Your planting zone directly correlates with your growing season (the days between the first and last frosts of winter in your region). These dates are crucial because if you plant too early at the start of the season or too late at the end of the season, you will lose your tender plantings to the sweep of cold weather. Use the Frost Date Chart below as a guide.
The dates in the chart below indicate normal averages for a light freeze (32 degrees Fahrenheit). Note that local weather conditions can vary dramatically – there is a 50% chance of frost occurring after the spring dates and before the fall dates. Additionally, microclimates within zones can affect the expected frost dates. For instance, if a garden is surrounded by buildings or parking lots, these "heat islands" will make this garden considerably warmer than the rest of its zone.
Light freeze: 29 – 32 degrees Fahrenheit. Tender plants killed with little destructive effect on other vegetation.
Moderate freeze: 25 – 28 degrees Fahrenheit. Widely destructive effect on most vegetation. Heavy damage to fruit blossoms and tender and semi-hardy plants.
Severe freeze: 24 degrees Fahrenheit and colder. Damage to most plants.
Click on the chart below for a printable PDF.
Information on this page courtesy of the National Climatic Data Center and the Farmers Almanac.