Many plants have sprouted, bloomed or spread their leaves early this spring, which is a risky business.
“It’s not too late for a hard freeze or a snowfall,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum. “As lovely as it is to see those first flowers, the earlier that plants begin growing, the more vulnerable they are.”
Springtime swings from cold to warm and back to cold are nothing new in Chicago, but the climate has changed to make those swings even more frequent, extreme and unpredictable. Here are some tips from the Plant Clinic for protecting vulnerable garden plants.
Watch the forecast. “Pay attention when nighttime temperatures below 35 are part of the 10-day
weather forecast,” said Spencer Campbell, Plant Clinic manager. “If a forecast is in that range, a freezing temperature of 32 degrees is increasingly possible.”
Prepare for cold nights. “Most cold damage is done overnight,” he said. Even after a balmy day of T-shirt weather, nighttime temperatures can drop below freezing. At any temperature below 32 degrees, the water in plants’ tissues can turn to ice. When sharp crystals pierce the walls of plant cells and their water drains away, the tissue dies.
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Don’t try to protect everything. Some cold-hardy plants such as daffodils, hellebores and Siberian squill will probably be fine on their own unless the temperature is especially brutal. Others, such as flowering cherry or magnolia trees, are just too large for a homeowner to protect. “Focus on tender plants that you can reasonably expect to cover that are especially important to you,” Yiesla said.
Spread protection. On the day before a cold night is predicted, cover plants to trap warmer air around them and buffer them against cold, drying air currents. “Covering plants won’t save them in bitter cold, such as a dip in the polar vortex that brings single-digit temperatures,” she said. “But it can help insulate them against a few degrees below freezing.”
Use leaves if you have them. One of the easiest ways to protect low-growing plants is to rake dry leaves over them. The fluffy leaves do a good job of trapping air to insulate young seedlings and the crowns of sprouting perennials. “It’s one of many good reasons to save fallen leaves in autumn,” Yiesla said.
Spread sheets. Cotton sheets work well for covering plants. You also can buy floating row cover, a lightweight, nonwoven synthetic fabric that has minute holes for ventilation, at garden centers. Do not use the plastic sheeting that is sold for drop cloths and furniture covers: Sheet plastic will trap moisture around your plants, which encourages the growth of fungi and leads to disease.
Remove sheets during the day. A layer of leaves can stay in place, but all other coverings need to be removed during the day, when it’s warmer, so that light and air can get to the plants. Keep your covering material handy so you can still protect the plants overnight as long as the cold nights last.
Consider a cold frame. Since volatile spring weather is here to stay, some gardeners may want to invest in a cold frame — a sort of mini-greenhouse that provides a safer place for early seedlings and tender plants. “There are many designs for cold frames, both do-it-yourself and ready-made,” said Campbell. “Just remember to vent the cold frame during the day so plants don’t overheat.”
Don’t rush the season. Although our winters are becoming shorter and milder on average, that doesn’t necessarily mean it’s safe to plant earlier. In an increasingly variable and unpredictable climate, damaging cold is still possible all through spring. “If you want to gamble on planting earlier, think about how you will protect those plants against a cold swing,” he said.
Be on your toes. “We can’t rely on traditional dates for planting, transplanting, or bloom times,”
Campbell said. “Gardening now means following forecasts day to day and for the days ahead, watching the changes in your garden plants, and getting in the habit of preparing for the unexpected.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.