Think you don’t need to worry about watering the garden because it has rained? Think again.
“Keep checking the soil to make sure that it is actually moist,” said Spencer Campbell, Plant Clinic manager at The Morton Arboretum in Lisle. “Don’t assume there has been enough rain for the water to soak down to where your plants can actually use it.”
Parts of the Chicago area were still in a condition of moderate drought in mid-August, according to NOAA’s Drought Monitor.
Plants need a steady supply of moisture in the layer of soil where their roots spread out beneath the surface: in the top foot for trees and the top 6 inches for most other plants. If enough water hits the soil surface, gravity will pull it down through the root zone and plant roots will be able to absorb what they need. But that’s a big “if.”
Green or orange blobs that move across a radar map on your smartphone or the TV news are no guarantee of rain in your yard. “Rain does not fall evenly,” Campbell said. “It can fall heavily somewhere in the metropolitan area and pass you by entirely, or rain very lightly.”
Even lightning flashes and booming thunder don’t necessarily mean major rain. “It can rain hard for a short time without raining enough to make a difference to your plants,” he said.
Don’t be deceived by soil that looks dark and moist. A light rainfall may dampen just the surface and leave the soil bone-dry a few inches down, where your plants’ roots live.
“There’s only one way to know whether there really is enough water in the soil, and that’s to check the soil,” Campbell said. Use a trowel or soil knife to dig down at least 2 inches and touch the soil to see if it feels moist. “It doesn’t need to be soggy, but it needs to feel damp,” he said.
When you water, try to imitate a long, slow rainfall that gives the water plenty of time to soak down deep. “Let the sprinkler run a good, long time,” he said. If you use a hose-end nozzle, go slowly, and play the water over the soil several times in every part of each planting bed.
When you water newly planted trees and shrubs, aim the water right at the base of the plant, where the roots are. “New plants can’t absorb water from a wide area because their roots don’t extend very far,” Campbell said.
Water new plants often, because their small, still-developing root systems can’t store enough water to keep them going for long. “It’s like having a car with a tiny gas tank that has to stop often to refuel,” he said.
And if you want to know when you’ve watered enough? “Get out your trowel and check,” Campbell 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.
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