Hardy Zone Planting
The first step in accurately predicting whether a plant will be hardy is to find your USDA hardiness zone. If you don’t know your zone you can use the Hardiness Zone Finder to learn what zone you live in. Once you know which zone you live in, it is relatively easy to figure out whether a plant will be perennial or annual. Simply compare your zone to the zone or zones listed on the Hardy Zone Finder or this website.
If your zone is equal to or higher than the zone listed for the plant it will be hardy for you, it is perennial in your area. If your zone is lower than the zone listed on the tag then the plant will not be hardy for you, it is an annual in your area. An example, if the plant tag says a plant is hardy in zones 6 to 10 the plant will be an annual for me in zone 5.
The same zones that apply to annuals or perennials also apply to shrubs and trees.
Once you know what zone you live in and understand a bit about how hardiness zones work, deciding whether a plant is likely to act as a perennial and survive your winter becomes much easier.
There are some other things to consider as far as hardiness goes.
The hardiness zone map assumes that the plant will be planted in the ground not in a raised pot or window box. The ground (soil) remains slightly warmer and won’t freeze as solid as a raised pot and will frequently thaw out even in the winter. If you are planting in a pot you will need to choose plants that are two zones hardier than the one you live in.
You could also dig a hole in an out of the way spot, place the pot into the hole and fill with soil until it is level with the boundaries of the pot. This will keep your pot warm as the ground and allow the plant within the pot to survive the winter.
It is also sometimes possible to take a plant that is one zone less hardy than your zone and bring it through the winter. Areas protected from the wind are often warmer like next to your house and near brick and stone wall can retain warmth.
The moisture level, how well your soil drains, how much you mulch, when you mulch, and how much snow cover you have can all affect whether a plant is perennial or not. Many plants that should be hardy will suffer in soils that don’t drain well. Thick layers of mulch, properly applied, can increase hardiness as can a thick, insulating layer of snow. The changing weather conditions each year can allow some plant to ‘perennialize’ for a few years and yet we all know that sooner or later, the real climatic zone will catch up with the hardy annual.
What does this all mean to you? Hardiness zones are essential information for choosing which plants are likely to be perennial, however, by experimenting with placement and plants that are almost perennial, it is possible to expand the selection of plants that are perennial in your garden.