Can we convert USDA zones to Australian climatic zones of tropical, subtropical, temperate, arid and cool/alpine regions?
That’s a common question many gardeners ask when referencing plant information from the other side on the world.
Even though the two are quite different measurements based on rather different criteria, we can roughly approximate them for practical purposes.
The USDA Hardiness Zones
The concept of USDA Hardiness Zones was developed by the US Department of Agriculture based on the average annual extreme minimum temperatures recorded over a period of the past 30 years.
It’s important to note that this measurement is not referring to the average minimum temperature across the whole year, which is a different concept.
USDA Hardiness Zones are divided into 13 separate planting zones, with each growing zone being 10 °F colder or warmer than the one before or after it. Additionally, each of these growing zones is further divided into an ‘a’ and ‘b’ zone with a 5 °F temperature difference between them, as shown below:
USDA Zone Minimum Temperature Ranges
Zone: Average Annual Extreme Minimum Temperature Range
1a: -60 to -55 °F (-51.1 to -48.3 °C)
1b: -55 to -50 °F (-48.3 to -45.6 °C)
2a: -50 to -45 °F (-45.6 to -42.8 °C)
2b: -45 to -40 °F (-42.8 to -40 °C)
3a: -40 to -35 °F (-40 to -37.2 °C)
3b: -35 to -30 °F (-37.2 to -34.4 °C)
4a: -30 to -25 °F (-34.4 to -31.7 °C)
4b: -25 to -20 °F (-31.7 to -28.9 °C)
5a: -20 to -15 °F (-28.9 to -26.1 °C)
5b: -15 to -10 °F (-26.1 to -23.3 °C)
6a: -10 to -5 °F (-23.3 to -20.6 °C)
6b: -5 to 0 °F (-20.6 to -17.8 °C)
7a: 0 to 5 °F (-17.8 to -15 °C)
7b: 5 to 10 °F (-15 to -12.2 °C)
8a: 10 to 15 °F (-12.2 to -9.4 °C)
8b: 15 to 20 °F (-9.4 to -6.7 °C)
9a: 20 to 25 °F (-6.7 to -3.9 °C)
9b: 25 to 30 °F (-3.9 to -1.1 °C)
10a: 30 to 35 °F (-1.1 to 1.7 °C)
10b: 35 to 40 °F (1.7 to 4.4 °C)
11a: 40 to 45 °F (4.4 to 7.2 °C)
11b: 45 to 50 °F (7.2 to 10 °C)
12a: 50 to 55 °F (10 to 12.8 °C)
12b: 55 to 60 °F (12.8 to 15.6 °C)
13a: 60 to 65 °F (15.6 to 18.3 °C)
13b: 65 to 70 °F (18.3 to 21.1 °C)
The way this USDA scale is used, if a plant is described as being “hardy to zone 10” this means that the plant can withstand a minimum temperature of 30 °F (−1.1 °C) the lowest temperature of Zone 10a through to 40 °F (4.4 °C) which is the upper limit of Zone 10b.
The concept of USDA Hardiness Zones has several limitations we need to be aware of. Since this model is based on the coldest temperatures experienced, it doesn’t incorporate heat levels during summer when determining the zone rating. This means that two regions with the same minimum winter temperatures but with different summer temperature would be assigned to the same hardiness zone.
The Australian climate zones take into account various factors such as temperature, rainfall, humidity, elevation, and specific regional conditions, so they don’t have a direct one-to-one correspondence with the USDA Hardiness Zones.
We can however roughly align the USDA Hardiness Zones with the Australian climatic zones as detailed below:
- Tropical Zone:
- USDA Zones: Generally, USDA Zones 10-11
- Explanation: The tropical zone corresponds to areas with consistently warm temperatures year-round. In the USDA system, Zones 10 and 11 cover regions with relatively mild winters and high temperatures, which can be similar to tropical conditions.
- Subtropical Zone:
- USDA Zones: USDA Zones 9-10
- Rationale: The subtropical zone has warm to hot summers and mild winters. USDA Zones 9 and 10 include areas with relatively mild winters and are thus similar to subtropical climates.
- Temperate Zone:
- USDA Zones: USDA Zones 4-8
- Rationale: The temperate zone encompasses a range of climates with distinct seasons. USDA Zones 4-8 cover a variety of climates with cold to mild winters and warm to hot summers, which can align with temperate conditions.
- Arid Zone:
- USDA Zones: USDA Zones 8-11 (for regions with desert-like conditions)
- Rationale: The arid zone corresponds to dry and desert-like climates. USDA Zones 8-11 can include areas with low rainfall and high temperatures, which are characteristic of arid regions.
- Cool/Alpine Zone:
- USDA Zones: USDA Zones 1-6 (higher elevations)
- Rationale: The cool/alpine zone corresponds to areas with colder temperatures, often found at higher elevations. USDA Zones 1-6 include regions with cold winters and shorter growing seasons, which align with cool and alpine conditions.
We need to keep in mind that while these general correlations provide a rough approximation of USDA zones with Australian climate zones, they aren’t precise matches, so it’s best to check the growing requirements of any plants and their suitability to your location before planting.