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The question of plant hardiness The principal question asked by everyone with ambitions to grow hardy and semi hardy tropical plants in the UK is “How hardy is it?”. Most available information is either US based, or far too sketchy, or even contradictive. (For USA Zone Hardiness information please click here) However it is a complex question as to how much frost a plant will take and in my own 40 years of experience I am still regularly surprised by the damage or non damage to slightly tender plants after a winter has passed.

The other consideration is to grow jungle plants for tropical effect but treat them as tender perennials, either just for a single season, or over wintered in a frost free Conservatory, greenhouse or even indoors. Many of these grow very quickly from seed, given some early heat in February, making a spectacular show during the summer in a mixed border or as a specimen plant. It is important to remember that most tropical plants find our summers too cold to grow properly outdoors but there are species that fall in the semi hardy category that do well during our summer and I have tried to include these in my Web Site.

Going back to the question of winter hardiness and if for example the Chamerops Humilis (Mediterranean fan palm) will tolerate at the outside minus 10C what does that really mean for our recent winters? I have tried to simplify the question in the following tabular form.

 

Factors effecting temperature

 

First of all the country location is clearly a factor :- 

  • West Country, Southern England, Northern England or Central England

Then there are specific local climatic influences :-

  • Coastal regions, Large City habitats or Frost pockets

The specific orientation of the planting :-

  • Southern, Western or Eastern and Northern facing

On top of these is the question of a really localised “micro climate” and these can be influenced by :-

  • The nearness of a large body of water, or concrete apron, or the shelter of a wall or large building, all of which act as overnight heat sinks.

The following Table shows a rough rule of thumb as to how these combined factors might influence localised night winter temperatures around a plant.

Example Micro and Local Climatic Factors

Frost variance Table based on a typical recent winters worse case frosts of -10C in an Central England Frost pocket facing east, compared to a London House Front Porch

Degree

Variance

Area

West

Country

+5

Southern

England

+2

Central

England

0

Northern

England

-2

+2

Attitude Facing

South/West

+2

North/East

0

 

 

 

 

+2

Locality

Coastal

+5

Large Urban Habitat

+5

Frost Pocket

0

Non Frost Pocket

+2

+5

Micro Climate

Water or

Hard standing

+2

Garden wall

+3

Sheltered House Wall

+5

None

+5

Total Variance

+14

If the minus 10C in this case is compared to a front porch in London, we gain 2C for being in Southern England, 2C for a South facing position, 5C for being in a large Urban Mass and a further 5C for being against a sheltered House Wall with an overhang. The likely temperature on the same night on the London front porch was probably no worse than +4C i.e. a 14C hike in temperature! Which explains why geraniums often survive in London and flower outside all Winter but in my own garden some Hebes were killed by the frost and a number of “hardy trees” badly frost damaged!

The story is much more complex than that of course and to be accurate the health of the plant in question, the soil type, how dry or wet the conditions were, how long the temperature stayed below zero etc would need to be considered but the table above should give you a rough guide to have a chance of success. For example the Chamerops Humilis already mentioned would most likely prove to be completely hardy in London, or the West Country or elsewhere given the shelter of a south facing wall. In fact it has proven hardy so far albeit marginally in my Oxfordshire front Garden in a sheltered position but with minimal microclimate influences.

Use the Table below to factor the local temperatures  (assuming my minus 10C typical worse case) for your planting area to determine the viability of any plant, knowing its temperature hardiness frost tolerance. Just put the appropriate Degree variance for each row in the right hand column and compare the total to a typical worse case minus 15C for Central England. This will give a guide to determine your specific microclimate worst case winter temperature. I would like to state that this is only a guide, completely subjective and based on my experiences with no evaluated scientific basis.      It is still up to you to experiment!

Micro and Local Climatic Factor Table

 

Degree

Variance

Area

West

Country

+5

Southern

England

+2

Central

England

0

Northern

England

-2

 

Attitude Facing

South/West

+2

North/East

0

 

 

 

 

 

Locality

Coastal

+5

Large Urban Habitat

+5

Frost Pocket

0

Non Frost

Pocket

+2

 

Micro Climate

Water or

Hard standing

+2

Garden wall

+3

Sheltered House Wall

+5

None 

 

Total Variance