When is it safe to plant?

For Northerners who garden with annuals, May is always a fraught month. When is it safe for us to plant?

The usual answer is around May 15 or Mother’s Day, when frosts are no longer expected. But even if a frost isn’t in the forecast, temperatures in the 40s can stunt warm-weather annuals. As the Chicago Botanic Garden says on its website, “In May, it pays to be patient and flexible.” 

I had targeted Memorial Day weekend for planting the balcony containers. But what a glorious day last Tuesday, May 13, was. I had nothing on the calendar. No harm in getting the planters ready with new topsoil. 

When that was finished, I checked the weather forecast for the rest of the month. Nights in the 50s, some into the 60s. The sweet potato vines I’d brought in for the winter were dying off indoors. The few remaining ones might be happier outside. As long as the temperature didn’t dip below 50 degrees, they should be OK.

The sweet potato vines planted, I couldn’t stop. I got on a bus with my grocery cart and went to Home Depot. The orange-flowered impatiens I came home with were planted immediately, since they also survive above 50 degrees. 

Wednesday was even warmer. More than once I went out on the balcony to admire the plantings.

Then on Thursday evening came severe thunderstorms, with 60 mile-per-hour winds. On Friday Chicago got a rare dust storm, with strong winds again. Lows dropped into the 40s overnight.

When I stepped onto the balcony Saturday morning, I felt crestfallen as well as cold. Two of the sweet potato vines were dead. The leaves of the impatiens were shriveled. I gave the plants a good watering — they were dry from the wind, despite rain — but don’t feel hopeful after seeing the revised weather forecast. Overnight temperatures in the 40s were predicted for each of the next eight nights.

“Impatiens simply won’t survive cold temperatures, so be sure overnight temperatures don’t dip below 50 degrees Fahrenheit,” says the website doityourself. Although that sounds like we can control the temperature, it is a recommendation to forestall planting until lows above 50 degrees are predictable. 

I remember planting in mid-May last year and not suffering any consequences. That was lucky. May temperatures can vary wildly, with 40-degree swings in the same week and lows below freezing. Not until the end of May are consistent temperatures above 50 degrees expected. 

“It may be best to plant warm-season plants at the end of May or even early June,” advises the Chicago Botanic Garden, mentioning impatiens specifically. “Cautious gardeners often wait until Memorial Day.”

Next year I’ll try to be a cautious gardener. There are only a couple of weeks between mid-May and Memorial Day. If I have to replant this year, it will be the end of the month. I’ve given the scraggly plants until then to revive.