By: Mary H. Dyer, Credentialed Garden Writer
If you’re a gardener in the United Kingdom, how do you interpret gardening information that relies on USDA plant hardiness zones? How do you compare UK hardiness zones with USDA zones? And what about RHS zones and hardiness zones in Britain? Sorting it out can be a challenge, but understanding zone information is important because it helps you select plants that have the best chance of surviving in your particular climate. The following information should help.
USDA Plant Hardiness Zones
USDA (U.S. Department of Agriculture) plant hardiness zones, based on minimum ten-year average temperatures, were created in the 1960s and are used by gardeners around the globe. The purpose of the designation is to identify how well plants tolerate the coldest temperatures in each zone.
USDA zones begin at Zone 1 for plants that tolerate severe, sub-freezing temperatures to tropical plants that thrive in Zone 13.
RHS Zones: USDA Zones in Great Britain
RHS (Royal Horticultural Society) hardiness zones begin at H7 (temperatures similar to USDA Zone 5) and are used to designate very hardy plants that tolerate sub-freezing temperatures. On the opposite end of the temperature spectrum is zone H1a (similar to USDA zone 13), which includes tropical plants that must be grown indoors or in a heated greenhouse year-round.
Does Britain Use USDA Hardiness Zones?
While it’s important to understand RHS hardiness zones, much of the available information relies on USDA zone guidelines. To get the most benefit from the wealth of information on the Internet, it’s a tremendous help to arm yourself with information about USDA zones in Great Britain.
Most of the United Kingdom is located in USDA zone 9, although climates as chilly as zone 8 or as mild as zone 10 aren’t uncommon. As a general rule, the UK is marked primarily by cool (but not frigid) winters and warm (but not scorching) summers. The UK enjoys a fairly long frost-free season that extends from early spring to late autumn.
Keep in mind that UK zones and USDA zones are intended to serve as guidelines only. Local factors and microclimates should always be taken into consideration.
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Read more about USDA Planting Zones
Whilst the USDA system of plant hardiness zones is well adopted across the world, it’s not without its drawbacks. The system only takes into account the extremes of winter temperature without consideration to the other parts of the climate. There is no indication of humidity, snow cover or summer heat.
This is easily explained by pointing out that the majority of Texas, USA is also a USDA Hardiness Zone of 9. The exact same as we are in the south of the UK. However, anyone familiar with both climates will attest to how dissimilar they are. Similarly, Sochi (Russia), Madrid (Spain) and Belfast (Northern Ireland) are also USDA Zone 9 all with varying environments.
Whilst average winter temperature may be similar, summer heat, duration, humidity, snow cover and even day length is completely different. All of these factors and more need to be taken into account for a plant to thrive in a specific area.
USDA Plant Hardiness Zones are not a definitive guide but a helpful extra bit of information to use in conjunction with other information such as ‘Indicator Plant Species’, geographic information and local information.