How well will a plant grow in an area similar to where you live? This information is generally available in the form of plant hardiness zones based on long-term climatic data, in contrast to day-to-day weather conditions.
Knowing your hardiness zone, however, is only useful as a very broad guide – you will need to interpret it, taking into account factors other than low temperature, such as demands for watering, high temperature, frost, and high humidity, as well as local knowledge.
Climate and Hardiness Zones
The USDA was the first to develop plant hardiness zones based on the average lowest winter temperature over a specified time. There is still a fifty percent chance, however, that winter temperatures could drop lower than the designated zone for a particular area, and likewise a fifty percent chance they could rise higher.
Furthermore, hardiness zones do not have absolute boundaries like rivers and mountain peaks. Instead, their outer regions are transitional and merge into surrounding zones. Larger-scaled maps (e.g. zones of Michigan) show more detail than smaller-scaled maps (e.g., world zones).
Geography of Hardiness Zones
In general, average temperatures drop toward the poles and rise toward the equator. If this were absolutely true, however, the different hardiness zones would parallel the latitude lines – and they don’t.
By way of example, consider European migration to North America. In the late 1800s, my tenant-farming grandparents migrated from Dumfries in Scotland (55°N) to North Dumfries Township in Ontario (43°N). Based on latitude, they might have expected considerably warmer temperatures in Canada. In reality, the more northern Dumfries in Scotland (Zone 8) is warmer than North Dumfries Township (Zone 5) in Ontario.
Higher latitudes but higher temperatures… why? Two big reasons are the influence of land masses (continental climates) and in contrast, water and ocean currents (maritime climates).
Ocean currents, for instance, are generally cold flowing away from the poles and warm flowing from the equator.
A glance at a map of the world’s ocean currents shows that the heat of the warm Gulf Stream flows toward Scotland, aided by the prevailing westerly winds. North Dumfries, in contrast, is more continental and exposed to polar air masses, although the Great Lakes also influence the local climate to some degree.
A good example of the effect of a continental location is the contrast between relatively coastal Washington D.C. and the geographic center of the continental United States, Kansas City. At relatively the same latitude Washington (39°N), influenced by the coastal waters but with the prevailing westerlies blowing offshore is in Hardiness Zone 7 while continental Kansas City is in Hardiness Zone 6.
National Plant Hardiness Zones Systems
This discussion has visited the effect of several climatic influences on plant hardiness zones on a world scale. But world systems, while related to climate factors, do vary between nations and world regions.
When browsing information from other countries, it’s helpful to reference a climatic comparison of national hardiness zones systems to determine whether a plant native to another area might survive in yours.