Zoning in on Hardiness

One of the most commonly asked questions by gardeners has to be “Is this plant an annual or a perennial?”  This deceptively simple question is a bit more complicated than it sounds.  Perennials are generally described as plants that return year after year without replanting.  Whether a plant is a perennial or an annual is based on whether that plant can withstand the cold temperatures of winter. 

What are Hardiness Zones?

This sounds simple and in some ways it is simple.  The main factor complicating things is that winter temperatures vary widely across North America.  In an effort to help gardeners understand which plants are hardy in which areas the USDA developed and published a plant hardiness zone map.  This map was first published in 1960.  It was updated in 1965 and again in 1990.

This USDA Plant Hardiness Zone Map divides the US and Canada into 11 different zones. These zones are based on the lowest average temperature each area is expected to receive during the winter.  The averages used to develop the current map are based on winter temperatures from 1974 through 1986.  

The first step in accurately predicting whether a plant will be hardy for you is to find out what your USDA hardiness zone is.  If you don’t know your zone you can use the U.S. Hardiness Zone Finder to learn what zone you live in or use this map for Canadian zones. 

Choosing Cold Hardy Plants for Your Zone

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 plant tag or website or within a gardening book. 

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.  An example, I live in zone 5.  If the plant tag says a plant is hardy in zones 5 to 9 the plant will be perennial for me.  It would also be perennial in zone 6, 7, 8, and 9.  If I lived in zone 4, then the plant will not be perennial for me, since zone 4 is colder than zone 5.

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.

What if My Zone is Too Warm?

While hardiness zones are most often used to determine whether a plant can survive cold winter temperatures, they also determine if your zone is too warm for plants to thrive. If your zone is too cold for a plant, it won’t survive through the winter. Death is a pretty definitive answer to whether a plant is cold hardy.

Planting a cold hardy perennial in a zone that is warmer than the zone range specified for the plant is less cut and dry. If a plant is hardy in zones 5-9 and you plant it in zone 10, the most likely impacts are a lack of vigor, a lack of flowers and if the plant usually sets fruit, it is unlikely to set fruit. In other words, plants grown climates that are too warm tend not to thrive and waste away over time.  Sometimes the plants do grow decently, but don’t flower or set fruit as you would expect them to.

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.

Additional Considerations

1. 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 will.  The pot will frequently thaw out from time to time even in the winter.  It is this thawing and re-freezing that will cause many perennials and even some woody shrubs to be less hardy in a container than the landscape.  The zones listed in catalogs and on tags and websites refers to the zone a plant is hardy to if planted in the ground.  If you are planting in a pot you will need to choose plants that are two zones hardier than the one you live in.

   

     So for me to have a plant hardy in a pot it would need to be hardy to at least zone 3 rather than zone 5.
   
   You could also dig a hole in your vegetable garden or an out of the way spot in your garden.  Place the pot into this hole and fill in soil until it is level with the lip of the pot.  This will keep your pot as warm as the surrounding ground helping the plant within the pot survive the winter.
   
   
2. Plant breeders are constantly working to select plants that are hardier than the regular species.  It is possible that a newer, named cultivar could be hardier than the regular species.
   
   That being said, it is also possible that a newer, named cultivar is less hardy than the regular species.  This will sometimes happen when a new flower or leaf color (or pattern) is discovered.  Sometimes these genetic breakthroughs are initially linked with less hardy plants.  So if a plant tag lists a zone that is either hardier or less hardy than the general species it isn’t necessarily a mistake.
   
   
3. It is also sometimes possible to take a plant that is one zone less hardy than your zone and bring it through the winter.  If you live in zone 5, a zone 6 plant could be perennial for you.  There are several reasons this could be true.  First the published zone information might be incorrect.  Second, it is often possible to find spots within your garden or even within your community that stay a few degrees warmer than the rest of the general area.  For instance, areas that are protected from the wind are often warmer.  Areas next to your house, especially south facing exposures, or those near a brick or stone wall can retain more warmth also. 

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?  

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.