Before you begin planting your garden, you need to know what the frost dates are in your region. The term "frost dates" is shorthand for when the last frost will occur in the spring and when the first frost will happen in the fall or winter. A frost can damage newly planted growth and even some established plants, so you want to be sure you are past the date (for spring) or prior to it (for fall) before planting or harvesting.
There are a few different sources you can turn to in order to find out your frost dates. In addition to our map and suggested websites, the local gardening center may be your most reliable source. But first, understand exactly what the frost dates mean.
Frost Date Factors
Frost dates are typically defined by the day in which there is a 50 percent chance of being frost-free, which means there is also a 50 percent chance there could be frost. So if you want to ensure the safety of your plants, you may want to adjust these dates, perhaps by a full two weeks—two weeks forward for spring, and two weeks backward for fall/winter. Choose this date for when to begin planting and/or harvesting, as well as for protecting your winter plantings from the cold weather.
Determining 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 look at, 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 in your area. In addition, climate change has not completely been taken into account when determining these dates.
Using Your Hardiness Zone
The Plant Hardiness Zone Map (PHZM) will show you which planting zone you live in and is designed to guide gardeners as to what is best to grow in their area based on what will survive in that temperature range. This information is useful both in terms of knowing when to sow seeds as well as figuring out how late in the season you can plant perennials, trees, and shrubs.
To find your planting zone, refer to the official USDA zone map. The PHZM has been completely revamped, and is now GIS-based, meaning it uses specific data relating to positions on the earth's surface, comparing and contrasting information about a precise location. This is greatly improved compared to the old model which simply showed the average maximum extreme temperatures for wider areas, which, due to microclimates, could actually range a bit more.
Since the old zones covered a large area, first and last frost dates were not exact for each location. The updated zone map now has several more zones, having broken down each numeric zone into two, creating an "a" (or northern) and "b" (or southern) subset of each zone, as in zones 7a and 7b. In addition, the PHZM added two new zones for Hawaii and Puerto Rico.
First and Last Frost Date Charts
Although the PHZM has updated its zone map, the frost date charts currently do not reflect the new zone subsets. In addition, the dates are basically the middle of each month, but frost can arrive anywhere from the beginning to the end of the month. Thus, some of these charts are not that helpful. Instead, look at the National Oceanic and Atmospheric Administration's website; once you choose your state, you will find a list of the freeze/frost dates by probability (90, 50, 10 percent), as well as three different temperature thresholds.