Before you begin planting your garden, you need to know the last and first frost dates in your region so that you plant at the right time. What is a frost date? A frost date is the average date of the last frost in spring or the first frost in fall or winter. Frost occurs when air temperatures reach freezing: 32° Fahrenheit (0° Centigrade). However, a frost can occur even when air temperatures are just above freezing. Frost can damage young and newly planted plants, the new growth on established plants. and can affect harvesting. If you are planting tender or warm season plants, you should do so after the last frost date in spring and before the first frost in fall or winter.
You can turn to a few different sources to determine the average frost dates in your area, as noted. In addition to our map and suggested websites, your local gardening center and your personal experience can be your most reliable source. It's a good idea to jot down the actual last and frost dates in your area to keep an accurate record year by year.
Frost Date Factors
Regional last and first frost dates are an average estimate based on historical climate data and are not foolproof or exact. So in addition to knowing published frost dates, it's prudent to always be aware of your local weather forecast and be watchful for those nighttime temperatures.
To ensure the safety of your tender plants and fall harvests, you might want to adjust the average frost dates that are published for your region. Consider planting two weeks later in spring and two weeks earlier for fall/winter to protect your plants from unexpected and abnormal cold temperatures. And, it's important to know the difference between a frost and a freeze because some plants can tolerate a light frost but cannot tolerate a freeze.
Determining Frost Dates by Zip Code
Many believe that using your zip code will lead you to a more accurate frost date. There are a few different websites to refer to, including davesgarden.com, the Old Farmer's Almanac, and the National Gardening Association. Just keep in mind that these dates are an average and don't take into consideration any microclimates on your property. In addition, climate change has not been taken into account when determining some frost dates.
Know Your Plant Hardiness Zone
Another important garden planning tool is to be familiar with the USDA Plant Hardiness Zone Map, which shows the planting zone in which you live. The map divides the United States into cold hardiness zones based on average annual minimum temperatures. A plant’s cold hardiness zone indicates where a plant is likely to survive the winter. This information is useful both in terms of knowing when to sow seeds and the right time to plant annuals, perennials, trees, and shrubs that are likely to survive the winter. Use this map in combination with last and first frost dates to ensure you plant at the right time.
To keep informed about latest updates and possible zone adjustments, see the official USDA Plant Hardiness Zone Map website. This map is interactive and provides features that enable you to find and download cold hardiness zones at the local zip code, state, regional, and national level.
Another important source of weather-related information is the National Oceanic and Atmospheric Administration website, which you can use in combination with frost dates and cold hardiness zones to assist with selecting safe planting dates.