What Are First and Last Frost Dates? Here's How to Find Yours—and Why They Impact Your Garden's Timeline
As a gardener, there are plenty of things to keep track of, such as when to fertilize your plants, how much water they need, and their preferred soil pH. But one of the most important aspects of gardening is being prepared for your area's first and last frost.
"Knowing your frost dates minimizes the risk of cold weather damaging or killing your plants," says Kelly Funk, president of Park Seed. "The last frost date will guide you on when it's safest to plant and the first frost date will give you an estimated time of when to harvest." Frost can damage plants and hinder new growth, but there are plenty of resources available to gardeners who are unsure of what their local frost dates actually are.
Frost Date 101 and Classifications
According to Funk, a frost date is the average date of the last frost in spring or the first frost in fall. Essentially, the last frost of spring will tell you when you can resume outdoor planting, and the first frost of fall indicates when it's time to either harvest plants, bring them inside as houseplants, or simply let them die. While frost occurs when temperatures reach freezing—32 degrees—the ice crystals can even form at temperatures that hover around that benchmark. According to Funk, there are three main classifications of freeze temperatures.
1. Light Freeze: 29 to 32 Degrees
During a light freeze, non-hardy sensitive plants are damaged and often die back entirely. Funk says this also initiates leaf drop in many deciduous trees and shrubs.
2. Moderate Freeze: 25 to 29 Degrees
Marginally hardy plants and tender growth sustain moderate to severe damage during a moderate freeze. There is typically also some damage to hardy deciduous plants.
3. Severe Freeze: 24 Degrees and Colder
A severe freeze causes moderate damage to hardy plants, trees, and shrubs, as well as severe damage or even death to marginally hardy shrubs and trees.
How to Find Your Local Frost Dates
There are many resources online that provide frost date information based on zip code, including the Old Farmer's Almanac and the National Gardening Association. It's important to note that these dates are just an average based on historical climate data and your actual frost dates may vary year to year.
Melinda Myers, gardening expert and host of the Great Courses How to Grow Anything DVD series, recommends using your local university extension service, which she says will likely have the average last spring and first fall frost dates available for all parts of your state. Additionally, she notes that many seed companies have interactive maps that provide average frost dates for your area, as does the national weather service.
How to Prepare for Your First Frost Date
Since frost dates are just an estimated average calculated from past temperatures, Myers says to start watching the weather leading up to the average first frost. Once the ice crystals arrive, there are a few maintenance steps you must take to protect your plants.
Cover Your Plants
Myers recommends covering plants with a sheet in the late afternoon until temperatures rise above freezing the following day, which you can do until you're done harvesting—or if a warm period returns. She says she prefers using floating row covers to loosely cover her plants; she anchors the edges down. "This fabric lets air, light, and water through, but traps heat around the plants," she says. "It comes in different weights providing different levels of protection."
If you have any container plants growing outdoors, you can move them inside, rather than covering them.
Clean Up Your Garden
Your first fall frost is also a great time to get your garden ready for next year's planting season. Funk says to clean up your summer vegetable garden by removing expired edibles and weeds. "Dead edible can attract pests that will overwinter and cause issues for next year's crop," she says. "Clean up weeds, as well, to leave fresh, healthy soil for next year."
She adds that you should also apply mulch at this time to protect and insulate your plants' root systems.
Plants That Can Withstand Frost
According to Myers, certain plants have more dissolved substances, like sugars and salts, built up in their cells that decrease the freezing point of the liquid within—making them more resistant to the cold. There are a handful of plants that can survive in frost, such as cool-season vegetables, like broccoli, cabbage, radish, spinach, and especially kale (which Funks says is a superstar when it comes to surviving in cold weather).
Plants That Cannot Withstand Frost
Of course, not all plants make it through the colder months. "Many of the vegetables that you plant in your summer garden are susceptible to damage at the first sign of a cold snap," Funk says. "Plants like tomatoes, peppers, okra, eggplants, cucumbers, squash, and melons should not be planted until the threat of frost is well passed."
Additionally, though it may come as a surprise, winter squash cannot withstand freezing temperatures. "Winter squash are named because they are often harvested at the beginning of winter," Funk says. "Just like other summer crops, they can't handle winter weather."