Many gardeners are familiar with the USDA zone maps. These climate maps lay out the estimated coolest temperatures of each region. This allows gardeners to decide which plants will thrive in their particular area. For years, Baltimore has been in zone 6, along with westerns Maryland and Pennsylvania. But, now that the USDA published the 2012 edition of this map, Baltimore is in zone 7a. This puts it in the upper south along with the District of Columbia and parts of Virginia.
The previous USDA hardiness map was made in 1990 but temperatures in the Mid-Atlantic region have been gradually warming. The lowest range of temperatures in zone 7a is 0 to 5 degrees. As Baltimoreans know, we rarely reach those lows, much less the colder depths of zone 6, which can be as low as -10 degrees. Maryland has a wide range of temperatures on the map, from the colder zone 5B in Garrett County to warmer zone 8a in Salisbury, on the eastern Shore.
In revising the map, the USDA considered the average winter lows measured between 1976 and 2005. Some plants can slowly acquire hardiness as the days become shorter and the nights longer. They can also loose hardiness in the opposite case. The previous map only considered 12 years of data while the recent one considered 30 years. The USDA does not consider these new adjustments to be a result of global warming.
If a plant is doing well in your garden there is no need to take it up if it does not match what the new zone map says you can grow. Rely on your own experience and sense of what does well, as microclimates could also affect the temperature in your garden. A flat unsheltered area will likely be colder than a patch that gets reflected heat and sunlight from a nearby wall. Consider the new zone map a guide but don’t rely on it too strictly. More experienced gardeners only use the zone map as a tool after considering their own observations about the weather.
The USDA has no printed maps but you can see the complete USDA zone map online.
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