What is Your Plant Hardiness Zone?

Catherine Garner August 8, 2017 0
What is Your Plant Hardiness Zone?

To find your zone, use this interactive Plant Hardiness Zone Map (PHZM) -click here.  This map is generated and kept current by the United States Department of Agriculture (USDA).

When you are selecting the types of trees, scrubs, or flowers to be used for your landscape, you will need to know your Plant Hardiness Zone to ensure that you achieve the long lasting beauty that works well in your particular location.

Plant Hardiness Zones specified by the USDA are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future. Gardeners should keep that in mind when selecting plants, especially if they choose to “push” their hardiness zone by growing plants not rated for their zone. In addition, although this 2012 edition of the USDA Plant Hardiness Zone Map is drawn in the most detailed scale to date, there might still be micro-climates that are too small to show up on the map.

Micro-climates are fine-scale climate variations which can be small heat islands—such as those caused by blacktop and concrete—or cool spots caused by small hills and valleys. Individual gardens also may have very localized micro-climates. Your entire yard could be somewhat warmer or cooler than the surrounding area because it is sheltered or exposed. You also could have pockets within your garden that are warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first. No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.

In addition, many species of plants gradually acquire cold hardiness in the fall when they experience shorter days and cooler temperatures. This hardiness is normally lost gradually in late winter as temperatures warm and days become longer. A bout of extremely cold weather early in the fall may injure plants even though the temperatures may not reach the average lowest temperature for your zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants as well. Such factors are not taken into account in the USDA PHZM.

Other Factors

Here in our Idaho location, the Plant Hardiness Zone is 6a.  However, I have discovered that very few plants specify a unique hardiness zone such as 6a. Instead they list a range such as 5-8.

Therefore, in addition to hardiness zones, you will also need to consider other environmental factors that contribute to success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival.

Light: To thrive, plants need to be planted where they will receive the proper amount of light. For example, plants that require partial shade that are at the limits of hardiness in your area might be injured by too much sun during the winter because it might cause rapid changes in the plant’s temperature.

Soil Moisture: Plants have different requirements for soil moisture, and this might vary seasonally. Plants that might otherwise be hardy in your zone might be injured if soil moisture is too low in late autumn and they enter dormancy while suffering moisture stress.

Temperature: Plants grow best within a range of optimum temperatures, both cold and hot. That range may be wide for some varieties and species but narrow for others.

Duration of exposure to cold: Many plants that can survive a short period of exposure to cold may not tolerate longer periods of cold weather.

Humidity: High relative humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for evergreens.


United States Department of Agriculture (USDA) -links are provided within content above

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