ecoscapes blog

What's Up with Winters Warming Up?

“Oh the weather outside is frightful. . .” So goes the opening lyrics of the holiday classic Let It Snow! Let it Snow! Indeed, there have been times this winter when heavy snowfall, single-digit temperatures and ice slick streets made ‘frightful’ an apt description.

Yet the conditions we used to think of as typical for Chicago winters have been few and far between. ‘Mild overall’ is more apropos when talking about this winter and the last several ones.

The long-term trend of warmer winters was confirmed by the United States Department of Agriculture’s (USDA) 2023 update of its Plant Hardiness Zone Map. The map indicates the coldest minimum temperature a dormant plant can withstand. Scientists consider plant hardiness1 to be one of the most critical factors in determining where plants can grow successfully.

Each numerical zone reflects a range of 10 degrees Fahrenheit. The numerical zones are further divided into 5 degree Fahrenheit (F) half zones denoted by the letter’s “a” and “b.”

Warmer winters mean the greater Chicago area is now in zone 6a (-10F to -5F) where it previously was zone 5b (-15F to -10F).

An article on the updated map by Illinois State Climatologist Trent Ford explains that the 6a expansion in northeast Illinois is “likely due to a combination of long-term winter warming and urban sprawl and development in the region.” Warming winters are “one of the most consistent changes in Illinois’ climate that is directly linked to human caused global warming,” he adds.

The upside to less extreme cold is reduced injuries to plants and wildlife. The downside is significant as far as how suitable our surrounding environment will continue to be for native plants and animals.

Warmer winters, accompanied by an overall increase in temperatures and precipitation, could “be more conducive to certain species of non-native (plants), invasive plants, pests, and plant and animal diseases,” notes Ford, pointing to the growing incidence of ticks and tar spot.

As our climate continues to get warmer and wetter2, sustainable gardeners will be challenged to adapt to shifting conditions.

Here are some actions you can take to help your garden thrive as conditions change:

· Use native plants whenever possible. Over thousands of years, natives have adapted to the extreme ups and downs in temperature and moisture that we experience in our area. That makes them hardier than most non-native ornamental plants. And most native plants can tolerate a range of hardiness zones. Common milkweed (Asclepias syriaca), for example, is rated for zones 3-9.

· Work towards increasing plant diversity. Greater diversity helps ensure that your garden won’t be devastated if one or more plants cannot withstand changing conditions.

· Select natives and/or cultivars of natives (‘nativars’) that are resistant to fungal diseases. Milder temperatures and wetter conditions are ideal conditions for fungi to thrive. Typically, a fungus outbreak won’t kill a plant, but repeated outbreaks will weaken it and may eventually lead to its demise.

Some native plants such as bee balm (Monarda spp.) and phlox (Phlox spp.) are particularly susceptible to powdery mildew, for example. Rather than eliminating fungus prone natives from your plant palette, consider nativars that offer many of the same ecological functions as the native versions. Cultivation practices are also important: follow plant tag recommendations for plant spacing. Remove and place diseased plant parts in the trash. And periodically divide plants to prevent overcrowding.

· Protect your plants by surrounding them with a thick layer of mulch. With warmer winters there is the potential for less snow cover for extended periods of time. As snow acts as an insulator, a lack of it could allow a bout of frigid temps to reach deep into the soil. That could potentially damage shrub and tree roots. Mulch—ideally leaf mulch—can help protect plants in the absence of snow.

· Monitor and remove invasive plants. As conditions become more favorable for some invasives they will likely proliferate and crowd out desirable plants. Here’s a list of common invasive plants in Illinois. To dive in deeper, check out The Management of Invasive Plants and Pests.

Climate change will undoubtedly make gardening more challenging. Yet those who keep abreast of changing conditions and institute sustainable gardening practices will be more likely to succeed.

1 Information on plant hardiness, plant characteristics and growing conditions can be found at nurseries and sources such as the Morton Arboretum and Chicago Botanic Garden.

2 ‘Warmer and wetter’ does not mean our area will receive more precipitation on a consistent basis. Rather, there will be longer periods of drought interspersed with more intense periods precipitation, particularly during the growing season.