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Will it survive the frosts? Hardiness ratings for your plants

As a youtube gardener, my garden is shared with a worldwide audience of keen gardeners. We share information, learn from each other and get to experience gardening from different perspectives. It really is such an amazing opportunity to learn and the support system is second to none. There is always a need, however, to remember that you need to be aware of your own garden’s needs, microclimate, and planting. And although I repeat this often, I don’t think it can be said enough. One demonstration of this I’m frequently faced with is winter, or rather working out what to grow in winter.

Because youtube, or generally the internet, has a high proportion of North American users, I receive questions on a daily basis from UK growers confused as they try to work out which “growing zone” their garden is in. I suppose it’s not helped by people like myself trying to demonstrate the US growing zones in relation to our gardens as it falsely implies that this system fits our needs.

However, we do have a system in the UK that you may just find a whole lot more useful and it’s the Royal Horticultural Society (RHS) hardiness ratings for plants. It’s plant-specific rather than garden specific and is a fantastic way to check quickly about which of your beautiful garden babies might need some extra protection over the colder months or indeed which of those stunning plants you see in the garden center, to avoid at all costs.

So, are you ready for a crash course in plant hardiness levels?

The RHS hardiness scale is for rating the hardiness of plants, kinda goes without saying really. It’s a way to measure of how well a plant can withstand cold temperatures and goes from H1 (not hardy at all) to H7 (hardy in all climates). Its focus is on the temperature that the plant will tolerate, meaning that if you know the likely coldest temperature in your garden over winter, you can plan for which plants will tolerate this. So next time you are seed shopping, you can have a quick search on the RHS website and find out if those plants are right for your growing area: https://www.rhs.org.uk/plants/search-results

The brackets at the right show the minimum temperature range for plants with that rating.

So, let’s start with the wimpy plants at the bottom of the scale.
H1 plants are total tender babies and need to be grown indoors or in a regulated greenhouse. They just can’t handle the cold at all. Think of these plants as the divas of the plant world.

  • H1a – Under glass (heated greenhouse) all year (>+15°C) – tropical
  • H1b – Can be grown outside in the summer but performs better in a greenhouse (+10 to +15°C) – subtropical
  • H1c – Can be grown outside in summer in most of UK when daytime temperatures are high enough. Think tomatoes, cucumbers or peppers (+5 to +10°C)

H2 plants are a little hardier, but they still need to be protected from cold and especially frost. We call these plants tender. They’re like the sensitive souls of the plant world – they need a little extra care and attention.

  • H2 – Considered tender – tolerant of low temperatures, but won’t survive being frozen and should be protected from frost (+1 to +5°C)

H3 plants can handle a bit of frost, but they still need some protection. These plants are like the students of the plant world – they can put on a really good show, but they need a little support to really shine.

  • H3 – Hardy in mild coastal and relatively mild parts of the UK (-5 to +1°C)

H4 plants are pretty hardy and can handle frost and light snow. They’re like the steady, dependable aunt of the plant world – they can handle a bit of drama, but they don’t really need any special treatment to thrive.

  • H4 – Hardy through most of the UK (-10 to -5°C)

H4 are the plants most likely to cope in my garden here in East Lothian.

H5 plants can handle very cold temperatures and heavy snow. These plants are like the tough, rugged outdoors types – they can handle just about anything Mother Nature throws their way.

  • H5 – Hardy in most places throughout the UK even in severe winters (-15 to -10°C)

H6 plants are hardy in all but the most extreme climates. They’re like the extreme sports enthusiasts of the plant world – they can handle just about anything, but they might need a little extra protection in the most extreme conditions.

  • H6 – Hardy in all of UK and northern Europe (-20 to -15°C)

And finally, we have the ultimate toughies: H7 plants. These plants are hardy in all climates and can withstand the coldest temperatures. They’re like the Royal Marine Commandos of the plant world – they can handle just about anything and emerge victorious.

  • H7 – Hardy in the severest European continental climates (<-20°C)

So there we go, it’s not your garden zone you need to be thinking about but the hardiness of your plants.

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