A cold hardiness zone is the maximum temperature range that a plant can survive in during the winter months. North American Zones range from Zone 1 (being the coldest), below -50F in Fairbanks, Alaska and some Northwest Territories of Canada to Zone 11 (the warmest), above 40F in Honolulu, Hawaii or Mazatlan, Mexico. To find your zone on the map, you can look here: USDA Plant Hardiness Zone Map
I live in Central Missouri, the bulk of which falls under cold hardiness zone 5b, which is -10F to -15F. Some nursery tags will picture a little temperate map showing your zone, others will simply list the cold hardiness temperature under a list of growing conditions. Perennial plants within this range should be able to return each year after a period of dormancy. Plants outside of this range will have to be planted as Annuals, meaning you’ll need to replace them each year.
Although Central Missouri is in Zone 5b, my specific area is in a small pocket of 6a to 6b. On gardening shows, they’re referred to as microclimates. I’m not sure if maybe this is an effect of the large lake here, but it means that I can often get away with growing plants that surrounding areas cannot. We have varying micro-climates in our yard as well.
For years I had grown a row of Elephant Ears along the front of our house (Zone 10 plant minimum), never bothering to dig them up in the fall and store them inside to over-winter as was recommended. I covered them with leftover plastic bags that my fall mulch came in and then left the fallen leaves to cover the plastic until late spring early summer. For nearly 7 years, they came back every June, in spite of everything I’d read to the contrary.
Excuse me… my daughter is in my houseplants eating dirt…
Ok, where was I? Oh yes, Elephant Ears. I experimented with some more out in the yard itself; these did not return the following year as the ones next to the house did. My guess is that the house itself radiates enough heat right up next to the foundation that it was enough warmth to keep the other bulbs alive, though dormant through the winter months. So even though most of my yard may be Zone 5-6, this one particular 2ft x 6ft area is more like Zone 8-10.
Of course you have to take care of how far you push your luck. I bought a Crepe Myrtle last year that said it would be fine in Zone 6, and then we had several particularly cold days in January that caused it to die back to the ground. I lost the entire top of the plant that wasn’t covered by dirt and mulch. If I don’t want a repeat performance during another severe winter, I’ll now have to move it to a large pot and over-winter it in the house along with my house plants. Since its roots will always be bound by the confines of a pot, it’s unlikely it will become the large, lush tree like those I admired growing in yards during our visit to the Carolinas and Georgia several years ago.
The best way I’ve found to figure out what works and what doesn’t in regards to cold hardiness is to just experiment. I recommend starting with something small, like a couple inexpensive bulbs, or maybe some perennials that you’ve found on sale and wouldn’t be too upset if you lose them. Once the plant has performed well consistently for several years, it’s probably a safe bet to assume the temperature will remain fairly constant there from year to year.