It’s not even officially summer, and already much of the Chicago area is in a drought. “Get ready for a summer with a lot of watering,” said Julie Janoski, manager of the Plant Clinic at the Morton Arboretum in Lisle.
The National Drought Mitigation Center’s U.S. Drought Monitor map shows that all or parts of Cook, Lake, McHenry, DuPage, Kane, Boone and DeKalb counties are in moderate or severe drought, while Will County is abnormally dry.
Rainfall has rarely been this scarce: The spring of 2021 ended as the third-driest on record in Chicago (going back to 1871) and the driest since 1934, according to the Illinois state climatologist.
With no relief in the forecast for summer, homeowners should plan to water regularly to help their plants make it through the season, Janoski said.
“The most vulnerable plants are trees, shrubs and perennials that you just planted this spring or in the last year or two,” she said. “They haven’t fully developed their root systems, so even in a normal year, they would need extra watering. And this is not a normal year.”
It’s not just new plants, though. In a drought year, all plants — even mature trees — need watering. “Don’t take older plants for granted and assume they can fend for themselves,” Janoski said. “A drought year can do them serious, long-lasting harm.”
Plants absorb water through tiny, fine roots that extend widely through the soil. If the soil dries out, those small roots also dry out and die. Then, when it eventually rains, the plants will no longer have all the roots they need to take in enough water.
“Trees that are damaged in a drought year can remain stressed for years and eventually die,” Janoski said. Other plants don’t take that long: A week or a few days of dry soil can doom some annual flowers, herbs or newly planted perennials. Containers, which dry out quickly, often need to be watered daily in hot weather.
For plants in the ground, long, slow watering is best. “If you’re applying water with a sprinkler, let it run long enough so the water can soak down at least 3 to 4 inches into the soil,” Janoski said.
For young trees, you can turn the hose down to a trickle and lay it on the soil near the trunk. For older, larger trees, which have a widespread underground root system, you will need to water a larger area. You can do that with a sprinkler, or by moving the trickling hose around.
How much should you water? A common rule of thumb is that most Midwestern landscape plants need the equivalent of 1 inch of rainfall a week. However, that’s just an average that doesn’t account for hot weather, and it’s hard to measure in a home garden.
“What really matters is whether there is moisture in the top 6 or 8 inches of soil, where most plant roots are,” Janoski said. “The easiest way to find that out is to dig down a few inches and feel if the soil is moist.”
Spreading a wide, even layer of mulch over the soil around plants helps roots stay moist by reducing evaporation and insulating the soil against the hot summer sun.
“Over the next few months, our plants will need us to do everything we can to help them avoid drought stress,” Janoski 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.