A garden zone defines the hardiness or category of plants that are able to survive and thrive in the minimum and maximum temperatures of that zone. 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 colder than the soil temperatures underground. Snow acts like an insulator raising the temperature by up to 10 degrees underground compared to the air above ground. Some other 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.
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, poor soil conditions and their proximity to strong winds. Plant around these factors to get the most out of your garden and to enjoy the weather conditions indicative of your area.
If you live in the USA find your garden zone here
If you live in Australia find your garden zone here