How Frost Dates Are Determined
The chart below shows the average dates for the last spring frost and the first fall/winter frost dates for each USDA Hardiness Zone. These dates are typically defined by the day in which there is a 50% chance of being frost-free, so if you want to ensure safety for your plants, you may want to adjust these dates by a full two weeks (forward for spring, backward for fall/winter).
Also, you should be aware that within an individual hardiness zone, the dates for first and last frost can vary quite a bit. Within a single zone, the average dates can vary by as much as a week. If you do not know your planting zone, refer to the official USDA zone map.
Hardiness Zone May Not Always Be Accurate
A more precise way to determine your planting dates might be to use one of the many interactive maps that allow you to determine planting dates based on your zip code. You simply type in your zip code to receive detailed information on average temperatures for your location. Many of these are available, but make sure that the one you use is based on data from an official organization, such as the USDA or National Climactic Data Center. These sources are kept updated constantly, an increasingly important thing as climate change affects average temperatures across the globe.
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.
First and Last Frost Dates, by Hardiness Zone
|USDA Hardiness Zone||First Frost Date||Last Frost Date|
|1||July 15th||June 15th|
|2||August 15th||May 15th|
|3||September 15th||May 15th|
|4||September 15th||May 15th|
|5||October 15th||April 15th|
|6||October 15th||April 15th|
|7||October 15th||April 15th|
|8||November 15th||March 15th|
|9||December 15th||February 15th|
|10||December 15th||January 31st (sometimes earlier)|
|11||No frost.||No frost.|