|Below -50||Zone 1||Below-46|
|-50 to -40||Zone 2||-46 to -40|
|-40 to -30||Zone 3||-40 to -34|
|-30 to -20||Zone 4||-34 to -29|
|-20 to -10||Zone 5||-29 to -23|
|-10 to 0||Zone 6||-23 t0 -18|
|0 to 10||Zone 7||-18 to -12|
|10 to 20||Zone 8||-12 to -7|
|20 to 30||Zone 9||-7 To -1|
|30 to 40||Zone 10||-1 to 4|
|Above 40||Zone 11||Above 4|
Hardiness Zones are what the United States Department of Agriculture (USDA) came up with to define average low temperatures within different regions of the USA.
They seem to be becoming universally applied throughout the gardening world as a guide to a plants hardiness.
This categorization is purely temperature related and nothing else.
Take the well know proverb: " Beat a man with a stick once and you will hurt him. Beat him long enough and he will die."
Well actually I made that up but it makes a valid point.
Let's say that a plant is classified as 'hardy to zone 9'. This plant may well handle occasional short blasts of icy weather. If that same plant had to endure endless nights at -4ºC, it may well declare that 'This was not in the brochure' and die.
Other significant variables relating to a plants ability to thrive outside its comfort zone are:
Rainfall. Winter sunshine. Summer sunshine. Soil conditions. Length of winter. Summer night/day temperatures. Winter daytime temperatures, and as stated before - minimum temperatures and duration of freeze.
For example, the Agave parryi. Considered to be one of the hardiest of Agaves with a cold tolerance of -29ºC. A couple of mild, but wet, winters, followed by rubbish summers were enough for these plants to no longer wish to be active members of the garden.
At the end of the day, these factors affect only which plants you can choose as permanent members of your garden. Most of the fast growing tropical plants though, will go all of a quiver, just looking at postcards of Switzerland.