When will it be safe to plant gardens, flowers, trees and shrubs?

FARGO — There’s a lesson that many gardeners learn the hard way: A warm April can’t be trusted, and being seduced into planting flowers and vegetables earlier than advised often ends in the heartache of May frost.

During years when April is warm and balmy, new gardeners can be deceived into thinking it’s time to plant. More often than not, as shown by statistics, Mother Nature reminds us who’s in charge by blanketing the region in May frost.

April has been cold and snowy this year, leaving little question about whether it’s too early to plant. Even though April can be luxuriously warm, it’s an unreliable month for widespread planting.

I’m often asked, “When is it safe to plant flowers, vegetables, trees and shrubs?“ As gardeners gain experience, they realize the wisdom of delaying planting until it’s statistically less likely that temperatures will drop to 32 degrees Fahrenheit or below. Knowing the history and probability of spring frost minimizes the chance that freezing temperatures will kill newly planted flowers and vegetables.

David Samson / The Forum

Naturally, the likelihood of frost lessens with each passing day in May. We often hear stories of our parents and grandparents waiting to plant gardens until Memorial Day, which was observed traditionally on May 30 until 1968.

WDAY StormTracker Chief Meteorologist John Wheeler emphasizes that the average date of spring’s final frost doesn’t really mean much. The average is produced from dates that swing widely, rather than dates that are similar each year.

Wheeler provided examples of the wide variation in Fargo’s final frost of spring. In 2007, it never froze again after April 17. In 1969, the last spring frost arrived on the morning of June 20.

In 2020, temperatures dipped to 22 degrees on May 12. Last year, in 2021, Fargo registered 31 degrees on May 28.

Attempting to find an average date when it’s “safe” to plant frost-sensitive plants is all about risk percentages. Since records began in the 1880s, May 13 is the overall date on which Fargo has a 50-50 chance of not going below 32 degrees. If we examine just the past three decades, May 9 is the 50-50 date, meaning there are equal chances of having frost and not having frost on that date.

To further examine Fargo’s frost risk assessment, on May 5 there’s a 66% chance temperatures will drop to 32 degrees or below. On May 10, there’s a 47% chance of still having frost. By May 15, there’s a 30% chance of frost, and by May 20, the probability of having frost drops to 13%.

Keeping frost probabilities in mind, there are some vegetables, termed cool season crops, that can be planted early because they’ll survive freezing temperatures down to 28 degrees, including broccoli, cabbage, carrots, lettuce, onions, peas, radishes and potatoes. Besides tolerating freezing air temperatures, they will germinate and grow when the soil temperature is still chilly.

Vegetables termed warm season crops are easily damaged at 32 degrees, including tomatoes, peppers, eggplant, beans, corn, cucumbers, melons, pumpkins and squash. Planting about May 20-25 is best unless you’re prepared to protect plants. Air temperature isn’t the only factor, however, as warm season vegetables can be permanently stunted if planted in cold soil. Planting before the soil temperature reaches 50-55 degrees is counterproductive.

When do my wife, Mary, and I plant our flowers and vegetables? We consider the 10-day window from May 15 to 25 to be our favorite planting time. We adjust a little depending on the year, but by those dates the likelihood of frost has lessened, and the soil is warming up.

If you purchase flower or vegetable plants but aren’t ready to plant them, is it all right to keep them in the garage for a week or two until you are ready? No, it’s really not. Greenhouse plants are grown in sunlight and if stored in a dim garage all day, the plants will weaken. Instead, keep sun-loving plants outdoors in a sheltered spot in full sunshine and move into the garage at night if necessary.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.