On the back of seed packets, there is usually a range of numbers. To the beginning gardener, they seem a lot like gibberish, but they are actually very important. These numbers indicate the Hardiness Zone a plant will do the best in. The zones range from 1 – 13b, and they are based on a minimum winter temperature, divided into 10° Fahrenheit zones. For those of you wondering, yes – the addition of zones 12 and 13 is relatively new (2012). This will prevent you from trying to grow oranges outside in Idaho, for instance. Citrus usually does best in hardiness zones 8-10, while Idaho ranges from 3 – 7b.
There is, of course, some deviation that is okay as long as you are aware what hardiness zone your plant will thrive in. If you know this, you can re-create the conditions it needs to be a healthy, happy plant. But, here are some general recommendations for what to grow in your hardiness zone. I found a wonderful plant finder on this website: http://www.hgtvgardens.com/plant-finder/. All you have to do is type in your hardiness zone (or a range of them) and it comes up with an entire list of plants to grow. If you have certain plants in mind it’s easier, simply because you can see if they will work for your zone. If you are just looking for ideas, I love this tool. Also, you can search if a certain type of plant (for example, oranges) will do well in your hardiness zone.
Zones 1-4b (coldest part of winter is between -60° and -20° Fahrenheit – brrrrr). Best plants to grow include Alaska Cedar, Black-eyed Susan, Foxglove, Daisies, Hollyhock, Clematis, and Iris varieties.
Zones 5-9a (coldest part of winter is between -20° and +25° Fahrenheit). Best plants include Aloe Vera (an awesome plant to use for sunburns, btw), Flowering Kale, Peace Lily, Sweet Potato Vine, and Absinthe.
Zones 9b-11b (coldest part of winter is between 30° and 50° Fahrenheit). Best plants include Acacia, Agave, Strawberry, Tangerines, Oranges, Grapefruits.
Zones 12-13b are pretty hostile for growing. Coldest part of winter is 50°-70° Fahrenheit. Best plants include Azaleas, Heliconia caribaea (super-cool), Sugar Cane, and Tobacco.
These are only guidelines, of course. Many plants do well across multiple hardiness zones, and you can always use a season extender (aka greenhouse) in your colder climates, and shade cloth in the warmer. This is just a handy, hopefully informative general guideline to hardiness zones.