What is my plant hardiness zone?

A wonderful web resource for finding out what plant cold hardiness zone in the US you are gardening in:

Plant Maps.com
Go the the page and type in your zip code, or select your state in the index list.

For instance, when I looked at the map of Pennsylvania, I was able to see that it is not all my fault that some of my plants did not survive the winter! Negative 20 degrees F (-28 C) is pretty cold!! Since I am in zone 5, I can see that I should expect to use plants that will survive temperatures down to negative 20 degrees F.

The caveat to counting on plant hardiness zones is that one must remember that they are averages. There will be times and places which will experience colder or warmer temperatures from the rest of the area.

For example, if you garden way out in the country it will get colder than if you garden in town.
In town, with lots of buildings and paving around your lot, you might be a zone warmer.
Another twist might be that a sheltered area in a warm spot of your yard (called a microclimate) might only be warmer for summertime; it may also be a frost pocket because the winter wind funnels into it (advice stolen from one of my favorites, C. Lloyd).  

Each garden - and even each spot in your garden - has its own characteristics.

But still, this is very useful information for getting you into the ballpark, so to speak.

Gilmore Gardens in October in Zone 5