How to find your average last frost date.
Some crops thrive in cool weather, while others only grow well when it’s warmer. So how do you know when to plant what? The key factor that should guide your decisions is your average last spring frost date. Most cool season crops, like cabbage, broccoli, lettuce and many others, can tolerate a light frost and will grow best when sown a couple weeks before your last spring frost. Some, like peas and spinach, are so cold-hardy they can even be planted “as soon as the ground can be worked,” as many seed packets say. But warm season crops like squash, cucumber and basil will be killed by frost if your seeds come up too soon. Ditto for warm season transplants such as tomatoes, peppers and eggplants — if you don’t wait until danger of frost has passed before you set them out, a late frost will kill them.
Thus on seed packets you often see “Plant after all danger of frost has passed.” So, how do you find the average last spring frost date for your area? There are U.S. maps that show last frost dates, but it’s hard to find your exact local dates on them. Your best bet is the National Climatic Data Center. Choose your state and then locate the city nearest you, and it will show your average last spring (and first fall) frost dates, based upon weather data collected by the National Climatic Data Center from 1971 through 2000 from that location. You can choose between a 50/50 probability of frost after the given date, or you can play it safe and choose the 10 percent date, which means there’s only a 10 percent chance of a frost after that date. The Freeze/Frost Occurrence Data charts also provide average dates for 36 degrees Fahrenheit, 32 degrees and 28 degrees; for most crops gardeners should use the 32 degree dates.
Another great tool to find your average frost dates is the MOTHER EARTH NEWS Vegetable Garden Planner. The Planner will even send you customized planting reminders for which crops need planting based on your frost dates and location.
Here’s a summary of which crops to plant early, and which ones not to plant until after your last spring frost date:
For more information on planting dates, organized by region, see What to Plant Now.
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