If like me, you read articles about gardening written by our US cousins, you’ll probably find reference to the United States department of Agriculture hardiness zone.
You might immediately switch off as there’s no equivalent map for the UK and so you’ve no idea which one you are in. Here’s a bit of help and then you can read these articles too. Perhaps if enough people ask, I’ll try to work out an equivalent UK map and post it here, let me know.
So, what is it? Their text says this:
Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.
And happily, the map has both Celsius and Fahrenheit.
The USDA has just updated its map (January 2012), see press release here:
The map itself is here: http://planthardiness.ars.usda.gov/PHZMWeb/
I would say that in my garden in North Bucks, the average lowest Winter temperature is around -8C, so that puts me in region 8B (between -9.4 and -6.7C).
So, now I know and I can read gardening articles from the USA with more understanding of the likely success of plants in mine, any plant that is considered hardy only in regions with a higher number than 8B are likely to be a problem for me.