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The level of resistance to the cold varies according to the plant
All plants are not equal when it comes to the cold. You only need a pot of Pelargonium (balcony geraniums) outside one night when the temperature falls below zero and you will quickly realise! Plants that do not tolerate the slightest frost are said to be frost fearing or “non hardy”. It is the case for nearly all houseplants and flowers that are grown as annuals. Plants that need to spend winter protected from the cold in a sheltered spot are also non hardy, examples being Evening fragrance (Brugmansia) and Bougainvillea.
Frost fearing plants
For the other plants things are not that simple. They can resist frost but to differing degrees. Let’s take the example of the olive tree: it can resist temperatures as low as -12°C. If the winter is colder than this then it risks dying. But a lot depends on the way in which it has been planted and the type of cold that it suffers. A plant that was put in late in the season has had less time to form a proper root system: it is not well prepared for winter and will start to suffer as soon as the first moderate cold spells arrive. If your soil remains damp the plants will suffer even more from the cold: a wet soil combined with a long period of cold will prove fatal for many plants. The same plant in identical conditions but with a well drained soil would suffer a lot less.
Make the right choice
Choose your plants based on their resistance to the cold, and do not forget that all regions suffer from freak weather conditions. If it practically never falls as low as -15°C in your area then plants that are resistant to -20°C have nothing to fear. But if in your area you don’t normally get below -12°C and you put in a plant that is tolerant to only -10°C (a agapanthus, for example), then there is a high risk that the plant will suffer frost damage one day or the other. Hardiness is not an exact science: indications of the temperatures that plants will tolerate are a useful indication but not a true guarantee!
You cannot increase a plant’s natural resistance to the cold except by altering it genetically! It is also a waste of time to think that you can grow a palm tree from the Canaries outside at an altitude of 1000m. However you can protect your plants during the winter by various methods: cloches, fleeces, layers of dead leaves etc. - Photos (2)
The hardiness of plants
level of resistance to the cold varies according to the plant
Author: Jean-Michel Groult
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The hardiness of plants
Make the right choice
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