Planting zones in the United States, which are also known as Hardiness Zones, can be thought of as the rate of survival to the plant life in that particular region during cold periods, such as winter.
In effect it is the survival rate ability of a plant during the winter months that can fruitfully live on into the next growing season.
The zones were first developed by the United States Department of Agriculture, or the USDA.
After the USDA developed these zones, they were subsequently adopted elsewhere.
They are categorized according to the mean of the lowest temperature recorded each winter, termed the "average annual minimum temperature".
The USDA Hardiness Zone Map divides North America into 11 separate zones; each zone is 10°F warmer (or colder) in an average winter than the adjacent zone.
Probably the most important consideration in determining what garden and landscape plants you can grow in your garden is whether or not they will be able to survive the climate in your growing area.
The USDA Plant Hardiness Zone Map factors in average winter minimum temperatures.
This collection of information starts with Zone 1, where minimum temperatures can go down to minus 50 degrees Fahrenheit, and ranges to Zone 11, for the prospering gardeners in Hawaii and extreme Southern Florida where lows don't drop below 40 degrees Fahrenheit.
There are some criticisms to effectiveness of Hardiness Zones though.
One of the most significant drawbacks is that it doesn't incorporate summer heat levels into the zone determination.
Another deficiency with the zones is snow cover effect.
Snow is actually considered to have an effect on the ability of plants to survive because the snow acts as an insulator against extremely cold weather that can attack the seeds and roots of the hibernating plant during the winter months.
Some other factors that may affect your plants growing patterns are humidity, an odd timed cold snap, the amount of frost on the ground, and soil moisture.
Another thing to think about is, a plant may naturally grow in an unnaturally cold or warm area than the plant is used to.
It may not bloom because the exterior conditions do not favor the plants optimal growing conditions.
Another thing to remember is, many plants will survive in a locality but won't flower if the daylength is inappropriate or if they require vernalisation (a particular duration of low temperature).
Luckily an alternative system for describing plant hardiness is also available, indicator plants.
Check with the USDA who also publishes a list of these to go with their map.