On virtually every seed package or plant that you buy it is likely you will see the term USDA garden zone, also known as a hardiness zone, with a number from 1 through 11 associated with it. If you’ve completed our online profile there are questions that help us to determine the zone you live in so it is obviously of some significance, but do you really know what the term means and why it is important? Education is one of our main goals at Blooming Secrets and understanding this concept can really help you garden more effectively and perhaps a little more bravely too!
The textbook definition of a hardiness zone/garden zone is a geographically defined area in which a specific category of plant life is capable of growing, as defined by climatic conditions, including its ability to withstand the minimum temperatures of the zone. In a nutshell, it means that a plant that is considered "hardy to zone 10" can survive a minimum temperature of 30 °F as determined by the United States Department of Agriculture (USDA). Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period, not the lowest temperature that has ever occurred in the past or might occur in the future. The lower the zone number in which a plant can survive the more resilient that plant is considered to be. This hardiness is usually expressed in a numerical range. For example, a plant that is determined to be "hardy in zones 3 to 9" tolerates colder temperatures better than a plant whose hardiness range is only 7 to 9.
These zones were first developed by the USDA in 1960, and they have been revised several times since then. The 1990 hardiness zone map showed 10 different zones and it further subdivided zones into a/b zones for greater accuracy. Later, the USDA introduced zone 11 which represents areas that have average annual minimum temperatures at or above 40 °F and that are essentially frost-free. The use of these hardiness maps have also been adopted by other countries.
While the hardiness zones are informative they do have a number of drawbacks. The zone map does not take into account the reliability of snow cover as an insulator against extreme cold. This insulation can protect the root system of dormant plants and if an area has reliable snow cover it may actually be able to sustain plants that might not ordinarily grow in that garden zone. For example, location A might be located in zone 4, but it can rely on a significant snow cover every year and it may be possible to cultivate plants normally rated for zone 5 or even zone 6.
Microclimates are also something not contemplated by the zone map. An example of a microclimate might be radiating heat from blacktop and concrete which allows the air to be warmer than normal or valleys where colder air, which is heavier than warmer air, settles. In my yard the rear of the house faces south and the radiating heat from the foundation causes this area to be much warmer than other places in my garden. Many other factors not contemplated in the hardiness zone map can and do contribute to the success or failure of plants. Wind, the type of soil and humidity can greatly affect the survival of plants as well as their size and overall health of the plants.
So what does this all mean to you? Hardiness zones are just guides. If you live in hardiness zone 3 and grow a plant that is considered hardy in zones 3 to 9 it means that you could experience a year with an unusual extreme cold snap and plants that have thrived happily for several years could be lost. Past weather records don’t guarantee that there won’t be variations in the weather in the future and this should be taken into account when choosing plants, especially if you choose to “be brave” and grow plants not rated for their zone. Most of all remember that no hardiness zone map can take the place of the detailed knowledge of your own gardening conditions.