Planting zones, also called hardiness zones and gardening zones, help you select plants that will grow in your climate. In this blog I will answer the following questions:
- What are planting zones?
- How do you determine your own hardiness zone?
- Are the USDA zones the only planting zones? What about other countries?
- How do you select better plants by knowing your planting zone.
What are Planting Zones, Hardiness Zones and Gardening Zones?
Planting zones, hardiness zones and gardening zones are all names for the same thing. USDA zones is another name for the same thing.
Consider the above picture, Berkheya purpurea, from my garden. Before I grew the plant, I didn’t know if it could survive our winters. How much cold can Berkheya purpurea take in winter? Planting zones provided a way for me to know this without buying the plant.
I know my planting zone is 5, or more precisely 5b. With this information I can look up the plant on the internet to find it’s hardiness zone. If the plant’s hardiness zone is the same as mine, or a smaller number, then it should grow here.
Plants generally have a range of hardiness zones. So a plant might have a zone range of 4 to 8 (usually written as 4-8). This means that the plant should grow in any planting zone between 4 and 8, inclusive. The smaller the number, the colder the climate – just like a thermometer. So if you live in zone 3, a 4-8 plant will probably not make it through the winder.
If you live in a warm planting zone like 10, then this plant will also not do well because of the heat. It does not want to grow in a zone warmer than 8.
Finding Your Planting Zone
There are two ways to find your planting zone. You can ask an experienced gardener in your area. They will almost certainly know their planting zone and if you live nearby – yours will be the same.
You can also look on the internet for a planting zone map for your area. Some magazines and books also have this information but older written material may be out of date.
USDA Planting Zones
The USDA has created a number of plant zone maps over the years, with the latest one being released in 2023. It’s official name is the USDA Plant Hardiness Zone Map. The map is based on the average annual minimum winter temperature, and is divided into 10-degree zones (F).
Each zone is further divided in half to have an ‘a’ and a ‘b’, with ‘a’ being 5 degrees colder than ‘b’. To be honest, very few sources of plant information bother with the ‘a’ and ‘b’. They will just report the number.
If you live in the US, you can get your planting zone here: http://planthardiness.ars.usda.gov/PHZMWeb/
Canadian Planting Zones
Canada has taken a slightly different approach. In addition to the minimum winter temperature, they also take into account other variables like: length of frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed. In theory this makes the maps more accurate than the USDA zone maps.
The latest map is from 2000 and it can be found here: http://www.agr.gc.ca/atlas/agpv?webmap-en=78529700717d4cab81c13e9f9404ef10&webmap-fr=c1b454842d3748b0bb0807d7817d34c2 Unfortunately, these maps use stupid colors so it is hard to tell one zone from another.
An easier way to find your Canadian hardiness zone is to look your town on this list. The headings are bit confusing. The first column is the old zone number, the second the new and current zone number.
Be careful with older Canadian information. Prior to the 2000 update, the Canadian system used a different number scheme. In my case I used to be a 6, and now I am a 5. They did this to harmonize the numbers with the USDA maps.
Note added Dec 23, 2015: I compared the US maps to the Canadian zone map of 2,000, and found that Halifax, Quebec City, Edmonton and Guelph had the same zone number on both maps. Winnipeg and Richmond BC were not the same. It is clear that not all parts of Canada are harmonized with the US system – you need to check this for yourself.
Planting Zones for Europe
The most used planting zones for Europe follow along the same lines as the USDA Plant Hardiness Zones. A map for Europe can be found at: http://www.houzz.com/europeZoneFinder
Planting Zones for the UK
The UK uses the same zones as Europe – sort of. Planting zones in the UK range from 7 to 9. If you read any plant information from the UK, they really only have two types of plants; hardy and not hardy. Since almost everything grows in zones 7 and 8, most plants are hardy.
Trying to get hardiness information on a particular plant from UK sources is, for the most part, a waste of time.
