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If you’ve ever looked at a seed packet, growing guide or gardening video from the USA and wondered what on earth they mean when they say “I’m in Zone 6”, you’re not alone.

Gardening zones can be confusing, particularly when you’re reading advice from different countries. A plant might be described as suitable for Zone 5 in one article, while a UK gardening guide uses a completely different numbering system.

So, what exactly is a gardening zone, why does it matter, and how can you use one without getting completely bogged down in numbers?

The good news is that you don’t need to become an expert in climate data. Understanding the basics around zones can help you make better decisions about what to grow, when to sow, and when it is safe to transfer precious seedlings outside.

What is a gardening zone?

A gardening, or more appropriately, a hardiness zone, is essentially a way of describing the climate in a particular area or region. It indicates how cold it is likely to get during winter.

The idea is to divide a country or region into areas with broadly similar minimum temperatures. This helps gardeners work out which plants are likely to survive their local winter.

This is particularly important with perennial plants (one that lives more than two years), shrubs and trees. If a plant is described as hardy to a particular zone, it means it should be able to survive the typical minimum winter temperatures found within that zone.

For annual vegetables, however, zones are only part of the story.

A tomato doesn’t really care which zone you’ve been assigned on a map. It cares whether the soil and air are warm enough, whether there is enough light and, most importantly, whether it’s going to get hit by a frost and die.

USA gardening zones

The gardening zone system you’ll most commonly come across online is the USDA (United States Department of Agriculture) Plant Hardiness Zone system.

The United States is divided into zones based on average annual extreme minimum temperatures. The zones run from very cold areas, such as Zone 1 (Alaska), through to much warmer areas, such as Zone 13 (Puerto Rico).

Each zone is further subdivided into a and b, which allows the system to give a slightly more precise temperature range.

For example:

• Zone 6a: -23.3 to -20.6°C (-10 to -5°F)
• Zone 6b: -20.6 to -17.8°C (-5 to 0°F)
• Zone 7a: -17.8 to -15°C (0 to 5°F)
• Zone 7b: -15 to -12.2°C (5 to 10°F)

These figures refer to the average annual extreme minimum temperature, rather than the average temperature throughout winter.

That distinction is important, because if an American gardener tells you they’re in Zone 7, they’re basically giving you information about how cold their winters are likely to get. They are not telling you exactly when they should sow their carrots or plant out their courgettes. The system gives you an indication that a certain veg may not survive a winter sowing in that particular region.

The USDA system is particularly useful when you’re researching plants from American gardening websites, seed companies and YouTube channels.

But if you’re gardening in the UK, you’ll also come across a different system.

RHS hardiness ratings in the UK

The Royal Horticultural Society (RHS) uses a hardiness rating system for plants in the UK.

Rather than using the same numbered zones as the USDA, the RHS uses H ratings, ranging from H1 for plants needing very warm conditions through to H7 for plants that can tolerate very cold conditions.

For example, you’ll often see plants described as:

• H4 – hardy in most of the UK
• H5 – hardy in most of the UK, including colder areas.
  

For the full RHS list see here https://www.rhs.org.uk/advice/rhs-hardiness-rating

The RHS system is designed specifically for UK growing conditions, so it can be more useful when choosing plants here.

It also takes into account the fact that the UK has a relatively mild climate compared with many parts of North America. This is where comparing the two systems can get a little messy.

Can You Convert a USA Zone to a UK Zone?

The answer to this is ‘well, not really ‘- at least, not accurately. You will sometimes find websites offering a simple conversion such as USDA zone 8 equals UK H4. But I would say be wary of treating these as exact equivalents. If you need to do a comparison, I’d go with the chart on the RHS website referred to earlier.

Both systems are used to help gardeners understand plant hardiness, but they work slightly differently.

Basically, the RHS system gives plants a hardiness rating based on the minimum temperature they are likely to tolerate under UK growing conditions. i.e. H1 for plants needing very warm conditions to H7 for plants able to tolerate colder conditions.

The USDA system is based on the average annual extreme minimum temperature for an area – essentially, how cold it typically gets at its coldest.

So although both systems are talking about cold temperatures, you can’t simply look at a USDA zone and find an equivalent RHS rating.

And there’s another important point: a hardiness zone doesn’t tell you when to sow your vegetables.

