Can Hardiness Zone Maps Help your Garden

By Murray Anderson

We all know tropical plants can’t survive the winter in North Dakota, but how can you tell if the beautiful flowering shrub you saw in a magazine will thrive in your area? To survive a shrub, perennial or tree must be able to withstand the climatic conditions in your area year round. However, trial and error is time consuming and expensive if all you’re doing is buying plants and shrubs that die. Part of the answer is using the Plant Hardiness Map developed by the USDA (United States Department of Agriculture) in conjunction with the Arnold Arboretum (Harvard University).

What are hardiness zones?

• The hardiness map divides North America (including Canada and Mexico) into eleven zones with each zone being 10°F hotter or colder (on average) during the winter than the adjacent zone. Zone 1 is the coldest (where temperatures can go as low as -50°F) while Zone 11 is the warmest.  Knowing the zone you live in is important to understand which plants will survive in your area.
• Finding out what zone you live in is as easy as clicking on this link to the USDA web site, where you will find a colored map of North America showing the various hardiness zones. If you’re not good with maps, there’s an option to click on your state to find out your zone.
• http://www.usna.usda.gov/Hardzone/ushzmap.html

Using hardiness zones

• Once you know your zone you can use the information to make informed decisions about plants you can safely put in your garden..
• Shrubs and perennials sold at greenhouses and garden centers usually come with printed information that describes the recommended zone and necessary conditions for planting (and in reality most local greenhouses will only sell plants that will grow in your area).
• However, if you're buying perennial seeds or shrubs online or by mail, it’s up to you to know what zone you live in and look for that information in the plant description.
• The hardiness ratings are useful for perennials and shrubs and trees used for landscaping but aren’t really needed (or of much use) for annuals that only have a life span of one year

So, do zone maps tell the whole story?

• Unfortunately, not.  Other factors –besides how cold it gets in winter will affect a plant’s survival. Variables like the type of soil, the amount of rain, moisture in the air (humidity) and wind will all have an impact on how well a plant does.
• Hardiness zones have their best application in the eastern part of the US, where the terrain is relatively flat and the temperature lines can be drawn approximately 120 miles apart.
• However, as you move west, other factors come in play. For example, a mountain range can have a humid climate on one side and a much drier climate on the other, yet the temperature ranges on both sides are similar. Plus consider the role elevations can play in for example, low level Seattle and high elevation (and inland) Tucson. Both are in Zone 8 but many plants that will grow in Seattle won’t last in Tucson.

While hardiness zone maps are a good tool, the real solution to growing a great garden is knowing the microclimate on your own property.

Murray Anderson is an experienced freelance writer with over 500 articles published on the web as well as in print magazines and newspapers in both the United States and Canada. He writes on a wide range of topics and is a regular contributor to DoItYourself.com. He can be contacted at murand@lycos.com.