Hardiness or tolerance to cold

The plants we grow in our gardens come from all over the world so it is not surprising that the sorts of winters that they are used to vary, you could say they are all hardy for where they come from! but the term "Hardy" is used to indicate that the plant should survive a normal UK winter, other countries have different systems like the USA's zone system (this tends not to be much use in the UKs maritime climate

Hardy (or Fully Hardy) will take temperatures down to -15c 

Frost Hardy will take temperatures down to -5c

Half Hardy will take temperatures down to 0c

Tender must stay above 0c

Many factors effect plant hardiness these include
Age of plant, some times young plants survive better sometime older plants do, it depends on the type of plant.

How wet it is when its cold, many north American plants survive -30c in north America but die here at -3c because our winters are wet while at -30c the ground is frozen meaning its also dry.

How hot the summer is, summer heat ripens wood enabling it survive much lower winter temperatures.

How quickly the temperature recovers after the frost, in a typical UK winter this is slow while in sunny desert areas it can be extremely rapid enabling Date Palms to survive night temperatures far lower than we get here in the UK.

Then there is the whole area of Micro climates, these are places within your garden that enjoy a different level of cold, due to the lie of the land, proximity of walls, or overhanging tree cover.

Confused? it can take many years of gardening to get the feel of what you can get away with. My advice would be to form the basic garden out of reliable hardy plants. Stick to old favorites. Use the more exotic planting where you have good spots in the garden and the sheltered south walls.

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