Freezing weather may make it hard to believe, but many Chicago-area gardeners — and their plants — are now living in a slightly warmer world, according to a new version of the USDA’s plant hardiness zone map.
“The map is a crucial tool for choosing plants because it shows the average low winter temperatures they will need to live through,” said Spencer Campbell, manager of the Plant Clinic at The Morton Arboretum in Lisle. Plant labels, catalogs, books and websites all classify plants according to the USDA hardiness zones.
On the new map, Chicago is in Zone 6 rather than the slightly colder Zone 5. The new Zone 6 area takes in roughly the eastern halves of Lake County and DuPage County, including much of the Arboretum and extends far into Will County.
The change doesn’t affect everyone. “Most of northern Illinois, including much of the suburban area, is still in Zone 5,” said Sharon Yiesla, plant information specialist in the Plant Clinic. The state is in Zone 5 as far south as Peoria.
To find out which zone includes your garden, search by ZIP code on the map at planthardiness.ars.usda.gov map.
The zone shift is the latest confirmation of the effects of climate change. All zones across the country moved north on the new map, the first revision since 2012. The shift is attributed to the changing climate as well as to the heat island effect in urban areas such as Chicago and its suburbs.
Does the zone shift mean Zone 6 gardeners needs to replace Zone 5-hardy plants? Not at all. “Any plant that is winter-hardy in Zone 5 will certainly be hardy in Zone 6,” Campbell said.
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It also doesn’t mean you can assume winters will be uniformly warmer. The USDA zones are based on average low temperatures in each location — and averages can mask major variations. “Spikes of bitter cold are part of the average,” he said. “But they kill lots of plants.”
Not only bitter cold but major snow and ice are still within the range of possibilities in Zone 6. Although the changing climate is making winters warmer on average, it is also making Chicago-area weather more volatile, with plenty of temperature swings from day to day and year to year. “There will still be periods of really rugged winter weather all over the area,” Campbell said.
That means that even if your garden is now in Zone 6, it will still pay to be cautious. “Keep plants healthy during the growing season so they’re well prepared for winter dormancy, and cover their root zones with mulch for insulation in winter and early spring,” he said.
Much of what we think of as winter damage is not actually caused by low temperatures but rather by swings between warmth and cold in late winter and early spring, Yiesla said. Warm spells that tempt plants out of dormancy can be followed by freezes that damage new growing tissues.
“Up-and-down temperatures in that time frame can do a lot of damage even to zone 5 plants,” she said.
Some gardeners may be delighted that they have a slightly different choice of plants now that they live in Zone 6. Yet in both Zone 6 and Zone 5, warmer winters are not entirely good news.
“It means that some diseases and damaging insects that used to be killed by colder winters can survive,” Campbell said. Fruit crops are more at risk because if trees bloom earlier, their flowers and therefore fruit can be destroyed by late frosts. Weeds can thrive because winters are also becoming shorter, giving these opportunists more growing time in fall and spring.
“It will take years to understand and adapt to these climate and weather shifts in our gardens,” Yiesla said. “Big average shifts won’t make the day-to-day swings go away. The smart thing is still to play it safe.”
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.