Understanding the USDA Plant Hardiness Zones

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A plant hardiness 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.” The USDA provides a plant hardiness map to gardeners and growers. There are several factors that determine a plant’s ability to survive – wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine – but also the way plants are placed in the landscape, how they are planted, and their size and health.

Here is the map of the US Plant Hardiness Zones:



These are factors that can effect your plant’s survival that are not necessarily taken into account for the plant hardiness map. Make sure you consider these factors as well.

  • Light: Light is an obvious factor in plant growth. If your plant gets too much sun in the winter time (when they normally require partial shade), it could cause changes in the plant’s temperature and end up killing it.
  • 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.

Here are a few winter hardy plants to try out this year!