As a gardener it is a good chance that you know what hardiness zone you live in. You know that you need to choose perennials, trees & shrubs that are hardy to your zone or a zone colder. You also know that plants that are in a warmer zone will not overwinter where you are.
So where did this map come from and why do we use it? Before the climatic zone map was introduced gardeners and farmers relied upon information passed down from family and friends through trial and error with what work and what didn’t. It wasn’t until the late 1920’s that a group of people tried to quantify growing conditions across the U.S. Alfred Rehder and his colleagues were the first to introduce a hardiness zone map in his publication Manual of Cultivated Trees and Shrubs. This document was groundbreaking because it was the first attempt to share with people the limitations that cold has on plant survival and the boundaries where these plants will grow.
In the mid 1930’s Donald Wyman took the map a step further and incorporated the weather data that was collected from the 1895 through 1935 and based the zones off the average annual minimum temperature. Wyman’s map wasn’t perfect and there was some inconsistency with the hardiness zones. Some zones had as much as 15 degrees difference in stead of a consistent five or 10 degrees that we know today.
It wasn’t until the 1960’s when the USDA standardized the hardiness map and has become the go-to resource for knowing what plants will and will not survive in a given zone. The map has been updated several times to reflect the shifting boundaries from better weather observations and climate change.
We know that the map is a tool and every location has its own limitations due to various factors. Alfred Rehder first acknowledge these limitations in his 1927 Manual:
There are, however, many other factors besides temperature in winter which will influence the hardiness and growth of certain plants, as soil, its physical as well as chemical composition, exposure, rainfall, humidity of the air, shelter from cold winds…
We also know that plants more often die during winter due to the warm spells than the actual cold. We also know that snow cover and soil moisture also play major roles in the plants ability to survive.
We can assume as more weather stations are added and data collection improves that the map will again change to better establish boundaries. However, we need look at the map as a tool and not as the final decision maker. We also need to know that other factors besides the weather will have profound impact on plants.
To find your USDA Hardiness Zone check out this link: https://planthardiness.ars.usda.gov/PHZMWeb/Default.aspx
To read more about the history of the hardiness zone maps click here. 
Image courtesy of Arnold Arboretum Archives. First published in 1927 in the Manual of Cultivated Trees and Shrubs by Alfred Rehder
