS Garden merit plants are labelled under the new plant hardiness rating. This helps to avoid a situation where, after a bad winter in the spring, the border has gaps where plants and shrubs have failed to make it through the winter.

Frost Hardy Rating

The Hardy rating system was introduced by the RHS  in 2013 with an additional 7 classifications which enable the plant buyer to be more confident the plant is suitable for their planting area.  The RHS classifications of plant hardiness are as follows:

The Frost Hardy table rating

H1a harydy Rating

Plants which will only tolerate temperatures Above 15 degrees C - over 59 F

These are very tender tropical plants which require all year round protection in a heated greenhouse- not suitable for growing outside

Hardy H1b rating

Plants which will tolerate temperatures of a minimum 10- 15 degrees C - 50-59F

Subtropical plants which  need extensive winter protection -  usually in a heated greenhouse, but which can be placed outside during the summer.

Hardy rating H1c

These plants require a minimum temperature of between 5 - 10 C 41-50F

These plants can be grown outdoors during the summer. If a perennial plant it will need to be moved into a sheltered greenhouse/under glass over winter

Hardy rating H2

Plants with a temperature range of between 1-5C    34-41F

Tender plants requiring a frost free environment. These plants will tolerant low temperatures,  but not being frozen, and can only be grown outside in very sheltered areas (except during the summer months)

Hardy rating H£

These plants are hardy down to -5 and up to 1 C   and 23 - 35 F

Typically these plants are only hardy outside  in sheltered areas, coastal gardens and those providing mild winters. In other parts of the country they will require winter protection.

Hardy rating H4

Plants in this group are hardy -5 to -10C or betwen 14 - 23F

This group of plants is hardy in an average winter in UK, and in gardens which are not too exposed or too wet. These plants are not fully hardy and may not be hardy in exposed or northerly gardens.

hardy rating H5

Plants are considered fully hardy down -10 to -15C and between  5 - 14F

This is a hardy rating which is good for most of the UK, hardy in most areas, including in bad winters.  However, in very exposed and northern areas in cold winters, the foliage may suffer from cold and wind burn. 

Hardy rating H6

Plants which are very Hardy will withstand temperatures down -15 to -20  and -4 to +5F

Hardy to all areas of the UK and will withstand severe winters,  prolonged cold and frosts

hardy rating H7

Most hardy of all these plants will withstand temperatures down to over -20C  and  -4 F

Very hardy  will withstand any winter conditions in UK and severe winters hardy to any part of the UK.

The Old Frost Hardy Table

no stars

These plants cannot tolerate temperatures below 0C -  32F

These plants are not frost hardy, and will not withstand a temperature below 0 degrees. They will require winter protection.

frost hardy 1 star

One star denotes frost hardy down to 0C or 32 F

Often described as "half hardy" withstanding temperatures down to zero, but not below, or for prolonged periods. Requires winter protection.

two star frost hardy rating

Described as 2 star hardy will tolerate temperatures down to -5C    2 -3 F

Frost hardy in many parts of the UK but mainly in the more southern areas and sheltered gardens. These plants will not withstand a bad winter or exposed areas.

Three star frost hardy

Fully hardy will withstand temperatures down to -15C    5F

Described as fully hardy in all parts of the UK able to withstand severe winters

last updated 22.11.2021