If you looked out your windows early this morning, you may have noticed Chicagoland was covered in a cold, white glaze. Or maybe you were laying comfortably in your warm bed, listening to the sound of neighbors scraping ice off their car windows? Seems Jack Frost has come to visit us once again. It’s a common misconception that our gardens are done growing after the first frost of Fall. And that may be true for less cold-tolerant plants like patio tropicals and coleus. But species adapted to our area are hardy, and it takes more than a cold morning or two to knock them down. So let’s take a closer look at frost in the garden…
TWO FLAVORS OF FROST
Frost is made of ice crystals that form when the air temperature drops below the point of freezing (32 degrees Fahrenheit). The ground need not be freezing as well; in fact, as I write this article there is frost covering the lawn, yet soil temperatures are close to 40 degrees. This is also known as a light frost, and damage to plants can be minimal if it quickly evaporates under the warming effect of morning sunlight.
We use the term killing frost to define what happens at temperatures of 28 degrees or colder. It’s still possible to have warmer ground temperatures, but a killing frost is cold enough to not only harm plants, but stop their growth completely. As farmers know all too well, killing frosts don’t obey any particular schedule – they can come late in summer and ruin a harvest, or come again late in Spring to kill off freshly planted crops.
A FROST IS NOT A FREEZE
A hard freeze is different from a frost, in that the ground itself reaches freezing temperatures. Most plants, even those adapted to colder climates, stop growing at this point. But not all play by this rule: the roots of a Japanese Maple, for example. continue to grow as long as the soil temperature is above freezing. The same can be said of many evergreen species. This is important to know, because those roots still need moisture as long as they’re active. It’s entirely possible for these kinds of plants to suffer or die from dehydration over the colder months, when we tend to think of them as being dormant.
TIMING IS EVERYTHING
Plants like dahlia, calla, canna and colocasia (giant elephant ears) won’t survive a Chicago winter. If you haven’t already done so, now is the time to dig up their flower bulbs and bring them indoors. And if you’re thinking of pruning trees and shrubs around this time of year, you’re better off waiting till late winter or early spring next year when the plants face improving weather conditions.
PREPARING FOR FROSTS AND FREEZES
A two or three inch layer of mulch can be used to form an insulating barrier between the soil below and air above. The goal here is not to prevent the ground from freezing, but rather to protect it from the adverse affects of repeated thawing and re-freezing as sunlight warms the ground and air temperatures rise and fall.
Mulch also helps the soil retain moisture. As mentioned before, some roots are actively growing during the winter and still need hydration. If the weather has been dry as we head into winter, it’s a good idea to give your gardens a watering or two while the weather still permits. Just be sure to detach garden hoses and bring watering supplies indoors before a hard freeze occurs.
The Famer’s Almanac puts together lists of average first and last frost dates, by area. For Chicago, these are around October 24th and April 20th, respectively.
The Illinois State Water Survey keeps a record of soil temperatures. To find the current soil conditions,click here to visit their website.