The Importance of the Plant Hardiness Zone
A plant hardiness zone is a geographically defined area in which appropriate plants are capable of growing. It refers to the climatic conditions of the zone, most notably the minimum temperature. As minimum winter lows are often the limiting factor when growing plants in marginal areas, the climate zones are useful when planning a garden or farm. They don’t tell the full story though and while the plant hardiness zones are a useful tool, it is important to know their limitations.
History of the Plant Hardiness Zones
The original USDA plant hardiness zone map and zone parameters were made in 1960. They were designed specifically for the USA and its territories where climate ranges from crazy cold (Alaska) to very tropical (Puerto Rico). The scale accommodates this range, starting at 1a with a minimum temperature of down to 51.1°C (-60°F), and moving all the way through to 13b with a minimum low of 18.3°C (65°F).
Since the USDA’s first map, other countries have piggybacked on the idea. Most have simply drawn maps for their own countries based on the USDA climate zones. Others, including Australia, have made similar systems but with their own scales better suited to the local climate.
Australian Plant Hardiness Zones
The USDA hardiness zones are in use to a degree (haha) in Australia. The problem with the USDA system is that Australia’s coldest climate starts in Tasmania at USDA zone 7. The north of the country is off the scale.
In 1991 Iain Dawsome of the Australian National Botanic Gardens devised an Australian system more in keeping with Australian conditions. While Iain’s map has a more appropriate scale for Australian climate zones, the system doesn’t fix a lack of historical data and widely spaced weather stations. For these reasons the Australian map is nowhere near as detailed as the USDA hardiness zone map is. Perhaps this will change as we move into a period where anybody can setup a weather station with automatic recording and uploading of data. I certainly hope so.
What Don’t Plant Hardiness Zones Tell You?
It is all good and well to know the hardiness zone where you live, but that isn’t the full story. For starters they tell you nothing of temperatures above the minimum in a climate. For example a very hot climate with a few cold days every year is very different to a more stable climate of the same minimum temperature (and thus zone).
Zones are not a reliable indicator of frost or snow either. At my property (zone 10b) I get no frost and am able to grow tropical trees that get knocked back by frost at friends houses in the same zone. My climate is also incredible stable and so I do not get the infrequent cold snaps that can wipe out entire groves in the higher zoned Southern Florida.
There are other things to consider than just the minimum temperature when deciding what to grow at your place. Chill hours, humidity, stability and heat all come into play. The moral of the story is that the hardiness zone map is a good place to start when deciding what to grow, but nothing more.