Every 30 years, maps are published showing the average coldest annual winter temps which may not reflect the absolute coldest it has been in each 30 years. Zones are measured in 10°F temperature blocks and gives us the probability of winter extremes of coldness. The lower the zone number is, the lower the cold temperatures in that zone. Zones are further divided into “a” and “b” segments. This represent five degrees of temperature difference. 

For example, Zone 4 represents minimum temperatures between -30 to -20 degrees F (-34 to -29 C). The ‘a’ is -30 to -25 degrees F (-34 to -32 C) and and ‘b’ is -25 to -20 degrees F (-32 to -29 C).

There are some things the PHZM cannot show us:

• How long the cold will last or how humid a garden will be.
• How cold it could get, but not how cold it might get. It’s all about averages. 
• How suddenly winter cold will appear.
• Winter rain. Winter snow. Winter rain and snow. This is the north, anything goes.
• How hot it could get as PHZM only looks at lows.
• Soil preferences, nutrient needs, and so forth of the plant.
• Microclimates in your area or you may create in your garden. 

Microclimates can be small 'heat islands'—such as those caused by blacktop and concrete— 'frost pockets' (cool spots) caused by small hills and valleys, even a side-yard that receives little direct sun. Microclimates are not unusual and can change from yard to yard and even within your own yard.

Zone maps are not absolute and do not take the place of the detailed knowledge that gardeners learn about their own gardens through hands-on experience. By knowing and understanding your specific zone, you can make informed decisions about plant selection, timing, and appropriate gardening practices.