What Garden Zone Am I In?

Great Britain has one “temperate oceanic / maritime” climate and isn’t very big, but she is still varied enough that it helps to know where your garden stands in terms of how cold it gets in Winter.

Plant Hardiness Zones, garden zones for short, are areas with minimum winter temperatures within a certain band.
This helps you to choose the right plants, and to know if a plant is too tender to grow normally outside in your beds.

Find your USDA garden zone using this interactive map
Compare your USDA zone to the RHS Hardiness Rating in the table below
Look up the plants that interest you in the RHS Plant Finder to see their Hardiness Rating.
Most likely, you are in USDA 8b/9a, where plants rated by the RHS as H4 and above should be fine outdoors, and plants rated H3 might be OK if they are sheltered, near a warm house and sunny wall that keeps the worst of the frost away.
If you’re on the coast or in the South West, you are probably in the warmer USDA 9b zone, where plants rated by the RHS as H3 and above should be fine outdoors, and plants rated H2 might be OK if they are sheltered, against a warm house and sunny wall that keeps any frost away.
Remember that plants growing in pots are more exposed than those in the soil, so they might lose a hardiness rating point.

USDA Zone	RHS Rating	Min. Winter Temp. range ºC (ºF)	Category	Definition
6a-1	H7	below -20C (< -4F)	Very hardy	Hardy in the severest European climates, including exposed upland locations.
6b/7a	H6	-20 to -15C (-4 to 5F)	Hardy: very cold winter	Hardy throughout Northern Europe. Plants grown in containers may still need protection.
7b/8a	H5	-15 to -10C (5 to 14F)	Hardy: cold winter	Hardy through most of the UK even in severe winters. May not withstand open or exposed sites or central / northerly locations. Many evergreens suffer foliage damage. (Many herbaceous & woody plants, some brassicas, leeks.)
8b/9a	H4	-10 to -5C (14 to 23F)	Hardy: average winter	Hardy except in inland valleys, at altitude, and central / northerly locations. May suffer foliage damage and stem dieback in harsh Winters & frost pockets. Some normally hardy plants may not survive long wet winters in heavy or poorly drained soil e.g Lavender. (Many herbaceous & woody plants, winter brassicas, leeks.)
9b/10a	H3	-5 to 1C (23 to 34F)	Half-hardy: unheated glasshouse / mild winter	Hardy in coastal / mild areas except in hard Winters, when it may be damaged / killed, particularly with no snow cover or if pot grown. At risk from early frosts. May be hardy elsewhere with wall shelter or good microclimate. Can survive with artificial winter protection. (Many Mediterranean plants, spring sown vegetables for later harvesting).
10b	H2	1 to 5C (34 to 41F)	Tender: cool or frost-free glasshouse	Tolerates low temperatures, but not being frozen. Needs glasshouse conditions in Winter until the last frost has passed. Might overwinter outside in frost-free inner-city areas or coastal extremities (Most succulents, many subtropical plants, annual bedding plants, many springsown vegetables.)
11	H1c	5 to 10C (41 to 50F)	Heated glasshouse: warm temperate	Grows outdoors in Summer in most areas, while daytime temperatures are sufficient for growth. (Most bedding plants, tomatoes and cucumbers.)
12	H1b	10 to 15C (50 to 59F)	Heated glasshouse: subtropical	Can grow outdoors in Summer if sunny & sheltered. Generally best indoors, or under glass year round.
13	H1a	above 15C (>59F)	Heated glasshouse: tropical	Grown indoors, or under glass year round.

There are two main differences between the USDA hardiness zone and the RHS hardiness rating approach:

The USDA zone system is based on average minimum winter temperatures. The RHS rating system prefers to look at maximum minimum winter temperatures.
The USDA zone system is focussed on the location and its temperature. The RHS rating system is focussed on the plant and its requirements.

Well seasoned and sober readers will recall life before 2011, when the RHS had only 4 hardiness categories. That simply wouldn’t do, so it was updated in favour of the above table.
As this pdf article on the change notes, “a plant’s survival and success depends upon many other factors : climatic, edaphic (soil), cultural and genetic.”

So your garden zone, meaning your minimum Winter temperature, is only one important factor in whether a plant will thrive or not.
As you might expect, a strong, healthy plant is likely to tolerate slightly lower temperatures, and vice versa a weak, unhappy plant might die in Winters that should not kill it according to the table above.

Looking at the 2006 USDA garden zone system map for the UK, which is close enough for government work, using the basic numbers 7-10 (each number is a 10 degree band, normally split in half into 5 degree increments, marked as a/b), we see:

Most places in the UK are USDA zone 8.
Most of the coast is zone 9, as is London
Zone 7 is the coldest inland Northern and Scottish areas

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What Garden Zone Am I In?

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What Garden Zone Am I In?