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Plant hardiness

02-Aug-2020

 

A guide to plant hardiness in the UK

 

When buying plants, you might see a number of terms relating to the hardiness of the plants and it can often be difficult to know which are suitable for your garden and the area you live in. We’re taking a closer look at what plant hardiness is and explain the ratings given so that the plants you buy have the best chance for survival when planted in your garden. 

 

 

What does plant hardiness mean?

Before you know which plants are hardy enough to be planted in your garden and where you can plant them, you’ll need to know what we’re referring to when we talk about the hardiness of plants. 

There is no widely agreed specific definition for plant hardiness but it’s usually thought of in terms of how the plant stands up to cold temperatures and, particularly in the UK, how well they withstand alternating periods of freezing and mild, wet weather. 

The plants we sell are adapted to tolerate cold temperatures at varying levels. 

To rate the hardiness of plants, the minimum winter temperatures they can withstand is used as a guide and the plants are then placed on a scale of least hardy to most hardy. 

 

What is the lowest temperature plants can tolerate?

The lowest temperature that plants can tolerate all depends on the plant you are buying and the conditions in your garden. 

Plants are usually divided into the following hardiness levels to help you understand how they might survive in your garden and if you should leave them in the garden over winter or move them inside.

Hardy - hardy plants are able to withstand temperatures down to -15°C and tend to thrive well in the UK because our winter temperatures rarely dip below -11°C.

Half-hardy - like hardy plants, half hardy plants can withstand long periods of cold temperatures but they’re unable to survive through frost or temperatures below 0°C. These are the plants you would typically sow inside a greenhouse and plant them out in the garden later in the year once the risk of frost has passed. 

Frost hardy - As the name suggests, frost hardy plants can survive a frost but cannot survive in temperatures lower than -5°C.

Tender - Tender plants cannot survive cold and wet conditions and tend to be annual plants in the UK as they are from warmer regions where they survive for much longer.

 

What are plant hardiness zones?

If you buy plants regularly, you may be aware of the different hardiness zones that plants tend to be divided into. These zones will show you how well the plant might survive in your garden and which are best suited for growing in the UK. 

Plant hardiness zones were first developed by the US Department of Agriculture (USDA) but have since been applied to the rest of the world to indicate the best areas for certain plants to grow and thrive. However, it’s important to bear in mind that these zones are only guidelines and take the ability of plants to withstand cold temperatures in mind whilst ignoring the impact of microclimates, precipitation, humidity and summer heat tolerance. 

Despite this, it’s important to be aware of the hardiness zones in your area to make sure your plants can survive! 

As well as the USDAs plant hardiness zones, in the UK, the RHS has produced its own method for rating plants hardiness but this has not been mapped into zones for the UK. The UK climate can vary significantly day to day (and even hour to hour!) So, to get the most from your garden, understanding the local conditions of your garden and how it can vary is essential. 

 

What hardiness zone is the UK?

The USDA guidelines indicate that the UK lies between zone six and zone nine but with variations across regions and seasons. The UK’s climate is characterised by its cool winters and warm summers and the frost season is fairly long; ranging from late March to April through to early October and the end of November.

The below table should help you to understand the RHS’s guide and the USDA guidelines together.

 

RHS rating Temperature Hardiness USDA zones
H6 -20°C to -15°C Hardy
Very cold winter
6, 7
H5 -15°C to -10°C Hardy
Cold winter
7,8
H4 -10°C to -5°C Hardy
Average winter
8,9
H3 -5°C to -1°C Half-hardy 9
H2 1°C to 5°C Tender cool 10
H1c 5°C to 10°C Warm temperate 11
H1b 10°C to 15°C Subtropical 12
H1a 15°C Tropical 13

 

What's your plant hardiness zone?

A map of the UK’s plant hardiness zones 
This map is not entirely accurate but it’s a good indicator of the hardy plants that might survive in your garden.

 

 

Whilst the UK is small geographically, our winter temperatures fluctuate significantly depending on where in the country you are. 

For example, in western coastal areas, the impact of the Gulf Stream means some frost-hardy and half-hardy plants will survive outside throughout the winter as temperatures are unlikely to drop dramatically. However, in more mountainous and exposed moorland, less hardy plants are unlikely to survive the low temperatures.

When buying plants, we always advise that you take the microclimate of your area into account. As temperatures and weather conditions can change so quickly, a plant could be frozen one day, being blasted by icy gales the next and then basking in warm sunshine two days later. This means we need plants that are resilient in our gardens to be successful!

 

Can plants recover from frost?

Sometimes, a warm spell can prompt you to plant your half hardy or tender plants outside and frost can take you by surprise. It’s an easy to mistake to make and it doesn’t always spell the death of your garden. Often, preventing frost damage from happening is the easiest solution. If you’re planting them outside before the frost season is completely over, cover you tender plants with sheets or burlap sacks until you’re sure the cold season has passed completely. Make sure you remove them the following morning when the sun comes up so that the plants have time to grow and get some light. 

If some of your plants have suffered with frost damage, please don’t give up on them straight away! You’ll find that many plants are extremely resilient and might come back in the early summer. If you haven’t seen any signs of revival by then, it’s probably best to assume it’s not coming back and replace it. 

For more information on saving frost damaged plants, we have more guidance here. 

 

When should you plant hardy perennials?

If you want to plant your perennial plants, it’s better to do it in spring (March to May) or in the autumn (September to October) whilst the ground is fairly moist. If you miss your chance to plant perennials in the spring, you should wait until the autumn to give them the best chance of survival. This will allow them to establish themselves in the warm soil before the colder months take hold.

However, if you can, it’s much better to plant perennials in the spring. As the soil warms up with the outside temperature, it will encourage the plants to establish roots and start to grow. Once the full heat of summer arrives, your perennials will be well established and ready to thrive.

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