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 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 the conditions a plant will survive, as a plant which is not frost hardy, or half hardy, will not survive a UK winter without protection. It is time consuming to have less hardy plants which need to be moved under glass, or sheltered and wrapped up for the winter and 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.

This can be true of plants in the genus. The commonly grown Rosemary officinalis is commonly described as H4 hardy. Also widely sold is a form of trailing Rosemary, of the Prostratus Group, which is not fully hardy and needs winter protection. It is easy to mix them up.

 The original basic system of hardiness classification was just to demonstrate the level of cold and frost the plant could withstand and was in widespread use until relatively recently. It is relevant still as some plants sold are still utilising this system.

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, variation occurred in different areas and planting situations which suggested that 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 it's classification; equally most gardens have some more sheltered areas and micro climates.

With some plants their hardiness maybe more obvious as in the images above. The Orchid  (left) is known to most as an indoor plant, Sweet Peas (image center) are planted out in the Spring and are frost hardy to a point, and Heathers (image right) are very hardy, as tough as old boots growing as they do at  high altitudes in the North and Scotland. Under the old system most plants in the UK were classified as H4 or H5 which didn't taken into account enough variations in conditions.  For example, a Pittosporum tenuifolium which initially survived for several years in the garden, in a sheltered spot, even though it is H4 hardy under the old plant hardiness rating system, but it perished in a bad winter. Under the  2013 scheme Pittosporum would be classified as H3 indicating a more tender nature. In a more protected spot or in a different garden the Pittosporum may have survived.  Equally 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, dry and well drained soil, but it just loathes having it's roots in the wet which will kill it more quickly than the cold. The French lavender, Stoechas is borderline, (for more about choosing and growing lavender.)The precise degree of hardiness isn't always indicative of how a plant will survive, and a new system was introduced in 2013 to take into account more variables.

The practical result of the new plant hardiness classification system is to better inform the  buyer to choose a plant more suited to the conditions. 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 chose a shrub which has higher hardiness rating. All the RHS Garden merit plants will be 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 reappear in the following spring. 

Frost Hardy Rating

The new Hardy rating system introduced by the RHS  in 2012 with a more detailed 7 classifications enables the plant buyer to be more confident the plant is suitable for their planting area.  The frost hardy classification has been refined and does not correspond precisely with the previous rating. It is worth noting that the RHS Award of Garden Merit scheme, which is awarded to plants which are considered excellent of their type and not unduly subject to pests and diseases, should all be labelled in the new system from 2013 onwards when it was implemented. The new RHS classification of plant hardiness as follows:

The New 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 requires a minimum temperatures of between 5 - 10 C 41-50F

These plants can be grown outdoors during the summer and if a perennial plant it will need to be moved to be in 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 and for all most including in severe 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 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 hardywill 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