The middle of May is approaching. Does that mean it’s finally time to plant our gardens?
Maybe, said Julie Janoski, manager of the Plant Clinic at The Morton Arboretum in Lisle. Traditionally, gardeners are told to plant after the last frost, and the average date of the last frost is usually given as May 15 in the Chicago area.
However, Janoski said, “there are a few holes in that advice.”
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For one thing, the last frost date of May 15 is just an average that masks a huge amount of variation from year to year.
“Our last frost may come in late April, or it may be as late as Memorial Day,” she said. “You need to pay attention to what the weather has actually been this year and what you see in the weather forecast.”
The risk of frost lasts longer in some places than in others. Outer suburbs may have freezing nights two or three weeks later than Chicago neighborhoods near the lakefront, where buildings break the wind and temperatures are buffered by Lake Michigan.
“Weather varies across the region,” Janoski said. “What matters to your plants is the weather right where you live.”
Another major factor is that not all plants are the same. “Some kinds of plants are much more tolerant of cold than others,” she said. “There’s no one time to plant everything.”
So, what can you plant when?
Start with trees and shrubs, which are generally fairly cold-tolerant. “You can plant them from late March through June, as long as the soil has dried out enough to dig,” Janoski said. Rose bushes can be planted after the beginning of April.
For other plants, including vegetable seedlings that you may have started indoors, it’s important to know the cold tolerance of the particular plant species involved.
In late April or early May, it’s safe to set out seedlings or sow seeds of cool-season vegetables such as broccoli, Brussels sprouts, cabbage, horseradish, kale, kohlrabi, lettuce, peas, rhubarb, rutabagas, spinach and turnip. You can also plant onion sets, horseradish roots and potatoes.
However, some vegetables that originally came from warmer climates, such as tomatoes, basil, peppers, corn and beans, will not tolerate any cold.
“They can’t handle nights that might get down into the 30s,” Janoski said. “Wait to plant them until late May or June, when nights are consistently above 50.”
Most perennials that are winter hardy in Chicago can be planted or divided starting in late April or early May. However, many familiar annual flowers, such as petunias, impatiens, calibrachoa and begonias, come from tropical or subtropical climates and, like tomatoes, need to wait until nights stay above 50 degrees, which usually means mid- to late May. You could also plant them in containers small enough to be moved into a safe spot, such as an unheated garage, if a cold night is forecast.
Before any kind of plant is moved outdoors, it needs to be gradually acclimated, or “hardened off.”
“A plant coming from a warm, sheltered greenhouse or basement can’t go directly out into the weather,” Janoski said. “The shock will be too great.” Instead, introduce the plants to the outdoors for increasing amounts of time over several days, or use a cold frame to keep them sheltered at night and open it during the day.
“You can’t plant the garden all in one day,” she said. “It takes planning and preparation to make sure all your plants get safely into their summer home.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice, 630-719-2424 or [email protected]). Beth Botts is a staff writer at the Arboretum.