It’s planting time! For most things, yes, but perhaps not for others, according to Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. Some plants are vulnerable to the colder temperatures that can occur in late May.
Trees, shrubs and most hardy perennials are safe, along with cold-tolerant crop vegetables like spinach, radishes, carrots and kale. For other plants, such as tomatoes, peppers, basil, impatiens and petunias, it’s better to wait until late May or the first of June, when the risk of frost is unquestionably past.
Spring weather in the Chicago region is changeable, and “it’s not uncommon to get freezing nights even in late May,” Yiesla said. It can damage plants that come from places where it never gets that cold. Peppers, petunias and tomatoes are examples of these tender plants: They all come from subtropical regions of Central and South America.
The average last freeze date for the Chicago region is about May 15, but that’s just an estimate. The last freeze comes sooner in some years and later in others. A changing climate is gradually making our growing season a little longer, but it’s also making the weather more variable.
“It’s better to go by the weather that is actually happening rather than by lists of planting dates or habits from earlier years,” Yiesla said. Check the weather forecast on a regular basis throughout spring, and don’t plant tender vegetables or flowers until nighttime temperatures are consistently in the 50s.
Even temperatures in the 40s can be hard on tender plants, but the deadly risk is freezing. Late-spring freezes usually occur at night when the temperature drops to its lowest point. If it falls below 32 degrees, ice can form inside plants’ tissues, which are mostly water.
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“Hardy plants that come from places with cold winters have ways to protect themselves against a night or two of freezing weather,” Yiesla said. “But basil or tomatoes have no defense at all.”
Most well-chosen trees and shrubs that are winter-hardy to Zone 5 or a colder zone on the USDA hardiness map will not be fazed by a late freeze. On a plant label, a lower number means that the plant is hardy to colder places. The Chicago area is mostly in Zone 5. If the label says “Zone 4,” the plant is suitable for Zone 4 and warmer zones, including Zone 5.
While you wait for safe weather, keep any tender plants you have purchased in a sheltered place, such as an unheated garage, at night. Move them outdoors during the day so they get some light.
If you plan to move houseplants out into the garden for the summer, don’t rush. Most houseplants are extremely tender. They come from tropical and subtropical rainforests or deserts and cannot tolerate cold nights.
When a late freeze is predicted, it’s possible to cover vulnerable plants already in the ground with sheets or lightweight row covers, which may save them. Be sure to remove the cover in the morning so plants don’t overheat during the day.
“Keep your eye on the weather forecast while you decide when you’re going to plant,” Yiesla said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.