Community gardening in the heart of downtown Minneapolis
Know Your Zone
Do you know how to tell if a certain plant will grow in your climate or not? While geography is only a piece of the equation, it is important to understand, and the United States Department of Agriculture is there to help!
The USDA produces the “Hardiness Zone” map which breaks the United States down into Zones that can be used to understand if plants will be able to survive or not in a given region.
The first Hardiness Zone map wasn’t affiliated with the government at all. It was produced in 1927 by Dr. Alfred Rehder who used information from a survey about plant survivability to draw boundaries around “zones” that could support similar plants. In 1938, this concept was taken further by Dr. Donald Wyman who created a new map, but based the creation of the zones on 40 years of weather data (1895 – 1935) that was published by the US Weather Bureau.
It wasn’t until 1960 that the USDA got involved, and when they did, it caused a bit of a problem! They attempted to answer the same question, but with completely different data, resulting in two – often conflicting – maps. This conflict wasn’t resolved for 30 years, and during that times BOTH maps were maintained separately! This was finally resolved in 1990 when the USDA partnered with the National Arboretum to begin producing a definitive, standardized map, which is the version that is still maintained and in use today.
Today the Zones are based on a average of the minimum winter temperature in a region over a 10 year period. Which is why Minnesota (Zone 4) is listed as “-25 to -20.” There are obvious problems with this: namely that it tells how cold it gets, but it doesn’t tell how long it stays that cold. This also doesn’t take into account regional differences like humidity.
Even though it doesn’t take all critical factors into account, it is important to understand where your plants have a chance of surviving. In fact most plant and seed companies now print the Hardiness Zone information on their products. So when if you are buying plants or seeds, make sure they work in Zone 4!
To learn more, here are a few resources:
* This last one is a LOT to get through, but for the academically inclined, it is a fascinating read!