Plant hardiness zones are climate zones mapped across geographical locations that inform gardeners about the temperatures in their areas and the length of their growing seasons. The zones are numerically divided by 10 degree differences in minimum winter temperatures, and then further divided into 5 degree increments that are represented by letters. See the map below of the plant hardiness zones across the United States and the legend for the zones to the right of it.
Currently, New Jersey (NJ) contains three 10 degree hardiness zones: 6, 7, and 8. Including the subdivisions of the zones, NJ has four 5 degree hardiness zones: 6b, 7a, 7b, and 8a. Historically, NJ was only zones 6a, 6b, 7a, and 7b, but with climate change there has been a shift towards warmer temperatures which has affected the hardiness zones. NJ lost zone 6a and gained zone 8a. Overtime, NJ will probably lose zone 6 all together and perhaps part of zone 7.
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