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.
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 10°C what does that really mean for our recent winters? I have tried to simplify the question in the following tabular for m.
Factors effecting temperature
First of all the country location is clearly a factor :-
Then there are specific local climatic influences :-
The specific orientation of the planting :-
On top of these is the question of a really localised “micro climate” and these can be influenced by
The following Table shows a rough rule of thumb as to how these combined factors might influence localised night winter temperatures around a plant.
If the minus 10°C in this case is compared to a front porch in London, we gain 2°C for being in Southern England, 2°C for a South facing position, 5°C for being in a large Urban Mass and a further 5°C 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 +4°C i.e. a 14°C hike in temperature! Which explains why geraniums often survive in London and flower outside all Winter but in my own garden some Hebes are 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 10°C 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 15°C 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!