If you’re watching an American gardening video and the gardener says they’re in USDA Zone 8, that information can help you understand something about their winter temperatures. It doesn’t mean you should automatically follow their sowing dates or planting-out dates.

Your growing season may be quite different because of rainfall, summer temperatures, daylight hours, altitude, exposure and local weather conditions or microclimates.

That’s why, when you’re growing vegetables, local frost dates, soil temperature and what is actually happening in your garden are often more useful than trying to convert one country’s gardening zone into another.

In other words, use the zone as a clue or very rough guide about climate, rather than a direct instruction for what to do in your garden.

We also mustn’t forget that the climate is changing, making your own experience and that of other gardeners in your locality often more valuable than any zonal temperature guide.

So, what zone are you actually in?

If you’re in the UK, I’d recommend finding your RHS hardiness rating for perennial plants, particularly if you’re buying shrubs, herbs or ornamental plants that need to survive the winter.

Obviously, if you are in America, then the USDA zones will be most relevant.

What about frost dates?

For growing vegetables, I actually think frost dates can be more useful than gardening zones.

Your first frost date is the approximate date when you can expect frosts to start returning in autumn. Your last frost date is the approximate date after which you are unlikely to experience another damaging frost in spring.

Together, these dates give you an idea of your growing season – the period between the last spring frost and the first autumn frost. If you are in a colder climate, these dates will be closer together whereas in warmer climates they are likely to be further apart.

This is particularly useful for tender vegetables such as:

• tomatoes
• peppers
• chillies
• courgettes
• cucumbers

(not an exhaustive list by any means).

These plants really don’t appreciate being planted outside while the nights are still cold. For example, you might have a packet of courgette seeds telling you to plant outside after the last frost. That’s useful information – but when is your last frost?

That’s the bit you need to find out for your own area. This is a useful website: https://www.lastfrostdate.co.uk/map/

But don’t treat your frost date as gospel

This is where gardening gets wonderfully inconvenient. A last frost date is an average or estimated date, not a promise from the weather gods.

As with pretty much anything in the world of gardening, use frost dates as a guide rather than a deadline.

Generalities are useful, but nothing really beats keeping an eye on the weather yourself. So if you are growing tender crops and there is a frost predicted later than your given frost date. It’s worth erring on the side of caution and covering you planted out crops with a fleece or bringing them indoors until the risk is over if they are in pots.

Microclimates matter more than you might think

Your gardening zone and frost dates give you a broad picture.

But your garden will have a little climate all of its own.

This is called a microclimate, and it’s one of the reasons gardening advice can never be completely universal.

A few things can make your garden warmer, colder, wetter or drier than the area around it.

Think about:

• walls and fences
• sheltered corners
• exposed or windy positions
• slopes
• south-facing areas
• areas beneath trees
• patios and paving
• raised beds
• greenhouses and polytunnels
• low-lying areas where cold air collects

A brick wall can absorb warmth from the sun during the day and release some of that heat after dark.

A sheltered corner can be several degrees warmer than an exposed area.

Meanwhile, a cold hollow in the garden can act like a frost pocket.

This is one reason why two gardeners living only a few miles apart can have completely different experiences with the same plant.

And what if you’re gardening in a new area?

I’ve recently moved from England to Wales, and although that doesn’t sound like moving to a completely different climate, the growing conditions can still be noticeably different.

Altitude, exposure, rainfall, soil and local weather patterns all make a difference.

It’s a good reminder that you need to spend time getting to know your new growing space, rather than assuming what worked in your previous space will readily transfer to your new space.

If you have a new growing space, here are some of the things you might want to make a note of;

• Where does the sun reach?
• Which areas dry out quickly?
• Where does water collect?
• Which parts are sheltered?
• Where does the wind come from?
• Where does frost linger?

These observations are just as valuable as knowing your hardiness zone. If you want to read more about the importance of observing your garden, you can click here Master Your Garden: The Art of Observation

Final thoughts – The last piece of the puzzle.

Hardiness zones, frost dates, books, YouTube & Instagram gardening channels and the like are all fantastic. But they are not gardening law.

If all the gardening advice is not to sow carrot seeds in your area in November and you want to give it a go – why not? You do you. If it works, then great; if not, then perhaps some of the advice is worth listening to.

What I’m trying to say is don’t get too caught up in gardening zones, and frost dates, etc. They are rules for fools, but guidance for wise people 😊

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