The RHS also has their own system of H#. If you want more information about this system have a look here: RHS hardiness system.
Planting Zones Globally
Most other countries have adopted the USDA Plant Hardiness Zones, and some countries have developed their own system. It really makes sense to stick to the US system or at least keep their numbering system, because that is the information you will find in most reference material, including the internet.
Here are maps for some other regions/countries:
Planting zone for Africa: http://www.backyardgardener.com/zone/africa.html
Planting zone for China: http://www.ars.usda.gov/Main/docs.htm?docid=9815&page=2
Planting zone for Japan: A pdf for Japan
Planting zone for New Zealand: http://liddlewonder.nz/zones.php
Planting zone for Ukraine: http://www.ars.usda.gov/Main/docs.htm?docid=9815&page=3
Let me know in the comments below if there are maps for other countries.
The Limits of Planting Zones
Planting zone information can be extremely valuable when selecting plants. Most plants sold in North America have the USDA planting zone right on the label. Many websites that discuss plants also list the planting zones for a particular plant. All good stuff – but there are limitations.
Accuracy of plant zones
Who determines the hardiness zone for a particular plant? Newly developed plants don’t have a hardiness zone since few people have grown it. any reported values are a guess that gets fine tuned over time as more and more people grow the plant.
Sometimes the zones are determined by testing plants in various regions. A few years ago I was asked to trial a new cultivar called Liriope ‘Super Blue’. The growers who were introducing the plant wanted to know how well it would grow in zone 5b. They gave me 12 plants and asked that I grow them in various locations at Aspen Grove Gardens – my private garden. The story and results are fully documented on my GardenMyths blog in a post called Hardiness of Liriope ‘Super Blue’.
If you go to 5 websites and check on the plant hardiness numbers you are likely to get 2 or 3 different answers. Some sites are very conservative. Fine Gardening seems to always understate the hardiness of plants. Other sites overstate the hardiness – are they trying to sell more plants?
The plant in the first picture in this post is Berkheya purpurea. It is rated as a ‘6b’ plant. From experience I know 6b plants don’t survive our 5 b winters. I also know that many plants native to South Africa are hardier than reported. I grew this one from seed, and it has survived 4 winters so far, two of which were extremely cold. Maybe I have been lucky, or maybe this is not a zone 6b plant. It looks more like a 5b plant and other growers are now reporting similar findings.
I usually check several websites for zone information. If they all match, I am fairly confident in the numbers. If they don’t match, I usually go to Dave’s Garden. I find the data on this site to be the most reliable. The easiest way to find the information if you are not a member of Dave’s Garden, is to add the word “Dave’s” to your Google search.
Local Micro Climates
I don’t believe that you have different small micro climates (ie low temperatures in winter) around a normal small sized lot – although many sources claim you do. However, your zone might be different than a garden up the road if one of the two sites is located at a higher elevation or is more exposed to winds. Published zone maps don’t take this into consideration.
I live just outside of a city of over 100,000 people. Inside the city, 4 km away, the low winter temperatures are a couple of degrees warmer in winter than my place. Some of my shrubs die back to the ground every winter, while in town they don’t.
Snow cover also plays a big role. I have trouble growing some plants because we have limited snow cover most winters. North of here, it is certainly colder, but they have reliable snow cover. So their air is colder, but the perennial, hiding under the snow is warmer. Their effective zone is actually warmer due to snow cover.
Dry Can Be More Important Than Cold
Some plants can take very low temperatures; most alpines for example. But some can’t take wet conditions in winter. If you have warm spells in winter, and winter rains, it can spell death to a plant, even though it should be hardy in your area.
Experience Fine Tunes Your Planting Zone
Planting zones are not perfect, but they are a great help in selecting plants – make use of them. Use them as a guide, and over time, experience will tell you if you are in a slightly warmer or colder zone. You will probably have to kill some plants to really find out.
1) Photo Source: all pictures by Robert Pavlis