Hardiness zones - a few words  

This refers to a system devised by the US Department of Agriculture which describes all parts of America by taking the average annual minimum temperature reached there. These temperatures are then divided up into ranges or bands, thereby linking areas that experience similar figures for the mean low temperatures each winter.  Those zones most relevant to the UK are shown below.

  Limitations of the system

The annual minimum temperature is only one of several different factors that makes up the local climate.  It takes no account whatsoever of the resultant day time temperature rise, how long the low temperature lasts, whether it is dry or wet, sunny or grey and all these things make a profound difference to what plants will and won't survive - in addition to merely their ability to withstand a low temperature figure. 

Statistics on our side of the pond

In particular there are significant differences between most of the US and the UK that needs to be considered when thinking of these temperature zones.  If plotted on a graph our winter lows would all be fairly closely dotted together with most years seeing a figure within a couple of degrees of the average.  Historically, there are occasional cold winters but these are no more frequent than the occasional very mild ones and are in any case not exceptionally lower than the average.  In the US, the vast majority of the winter lows are considerably higher than the average, but this average is dragged down by the periodic extremely cold spells that they experience - 'arctic pulses' - about every 10 years or so.  The effect of this is that a usual winter for most years in any given zone will be warmer than the equivalent zone in the UK, with  occasional extremely cold winters.  This is only a simplified overview - I am no meteorologist - but outlines the principle.

How we can use the zones

I use the zones as a sort of 'coarse filter' : plants that are known to only grow in a half zone or more warmer in the US compared to my garden are pretty well unlikely to grow here.  I do make exceptions occasionally if I particularly want to try a plant or I can give it a good microclimate, but generally this gives a good guideline.  Also when dealing with plants that are untested outside of their habitat, if the climate data is available and the 'zone' can be worked out then this is also a useful guideline.  Many of the plants from South Africa fall into this category eg some aloes such as Aloe pratensis are given a rating of z7 - a very cold climate plant.

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