When gardeners talk about Hardy Plants, sometimes it becomes a little confusing as to what they mean. And when they trow in the terms 'Half Hardy' and 'Tender' it can be a little confusing for the novice gardener.
However Hardiness, and even Hardiness Zones' are actually fairly simple, it simply refers to the degree of cold and frost that plants can survive in. The added complication is how long the frost lasts.
Hardy PlantsHardy plants are those that will survive hard frosts, this does not mean that they will survive a freeze, some will other will not. Generally 4 degrees C is the limit for hardy plants to survive however a frost of - 2C should be OK.
Half Hardy PlantsHalf hardy need it warmer, 7 C and only light frosts to 0 C
Tender plantsTender plants need temperatures at 10C or above, and no frosts.
All of this is applied regularly to vegetables and perennials that gardeners plant each year, it determines when you can plant them outside.
The question is also asked, what evergreen plants are hardy. Here the rule refers to what plants can be grown outdoors, in the garden in which areas. So it is again about the degree of cold and frost that a plant can survive in.
Hardiness zones were developed in the USA where climate variation is huge. The zones range from 1 - 11. The coldest zone is zone 1 the warmest is Zone 11. In the UK most of the country ranges from 7-10 and those are divided into a and be, so 7a, 7b and so on.
If in doubt, ask the question is this plant hardy in our area, your garden center should be able to answer. If not, go somewhere else.