Hardiness

In the simplest terms ‘hardiness’ is a measure of the lowest temperature that a plant will survive reliably.  We use the terms fully, frost and half hardy but in the USA the country is divided into numbered Zones for the ranges of minimum winter temperatures.  Plant books and websites often give the hardiness zone for plants that we also grow in the UK, so here is a simplified table mapping the zone to the approximate minimum required temperature.

You can see that zone 10 is UK half hardy or tender, zone 9 corresponds to frost hardy and anything less than zone 7 will be fully hardy here.