USDA HARDINESS ZONES
This system was developed by the United States Department of Agriculture and is used to help understand what plants will grow in certain locations and the typical duration of a plant's growing season with monthly average temperatures. There are 13 zones, 1 being the coldest (-51.1 to -48.3 degrees Celsius in northern and central Alaska) all the way to zone 13 (18.1 to 21.3 degrees Celsius in Puerto Rico. See our conversion page if you use Fahrenheit. Due to the variations, zones are divided into A and B. Unless stated a full zone is referred to.
What it doesn't say
These temperatures focus on the average lowest winter temperatures as plants sit outside, although poly tunnels and greenhouses provide micro climates to extend the food production season. Information not taken into consideration since its conception in the 1960's are humidity, wind or extreme weather, flooding and summer temperatures. Most of England would be zones 8A/B (-12.2 to -6.7 degrees C) and 9A (-6.7 to -3.9 degrees C), with exceptions of some areas such as the peak district and other high places. Flash in the peak district is much higher than many other places and receives more snow and colder lows, where just miles away lower down those places are having rain and be warmer, therefore they are 2 separate zones due to the micro climates.
Some issues can be that zones 8 and 9 for England also include some areas of central/eastern Spain and western Turkey, which both have higher and summer temperatures and more sun year round, meaning a longer growing season for the warmer areas. Another comparison within the USDA data range would be zones 1-4 are in Alaska as well as Montana, which is much further south. Montana has summer peak temperatures close to 40 degrees Celsius, where as Alaska summer heat peaks between 15 - 27 degrees.
USDA map
For up to date zone information the United States Department of Agriculture site: https://planthardiness.ars.usda.gov/pages/map-downloads
The zones are updated periodically, with the 2012 map being collated by data between 1976 - 2005. Due to the more rapid changes in environments hopefully a new one will be be available sooner than 33 years time.
USDA zone map
Created by Agricultural Research Service ARS and Oregon Sate University OSU 2012. Visit the link above or the Oregon State University site for a downloadable map.
USDA zones across the world (generalised)
Zone 6
-23.3 to -17.8 degrees C
Alps, Austria, Canadian coastal regions, central and southern Iceland, Czech Republic, Hungary, Macedonia, Poland, Romania, Slovakia, western and southern inland Scandinavia and various US states. Some of Scotland's northern islands would fall in to this category.
Plants that grow well in zone 6 are apple trees, beans (cold varieties), cornflowers, dahlias, kale, lettuce, potatoes, pumpkins, radishes, sage, spinach and squashes.
Undercover - chilies, cucumbers, peppers and tomatoes.
Zone 7
-17.8 to -12.2 degrees C
Andorra, coastal Iceland, Scandinavia, Estonia, Denmark, east and southern Germany, eastern Europe, northern Greece, Peak district (UK), Scottish highlands and the Spanish mountains
Pants that grow well in zone 7 include all of the previous zone. Plus beetroot, broccoli, nasturtiums, peas, parsnips, rocket.
Undercover - aubergines, chilies, cucumbers, peppers and tomatoes.
Zone 8
-12.2 to -6.7 degrees C
Belgium, eastern Spain, northern France, Holland, Interior Ireland, Luxembourg, mountainous Adriatic coast, Northern Ireland, most of Wales and England, Scotland, Southern Argentina and Chile, Wales
Plants that grow well in zone 8 are from the above zones 6 and 7. Plus figs, pears, radicchio, salad leaves, spring greens, sunflowers, sweetcorn and tomatoes.
Undercover: Asparagus, aubergines, melons, olives
Zone 9
-6.7 to -1.1 degrees C
Southern Argentina and Chile, China (with exceptions), Greece (northern areas), Japan (most of), Italy, New Zealand, Mountainous areas of southern Africa, North Africa (inland), Portugal, Turkey, Western Spain, US coastal states, northern Texas, coastal UK, the channel islands, Irish west coast.
Some plants from zones 6 - 8 may struggle in zone 9 summer conditions depending how dry it is. Plants such as apple and pear trees need a lot of water. Plants that grow undercover in zone 8 may be planted in outdoor suntraps.
Undercover: Aubergines, banana trees, olives.
Argentina, Australia (cooler areas), Canary Islands (high areas), China, England (isolated south coast in microclimates, Scilly Isles, channel islands and Isle of Wight), sheltered parts of the Irish south coast, Madeira, none mountainous areas of the the Mediterranean (including islands and north African coast), parts of Mexico, New Zealand, Portuguese Altantic coast, Southern Africa, Tazmania.
Zone 10
-1.1 to 4.4 degrees C
Some plants from zones 6 - 9 may struggle in zone 10 due to the hot and dry summer conditions. Apple and pear trees need a lot of water in zone 10 locations. Zone 9 undercover plants can do well outdoors, including aubergines, bananas, watermelon, olives.
Zone 11 - 13
4.4 - 21.2 degrees C
Australia, Argentina (northern), Brazil, Caribbean, Central Africa, Central America, Chile, Southern China, Columbia, Ecuador, Mexico, Peru, Southern India, Indonesia, United states (California, parts of Florida and Hawaii).
Cool climate plants may struggle with heat, sun exposure and drought. Look for local heat tolerant breeds, use shade or a controlled indoor climate. These zones are suitable to grow tropical plants outdoors including bananas, coconuts, coffee, guava, okra, pineapple etc.
