The Plant Hardiness Zone Map (PHZM) is a useful tool for gardeners and plant enthusiasts. It provides information about the average annual extreme minimum winter temperature in different regions, helping you understand the suitability of plants for your area. Here’s a guide on how to interpret and utilize these maps effectively.
Understanding Zone Numbers and Colors
The PHZM divides the United States and Puerto Rico into zones ranging from 1 (coldest) to 13 (warmest). Each zone is further divided into half zones, labeled as ‘a’ and ‘b’. For example, zone 7a represents the colder half of zone 7, while zone 7b represents the warmer half. These designations help communicate and compare extreme winter temperatures.
The colors assigned to each zone follow the spectrum of a prism, including red, orange, yellow, green, blue, indigo, and violet. By overlaying these colored zones on the map, you can easily visualize and compare plant cold-hardiness across the country.
Using the Map for Growing Plants
While the PHZM is a valuable resource, it’s important to remember that it serves as a general guide for growing perennial plants. The zones are based on average lowest temperatures, not the lowest ever recorded. The current edition of the USDA PHZM uses weather data from 1991-2020.
If you decide to grow plants at the extreme range of the coldest zone they can tolerate, there is still a risk. A rare, extreme cold snap lasting only a day or two could occur, potentially harming plants that have otherwise thrived for years. Therefore, it’s crucial to consider other factors and consult with knowledgeable producers, gardeners, or extension services for specific advice.
Factors such as wind, soil type, soil moisture, humidity, pollution, snow, winter sunshine, and warm season heat and moisture balance all play a role in plant survival. Additionally, how plants are placed in the landscape, their planting techniques, and overall health can influence their ability to thrive.
Important Considerations
When growing plants, keep the following factors in mind:
-
Light: Ensure plants are placed where they receive the appropriate amount of light. Excessive sunlight during winter can cause rapid temperature changes within the plant, potentially injuring those that require partial shade.
-
Soil moisture: Different plants have varying soil moisture requirements, which can also change with the seasons. Plants at the limits of hardiness in your zone might be injured if soil moisture is too dry in late autumn, causing moisture stress during dormancy.
-
Temperature: Optimal temperature ranges vary among plant varieties and species. Some plants may tolerate a wider range, while others have a narrower range for ideal growth.
-
Duration of exposure to cold: While some plants can withstand short periods of cold exposure, longer periods of cold weather may be detrimental to their health.
-
Humidity: High relative humidity helps reduce moisture loss from leaves, branches, and buds, thereby limiting cold damage. Low humidity, especially for evergreens, can make cold injury more severe.
Additional Information
For more detailed information or examples on how to use the PHZM as a decision-making tool for planting, you can refer to the publication “Horticultural Applications of a Newly Revised USDA Plant Hardiness Zone Map” by M.P. Widrlechner, C. Daly, M. Keller, and K. Kaplan. It was developed for the 2012 version of the PHZM and can be found here.
By understanding and utilizing the PHZM effectively, you can make informed decisions about which plants are best suited for your specific region, increasing the chances of a successful and thriving garden. Happy gardening!