As I was going through my gardening books, I saw the Plant Hardiness Maps, and I recall that the Zone system for plants can be quite confusing for folks.
The Plant Hardiness Zones are numbered from 1 to 9, where 1 is the coldest climate, and 9 being the warmest. It is not only the winter temperature which is considered in the assignment of Zone numbers. Every factor that influences how well plants will grow is taken into consideration: winter temperatures, length of frost-free growing season, quantity of precipitation, degree of humidity, and winds. This results in general areas. A plant that is hardy for Zone 1 can grow in higher zones. But a plant that is hardy to Zone 6 will likely not survive in Zone 4 or lower. Some plants need the winter dormancy for survival.
According to the maps, my area is Zone 2 (a or b), although we are safe to consider ourselves Zone 3. The specific area where you live, e.g. high up on a wind-swept unprotected hill, or conversely, a south-facing, well-sheltered flowerbed close to a heated building, may provide you with a unique microclimate, which would influence the plant choices for that spot.
My sister in Sherwood Park had a lovely, sheltered, south-facing yard—she grew all kinds of Zone 5 and Zone 6 plants successfully. She was also fanatical about watering and mulching. One of our customers from Andrew (which is a zone 2 frost pocket) has also had good luck pushing the Zones in his sheltered yard.
You cannot assume that, if a greenhouse or nursery in this Zone sells certain plants, they are necessarily hardy for this area. Always check the tags for the zone—they are generally fairly accurate. Mail order seeds and plant catalogues generally do not give Zones. Some plants that are not winter-hardy in our Zone, are lovely plants if treated as annuals and enjoyed for only one season. The best example I can think of here is that of Tea Roses. With proper protection it is possible to winter tea roses, but probably only with 50% chance of success. So, buy them if you love them, plant them in a large pot and enjoy them on your deck or patio, and just like your hanging baskets, let them go in the fall. We sell only hardy roses recommended for Zone 2 or 3.
Some shrubs which are “funny” with regards to Zone are Forsythias, Azaleas and Rhododendrons, to name just three. These are listed as Zone 4 or 5. The reality is, the plants will likely survive our winters with sufficient snow cover, but the flower buds are not hardy to Zone 3, so if they bloom at all, the blossoms will be only on those branches that were covered by snow. Some shrubs like the Grace Smokebush will not die, but will die back to the ground every year, so they will never grow as large as we might see them in BC.
If you would like the challenge of pushing the Zone limits, don’t start with very expensive plants. Buy smaller sized or less expensive varieties, and if you find Zone 4’s or 5’s work for you, then try more expensive plants. Whether in-Zone or out-of-Zone, always water plants well in fall, and make sure there is enough snow cover or mulch. What often kills our perennials isn’t the coldest days (or weeks) of winter, it is the spells of warm weather which we often get in February and March before Old Man Winter returns with a vengeance—these warm spells may fool the plants into breaking dormancy too soon, only to succumb to the cold later on.
Use the Zones as a guide only: gardening is about experimenting and caring and bragging about your successes. In our Greenhouse and Nursery, we stay on the conservative side, selling only Zone 2/3 and a few Zone 4 plants that have shown promising results in this area.