Hardiness zones, what can I grow?

January is the perfect time for sitting with a mug of tea, a couple of biscuits (ok, ok a plate) and a pile of seed catalogues (or as Kate calls them, garden porn). Just browsing those wonderful blasts of colour and potential beauty is enough to keep me going all through the beginning of the year, but… I remember the first time I picked up the seed catalogues and I remember feeling lost in the jargon and abbreviations. So let’s make this part 1 about all the different pieces of information you’ll find in your seed catalogues, seed packets and in garden centres. Let’s start with hardiness zones.

When it’s time to decide what you’ll be ordering this year

Seed catalogues are full of options, wonderful varieties of plants you’ve never heard of with pictures that almost make you drool thinking of them in your garden. The thing is, not everything is going to grow well in your garden and working out what will and what types you want to buy is a bit of a labyrinth for any new gardener.

Hardiness zones

If you are a new gardener then hardiness zones might be a new term for you, but it’s something that’s really useful to know. It essentially gives you a rough way of looking at plants and knowing if they will thrive in your area, for example, there really isn’t much chance of tropical plants growing outdoors here in Scotland. It’s a wet and cold climate but would a palm tree? Well, let’s look at the hardiness zones and information that the RHS (Royal Horticultural Society) publish. This gives us the USDA Zones and some info about the temperatures in them but more importantly, the RHS hardiness rating which you are likely to see in catalogues and garden centres in the UK.

The zones

RHS Rating Temperature Hardiness USDA Zones Notes
H1a >15C Tropical 13 To be grown under glass or as a house plant
H1b 10C to 15C Subtropical 12 Can be grown outdoors in summer in warm, sunny and sheltered locations, but will generally perform indoors
H1c 5C to 10C Warm temperate 11 Can be grown outdoors in summer throughout most of the UK.
H2 1C to 5C Tender cool 10 Tolerant of low temperatures, but will not survive being frozen.
H3 -5C to 1C Half-hardy 9 Hardy in coastal/mild areas, except in hard winters. Likely to be damaged or killed in cold winters, particularly with no snow cover or if potted.
H4 -10C to -5C Hardy 8,9 Hardy through most of the UK apart from inland valleys, at altitude and central/northerly locations.
H5 -15C to -10C Hardy 7,8 Hardy in most places throughout the UK even in severe winters.
H6 -20C to -15C Hardy 6,7 Hardy across the UK and northern Europe.

So looking at the table we can see a few important pieces of info, let’s take USDA zone 9 as an example. This is the zone I live in. The table shows that it has an RHS rating of H3, so if we see that in the garden centre we know that means us. The winter temperatures are on average between -5C to 1C and it’s considered half-hardy. We can also see a bit of advice. “Hardy in coastal/mild areas, except in hard winters. Likely to be damaged or killed in cold winters, particularly with no snow cover or if potted.” So something that usually survives winter might not survive a harsh winter, especially in a pot.

Useful to know. Means I may want the ability to move a plant into the greenhouse or indoors during a bad winter.

Zones in the UK

So the first thing I want to point out is that Scotland isn’t just one zone. You can see the west coast is marked as zone 9, whereas if you head inland towards the Cairngorms National Park it’s a hardiness zone of 6. This is because in different areas you can expect lower temperatures over winter, different levels of frost and snow. Knowing which zone you are in can really help you decide which plants will do well in your garden. However, don’t take this as absolute fact, keep your garden journal up to date and get to know your garden. I know that even though I am in zone 9, there is a bit of a local microclimate around my area which these charts can’t predict.

So that palm tree… well maybe, most palm trees are tropical meaning no they wouldn’t grow here, but you can look for palms which are labelled as hardy. These could be grown in pots so that you could bring them indoors over winter or wrap the stems to prevent damage from our cold winters.

Take a look at the RHS plant search site and have a look at the plants recommended for your hardiness rating.

https://www.rhs.org.uk/plants/search-form