As a result, you may actually be in a hardiness zone that is colder than that shown on the map, and some marginal plants may not survive your winters. Cold valleys may also be prone to late spring frosts and early fall frosts, making your growing season shorter than indicated by the frost-free season map . If you put out tender plants too early, you might lose them. Low temperatures in winter are not as extreme, and these areas are less prone to late spring and early fall frosts. Smaller bodies of water also have the same effect, usually to a lesser extent.
Urban areas tend to have less extreme low temperatures than the surrounding countryside.
Buildings and paved surfaces absorb heat during the day, then radiate it back into the air at night, reducing the chances of frost and moderating low temperatures during winter. Buildings also offer protection from wind in many places. Urban areas may be a full Hardiness Zone warmer than rural areas just a few miles away. These warming effects carry over into summer, as well. Urban microclimates can trap heat, creating a scorching environment that can damage plants.