Have you ever looked through a seed catalog and somewhere in the description is says something like, “Appropriate for Zones 4-8?” And you start to wonder if it is some sort of secret gardener code or something? Well, it is a code of sorts, and after reading this you will be in on the secret!
The plant hardiness zone helps you determine if a plant can survive outdoors over the winter. The zones are based on the average low temperatures for an area. The lower the zone number, the colder the average low temperatures are.
How does that help you? If you know your plant hardiness zone you will be able to choose plants for your garden/yard that have a reasonable chance of survival. For example, if you live in Zone 3 and plant a palm tree that is appropriate for Zone 8 and above there is a pretty darn good chance come spring you’ll have one dead palm tree to cut down!
The current plant hardiness zone was released by the USDA in January 1990 and is pictured at the top of this article. To find your plant hardiness zone click on the Park Seed Zone Map square in the box in the upper right of this page.
You can fine additional information at the US National Arboretum USDA Plant Hardiness Zone page.
A very common misconception is that the plant hardiness zone tells you when you can set your plants into the garden. Not the case. Your frost date is what determines planting times, and we have covered that in a previous article Get to Know Your Frost Date.
One other thing to keep in mind is that these zones are not foolproof. They are based on average temperatures, so it is quite possible that there will be days too cold for a particular plant during any given winter. Use these zone numbers as a guide, but always remember they do NOT guarantee plant survival.
OK, so now you know that the plant hardiness zone tells us what can survive over winter in our area. You are in on the secret!
The rest of this article is going to talk about some interesting things that are happening to the hardiness zone map. A lot of what follows is drawn from unofficial sources and isn’t set in stone.
The old USDA Plant Hardiness Zone Map is almost 20 years old now. The USDA has stated that they plan to release a new zone map every 15 years. We are 4 years overdue! What gives?
Have you heard all the talk about global warming and climate change? Well, several years ago the USDA commissioned a new study but were not pleased with the results because the data showed an across-the-board temperature increase. Apparently they commissioned a new study and are supposed to release a new map sometime in 2009.
Interestingly, the Arbor Day Foundation took the results from the study the USDA wasn’t happy with and produced their own Plant Hardiness Zone Map:
2006 Arbor Day Foundation Plant Hardiness Zone Map
A visual summary of the changes can be seen here:
Differences Between 1990 and 2006 Plant Hardiness Zone Maps
{ 2 comments… read them below or add one }
That’s quite a visual difference over 20 yrs. You can clearly see the warm zones at the bottom expanding and all the zones shifting up. I wonder what exactly the new ‘official’ map will look like.
Great post!
@Coldprairie — It seems as if a climate change is happening, whether you believe that humans are responsible for accelerating it or not.
Another indication is our local frost date comes about 2 weeks earlier than it did 20 years ago. It used to be you planted your garden on Memorial Day weekend, which is the last weekend in May. Now the frost date is May 13.