If you grow vegetables from seed, the packet almost always lists how long before or after your average last frost date you should plant. It’s important to know your “frost-free” dates and what those mean for your vegetable garden.

Generally, the frost-free dates are those parts of the year when the odds of getting hit by frost are very low. The dates are calculated based on years of weather data, and may vary significantly across a state or region.

Dates Across Minnesota

For example, I live in Northfield, 40 miles south of Minneapolis and St. Paul. According to this extremely handy calculator from the garden forum, Dave’s Garden, on average my garden will not experience frost between May 3 and Sept. 30. I’m almost guaranteed it won’t frost between May 17 and Sept. 18. (I’m pretty much doomed to frost anytime between Oct. 11 and April 19.) So, my average growing season is 150 days, which means I don’t want to choose long-season melons or other crops unless I plan to nurture them in a cold frame.

If I lived up near Moorhead, however, my average last frost would be a week later than in Northfield (May 10) and my average first frost would be a week earlier (Sept. 24). In Moorhead, the absolute guarantee no frost date is May 25 and the growing season is 137 days, almost two weeks shorter than in southern Minnesota.

Pay Attention to Specific Garden Location

While knowing the frost-free dates is important, you also want to pay attention to your own garden. My vegetable garden is in a low part of my lot, and as such, it stays a bit cooler than other areas nearby, so I tend not to plant out warm season crops, such as tomatoes, until June. The plants do much better if I wait until the soil is warmed up.

Once you know your frost-free dates, you can figure out when to plant things. Cool-season vegetables—beets, lettuces, peas, broccoli— can be started a few weeks before the frost-free date, but tomatoes, beans, cucumbers and annual flowers should wait until after the frost-free date. Seed packets will usually recommend when to plant.

Using Nature’s Signs

Since our climate can be unpredictable, some gardeners have started to go by natural signs (plant and bird activity) rather than measurements based on years when the climate may have been cooler than it is now. Phenology is interesting to study and you might want to consider some of these recommendations in deciding when to plant:

• When crocus bloom, remove the mulch on your strawberries.
• When leaves first emerge on lilacs, plant lettuce, beets, spinach and other cool-weather crops.
• When lilacs are in full bloom and the barn swallows return, set out your tomato plants and basil

Do you use frost free dates to decide when to plant or nature signs?

Mary Lahr Schier is a Minnesota gardener, writer and editor and author of The Northern Gardener, From Apples to Zinnias, 150 Years of Garden Wisdom (Minnesota Historical Society Press, 2017), winner of the Silver Award of Merit from GardenComm in 2018. For 18 years, she edited Northern Gardener magazine, the publication of the Minnesota Horticultural Society and the only magazine dedicated to gardening in USDA Zones 3, 4 and 5..

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