Most recent USDA Plant Hardiness Zone Map can be accessed here.
Although the first map occurred in Alfred Rehder’s ‘Manual of Cultivated Trees and Shrubs’ in 1927, another version was created in 1938 by Donald Wyman known as the Arnold Arboretum map. Like our current map, both of these were developed by using cold weather data collected from various weather reporting stations. For a while both of these maps were in circulation at the same time, with some people using one map, and some the other. This is where problems started to arise. If you were in Zone 4 on one map, you could be in 5 on the other, and it was getting very confusing.
The U.S. Department of Agriculture (USDA) got into the picture in 1960 by developing its own map. More data was used to create this map, almost twice as much as the existing maps, and was considered to be the most accurate. It used 10°F temperature increments to delineate zones, which ranged from 1-10, with 1 being the coldest and 10 the warmest. Plants were now designated with numbers to correlate with a numbered zone on the map. You simply found where you lived, noted the correlating number within that particular region, and then looked for plants with that number or lower on the tag.
In 1990 the map was once again revised. This is when the map was segmented even further by dividing individual zones into A & B sections. An extra zone was also added, making the total number of zones now 11. This new map basically divided each area into 5°F segments. Originally Zone 4 had a minimum temperature average of -20 to -30°F, now 4a was -25 to -30°F and 4b was -20 to -25°F. Having more accurate data, the map also now contained areas of micro-climates, which showed pockets of colder or warmer areas within each zone. Many of us thought, at this point, that this would be the final map.
Surprise! The USDA, along with The American Horticultural Society (AHS), decided to change it again in 2003. The changes in this new version include: the number of zones increased to 15 instead of 11 to better include growing sub-tropical and tropical plants in those regions, the 5 degree a/b zones were dropped to make the map more readable and easier to understand, and is based on 16 years of data compared to the 13 years from the previous map. Now, are we finally done?
Now, let’s confuse you even further. There is also a Heat Zone Map, created by the AHS, which gives your plants a rating based on the amount of hot weather it can tolerate, consisting of the amount of days a plant can handle temperatures above 86°F. This map was created because plants can suffer just as bad from heat as they can from cold. Their thought was that people should use both maps, cold and hot, to select the proper plants for their region.
So, now that I’ve probably confused you even further, where am I going with all of this? Simple; don’t take these maps to seriously. They are designed for those who are new to an area or region, and even gardening itself, and need some general information and guidance on where to begin in plant selection.
When I’m choosing plants I like to drop the whole Zone lingo since these seem to change so frequently. I like to simply look at temperature. When looking at plant tags, just about every one I’ve seen these days uses both zone and temperature listings. If the tag says a plant is hardy to -20°F, I can relate to that and also know that it has a great chance for survival in my landscape. By the way, this is considered Zone 5 and some Zone 4 purists would probably avoid these plants because the tag doesn’t say Zone 4. Do you see why I don’t like the whole Zone lingo? Some people are missing out on some really neat plants because of it!
Also, don’t forget about micro-climates around your landscape. I’m sure you’re aware of the fact that the North side of your house is much cooler then the south side. Also, lower areas tend to be cooler then higher ones and areas away from your house are much colder then closer to it. It takes time to determine where these areas are located around your landscape, so be patient. Every landscape has warmer and colder, wetter and drier, and sunny and shady areas that help create micro-climates, so your job is to simply be aware that these exist, and take these into consideration when making plant choices.
One last thing; talk to your neighbors, especially the ones that have lived in the area for awhile. They can be great resources for finding out which plants survive best in your area. The experts at your local reputable garden center are also great resources. Don’t be afraid to ask, it’ll save you some disappointments in the long run. We are experts because we have already made the mistakes.
Now, keeping all of this in mind, I would like to address the issue of plant losses. The thing I hear the most when a plant doesn’t make it through the winter is: it must have been a hard, cold winter, or the plant wasn’t hardy enough. Around here, most of the time, that’s wrong! Because of our lovely clay soil, most plants are lost due to excess moisture in the soil, causing plants to rot out and not freeze out. This is why it is so important to improve our soil structure around here by adding compost or other organic matter, which in turn, aids drainage. By improving drainage you’ll see a huge reduction in plant losses right away. So, don’t jump to conclusions, it may not have been the cold.
All in all, these maps were designed to be general guidelines and not absolute rule, so use them that way. If you’re trying new plants, or are new to an area, they can be very helpful, but if you’ve lived in an area for a while, your personal experience and knowledge of your landscape will always trump the map. Also, there’s nothing wrong with trial and error when it comes to plant selection, to me, that’s half the fun. So, don’t get caught up in the whole Zone lingo and expand your horizons, there’s a huge plant world waiting for you out there. Happy Gardening!