A garden zone is used to define the hardiness or category of plants, that are able to survive and thrive in the minimum and maximum temperatures of a zoned area. Zones are labeled in numbers and letters, with each ascending number being 10 degrees cooler and each letter labeled sub category being 5 degrees cooler in that zone. For example, zone 7a can have low temperatures 5 degrees cooler than zone 7b .
With every garden zone, there are exceptions to the rule. For instance, a geographical area that gets plenty of snow cover over the winter, may be zoned to be colder than the actual soil temperatures underground. Snow acts like an insulator, raising the temperature up by 10 degrees underground, in comparison to the temperatures above ground. Some other elemental influences to consider are, where do you reside in the zone, are you in a low elevation spot or high elevation spot. Higher elevation tends to be cooler due to winds and cooler air temperatures at higher altitudes.
Also to be considered are rainfall and humidity, frosts that may come early or late, or if there is an unforeseen cold snap, blooming cycles may be interrupted leaving some plants out in the cold. Wherever you live in the world, it's good gardening practice to check what grows well locally. This information can be gathered by you from your surroundings and observations or found in local libraries, gardening books and online.
New York City inclusive of all it's 5 boroughs and it's surrounding areas such as Long Island, Jersey City and Hobokken are all zoned 7b meaning that plants can withstand temperatures between 5 - 10 degrees farenheit, that's -15 to -12 celsius. Remember plants will perform differently if they are not getting required light, in a drafty or humid spot, soil conditions and their proximity to strong winds. Landscape around these factors to get the most out of your garden and of the weather conditions in your area. Winter gardens are a beautiful thing and so under utilized.
If you live in the USA find your garden zone here
If you live in Australia find your garden zone here