Three Ways To Determine When To Plant

With our erratic springs in northern climates—70 degrees one day, 30 the next—it’s often a challenge to figure out when to plant your vegetables or ornamental plants. Most experienced gardeners use a variety of ways to determine when to plant their crops.

Here are three we’ve tried:

Using Frost Free Dates

Most seed packets will include a recommended seed sowing time based on your area’s last frost date or its frost-free dates. This is the date after which the odds of getting frost are extremely low. In most of central Minnesota, for example, the last frost date is usually around May 10, but we aren’t completely out of danger until the end of May—this is one reason many gardeners do not plant tomato plants outside until early June.

This post gives a lot more detail about frost-free dates and the vegetable garden. Frost free dates give you a general idea of when to plant safely. So, lettuce seeds and starts can be planted a couple of weeks before the last frost, but tomatoes and peppers should wait until a couple of weeks after.

Using Soil Temperature

A more precise way of deciding when to plant is by taking the soil’s temperature. Seeds for cool-season crops, such as lettuce, will germinate at temperatures as low as 40 degrees F. But—and this is an important but—they won’t germinate very fast, so it’s a good idea to wait until the soil temperature is over 50 or even 60 degrees to sow lettuce outdoors as seed.

Many experienced gardeners sow their lettuce and other cool season crops under lights indoors (or in a cold frame) then transplant them outside once they have a few leaves and are growing. At that point, the cool soil is not an issue. To take the soil’s temperature, you can buy a special soil thermometer, but you can also use a meat thermometer to check temperature.

Using Nature’s Signs

Mother Nature has been at this a lot longer than the rest of us, and some gardeners have taken to watching nature signs to determine the best time to plant. Phenology, as this study is called, looks at the timing of different events in nature and matches it with garden chores.

Traditional advice includes things like plant lettuce, spinach, beets and other cool season crops when the lilac leaves come out and when barn swallows return set out your basil and tomatoes. This advice was often developed in more moderate climates than ours, so it’s not 100 percent. To find out more about phenology and participate in this fascinating work, check out the National Phenology Network website.

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 State Horticultural Society and the only magazine dedicated to gardening in USDA Zones 3, 4 and 5..

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