In 1960, the United States Department of Agriculture developed the USDA Plant Hardiness Zone Map to help gardeners choose the right plants for their climate. Revised in 2012, the map determines an area’s zone according to its average annual extreme minimum temperature over the past thirty years.
While the map is a good guideline for what will and will not grow in your area, its usefulness is limited as there are many other factors that influence whether or not a plant will survive. Below is a list of other factors to consider when choosing plants for your garden.
Snow is very insulating and a consistent covering of snow will help plants make it through the winter. There is a big difference between an area with average winter temperatures of, say, 15 degrees with no snow cover and an area with the same average temperatures but consistent snow cover all winter long.
Freeze Thaw Cycles
Freeze thaw cycles are what create frost heaves, and as bad as they are for the road, they are worse for plants. When the ground goes through freeze thaw cycles plants get pushed up out of the ground to the point that their roots are exposed. Also, when the ground has thawed but it is still relatively cold outside, the soil can become water logged and roots can easily rot. Being in a very cold area where the ground freezes and stays frozen until it thaws in spring is easier on a plant than living in an area where the temperature hovers around the freezing point causing the ground to go through several freeze thaw cycles each winter.
It goes without saying that healthy soil produces healthy plants. As with pests and disease, healthy plants are less susceptible to cold temperatures. If you plant in good soil you will have stronger plants that can withstand the cold better.
Wind dries out the soil and displaces topsoil and mulch, often leaving plants dry and exposed. Wind also causes plants to lose moisture due to transpiration. Living in an area with very cold temperatures but no wind is very different from living in an area with the same temperatures that is windy.
The more sunlight a plant receives – up to a certain point – the more it can photosynthesize. The more it can photosynthesize the more energy it can store for periods of dormancy, or in the case of evergreens, semi-dormancy. In an area where there is a lot of sun during the growing season many plants will have a better chance of survival through cold winters than in an area where there is less sun during the growing season.
Most of the water a plant takes in is used for transpiration. Water is taken in by the plant roots and evaporated through the leaves. The more humid the air the more slowly transpiration occurs. Since the process of transpiration helps to cool the plant very high humidity can cause the plant stress. A very humid summer followed by a very cold winter can be especially hard on a plant.
Where a Plant Grew Up
You know how we say that people from warm climates are not as tolerant of the cold as people who grew up in cold climates? Well, the same is true of plants. If you have two plants of the same species and cultivar, both grown from seed, one in a warm climate and the other in a cold climate, the one that grew up in a cold climate will invariably be more cold hardy than the one from a warm climate. If you have a choice, buy plants that were grown locally.
In order for any plant to survive cold temperatures it must acclimatize. In other words, it must have a period when the weather gets gradually colder to acclimate to these colder temperatures. Even the most cold hardy plant will die if it is suddenly subjected to cold temperatures in the middle of July without any chance to acclimate. Therefore, areas that have early cold snaps and more volatile weather conditions make it more difficult for a plant to survive the winter months.
With all of these factors to consider what is a gardener to do?
For a more detailed map of hardiness zones consult the Sunset climate zone map created by Sunset magazine. Their map takes into account a myriad of conditions including elevations and ocean influence and divides the country into 45 zones as opposed to the USDA map’s 11 zones. Or follow Landscape Designer Kaveh Maguire‘s advice. In a conversation about the maps Maguire said to me, "The USDA maps are good for the information they provide but gardening is a lot of trial and error. The best thing for new gardeners to do is visit local public gardens to see what grows well for them and then to experiment on their own.”
I wholeheartedly agree.