A selection of Green Bay leaves on a bay tree that have no yet turned brown

A Guide To Plant Hardiness Zones In The UK

ByEve

In my recent article on the best cold hardy vegetables to grow in winter I mentioned the different plant hardiness zones in the US, but didn’t think to mention the way that the system works in the UK. Which is slightly different, and important for any UK based gardeners to be aware of.

And as a good percentage of my readers are UK based, it’s only right that I help you out too. So here’s a quick guide on plant hardiness zones in the UK and I’ve included an illustrated plant hardiness zone map further down as well.

What Are Plant Hardiness Zones? 

The plant hardiness zones dictate the average lowest temperature during the winter season for that particular region. There are a total of 13 hardiness zones in the category based on the classification defined by the United States Department of Agriculture (USDA). This category assists people from around the world in determining the type of plants compatible with their particular location so that their gardening endeavors are not in vain.

Please take note however that the UK utilizes a slightly tweaked version called the RHS (Royal Horticultural Society) hardiness planting zone. This revision is because the country as a whole experiences a rather specific climate. For example, the RHS hardiness zone incorporates H1a (equivalent to the USDA zone 13) up to H7 (equal to the USDA zone 6).

The UK experiences generally stable (albeit rainy – ha ha!) and cool weather all year round, hence revising the USDA hardiness zone to reflect their climate better and cut off the proverbial fat. Although the UK does suffer the occasional severe winter, the average minimum temperature throughout the winter stays relatively consistent, so UK peeps – like my Dad – can continue gardening throughout the winter by following the RHS zone system.

How Do The UK Zones Compare To The USA Zones?

The UK has a relatively particular climate that it experiences all year round, unlike the USA, which covers all four seasons, depending on the location. For this reason, the plant hardiness zone UK variation utilizes a slight variation of USDA’s model, but the former is still based on the latter’s official definition.

Is the difference between the two quite obvious? Perhaps, but to help clear things up further, here is a quick overview of the two hardiness zone models.

UK’s RHS Hardiness Zone

The Royal Horticultural Society (RHS) developed a hardiness zone model based on the USDA’s official model. The main difference with the RHS hardiness zone is that it covers the absolute minimum recorded temperature, not the long-term average. In total, there are nine hardiness zone ratings in the RHS version.

USA’s USDA Hardiness Zone

The USDA plant hardiness zone divides the map of the USA and Canada into 13 different zones based on the average minimum winter temperature. Zone 1, which covers the northern part of the map, is the coldest, while zone 13, in the southernmost section, is the hottest. This somewhat simplified model can help people narrow down the list of plants and vegetation that they can successfully grow in their region.

Of course, this is just a general guide, and other factors should still be considered, such as topography and urban heat island effects, which affect different climate zones in the same latitude. These notable factors are why some areas within the same hardiness zone still experience significantly different temperatures.

UK Plant Hardiness Zones

As stated previously, the UK’s RHS hardiness zone model has a different category based on the USDA model. As the UK doesn’t generally experience some of the more extreme climates in the USA and Canada, it pretty much redacted half of the categorical zones in the USDA model. Here is a quick overview of the difference between the USDA model and the plant hardiness UK version.

  • USDA zone 6 / RHS zone H7 / -23.3. 0°C to -17.8 °C
  • USDA zone 7 / RHS zone H5 and H6 / -17.8 °C to -12.2 °C
  • USDA zone 8 / RHS zone H4 and H5 / -12.2 to -6.7 °C
  • USDA zone 9 / RHS zone H3 and H4 / -6.6 to -1.1 °C
  • USDA zone 10 / RHS H2 and H3 / -1.1 to 4.4 °C
  • USDA zone 11 / RHS H1c / 4.4 to 10 °C
  • USDA zone 12 / RHS H1b / 10 to 15 °C
  • USDA zone 13 / RHS H1a / 15.6 to 21.1 °C

This model is a revision made by the RHS in 2012; the UK incorporated an RHS hardiness zone model consisting of H1 to H4 ratings only.

With the ongoing climate crisis, there is obviously a fluctuation in the lowest and highest temperatures being experienced in both the UK and the USA so it’s possible at some point that the maps may change in the future to more accurately reflect the climate change and regional average minimum temperatures too.

Hardiness Maps

To better help you know where you are located, I’ve provided two illustrated plant hardiness maps.

I hope you find the maps helpful in locating your plant hardiness zone in the UK. Let me know in the comments below which zone you’re in and if there are any struggles you find with gardening in the winter months!

And don’t forget to check out

Similar Posts