Frost Hardy - What does it mean?

Moth Orchids Phalaenopsi Sweet Pea Heather is fully hardy

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 an 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 and it is time consuming to have less hardy plants which need to be moved under glass, or sheltered and wrapped up for 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.

 The original basic system of hardiness classification had just four bands to demonstrate the level of cold and frost the plant could withstand and was in widespread use until 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 by different winters and planting situation which suggested that the previous classification with four bands was not sufficiently detailed for the wide variety of growing conditions in the UK.  More exposed Northerly or wetter gardens meant in effect the plant was less hardy than it's classification; equally all gardens have some more sheltered areas and micro climates.

With some plants their hardiness maybe more obvious as in the images above. The Orchid is known to most as an indoor plant, Sweet Peas are planted out in the summer and Heathers are as tough as old boots growing as they do at  high altitudes in the North and Scotland but the precise degree of hardiness isn't always that helpful and this is when the new system of classification should help. For example, a Pittosporum tenuifolium survived for several years in the garden, in a sheltered spot, even though it is ** hardy under the old plant hardiness rating system; but perished in a bad winter. In a more protected spot or in a different garden it 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 chosing and growing lavender.)

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 chosing a Lavender for a more exposed site to pick H5 not H4. The same applies when chosing shrubs allowing a buyer to chose a shrub which a 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 the recent bad winters experienced all over the country in the spring  the border is full off gaps where plants and shrubs have failed to reappear the following spring. 

The new system Frost Hardy Rating

The new system introduced by the RHS with it's more detailed 7 classifications instead of the previous 4 bands, 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, will all be labelled in the new system from 2013 when it was implemented. The new RHS classification of plant hardiness as follows:

 

The new system of Frost  Hardiness Rating  recommended by the RHS

 

Rating Temp Description
H1A +15 Very tender tropical plants requiring all year protection in a heated green house. Not suitable for outside planting.
H1B 10-15+ Heated greenhouse only suitable for planting outside in summer and only in a sheltered spot
H1C 5-10+ Tender plants suitable for growing outside in most areas of UK during the summer, such as bedding plants, toms, cucumbers requires a warm temperature.
H2 1-5+ Tender plant which will not survive any degree of frost but can be grown outside only during summer when frost passed
H3 1- 5- Half hardy plant suitable for outside planting only in mild areas such as coastal gardens. Will need winter protection and overwintering under glass unless in very sheltered spot. Will be killed by frost or snow unless adequately protected.
H4 -5--10 Hardy plant will take temperatures down to -10 which will be hardy in many areas of UK, but not all. In exposed windy, Northern or wetter sites, or higher altitude sites plant may not survive. May be killed by prolonged periods of frost and snow in severe winter
H5 -10-15 These plants are hardy through most winters and in most locations but not all. Can still be killed or damage if in exposed site particularly further North or in a container environment but safe to grow in most gardens.
H6 -15-20 Very Hardy plant will stand temperatures down to -20 will survive in UK through winter.
H7 -20 Will survive anywhere even in very exposed sites

 

For the record, as not all plants  are properly labelled,  until the new classification there were just four simple bands.

The old system of Frost Hardiness rating

        Frost tender not hardy below -5 degree C
*
half Hardy will wthstand tempretures down to 0 degree C
** Frost hardy will withstand tempretures down to -5 degree C
*** Fully Hardy will withstand tempretures down to -15 degree C

 

 

This classification will still be found on many plants and its clear to see why it is less informative than the new system implemented.

 

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