They say that there’s an exception to every rule. That includes plant hardiness zones. All plants have their hardiness zones listed on the tags in the pot or hanging from the plant. This is supposed to tell you whether the plants will grow in our climate.
In 2012, the USDA (United States Department of Agriculture) and the American Horticultural Society published a new hardiness zone map expanding the country to 26 zones from the previous 14. The zones are based on the lowest average annual temperature. The lowest numbered zones are the coldest. You can check out the map at http://planthardiness.ars.usda.gov/PHZMWeb.
The further south you get, the higher the zone number and the warmer the temperature. Most of our area is now in zone 6a (-10º to -5ºF). The area along the shore of Lake Ontario, however, is zone 6b (-5º to 0ºF).
With few exceptions, I recommend only zone 5 plants for our area. In most cases, these plants will flourish quite well here. Planting zone 6 or higher plants here can get a little “iffy.” Zone 6 plants may be too tender for our winds. You may need to cover them or spray them with an antidesiccant like Wilt-Pruf for them to survive a typical winter.
Hardy plants that may not be hardy here are those labeled for our zone but grown in a warmer climate. These plants may have been grown in the south, or even in a greenhouse, and never experienced winter. Once they get a taste of our cold weather, they give up the ghost.
I like to buy plants at a local garden center; your chances of getting truly hardy plants is excellent. Garden store owners know their sources, and can buy with the confidence that the plants can acclimate to our weather conditions. Those bought at big box stores may have come from anywhere, so they may have never seen snow, even though the tag says they are hardy here.
In most cases, buying zone-sensitive annuals is not as big a risk as buying perennials, shrubs or trees. You have less to lose if you buy inexpensive plants that are not acclimated. Besides, a six pack of annuals has probably spent its whole life in a greenhouse. Trees, shrubs, and even perennials, are major investments, increasing the risk. So, be sure to buy those plants at a reputable garden center.