If you’re planning to plant trees or shrubs this fall, the most important thing you can do to ensure their success is not to plant them too deeply.
“Roots need to be just underneath the surface of the soil,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “If you’re not careful, it’s all too easy to bury them so far that the roots are basically smothered and the plant can’t thrive.”
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One reason that tree and shrub roots are often buried too deeply is that the roots are already too deep when you buy the plant. Gardeners are often told to plant new perennials at the same depth as in the pots they came in, but that’s not good advice for trees and shrubs.
“With trees and shrubs sold in pots or wrapped in burlap, the roots may be several inches below the surface of the soil,” Yiesla said. Before planting, you need to remove any surplus soil to find the correct depth for the plant.
Why is the correct depth so important? After planting, roots will gradually grow out sideways from the original ball of roots. They need to be surrounded by soil, but they also need air, which can only percolate from the atmosphere into the soil for a few inches. “If the roots are below the reach of air, the plant will struggle and likely fail,” she said.
The right depth is determined by the root flare, the spot where a tree or shrub’s trunk just begins to widen out into its main, structural roots. “It’s a curve, like the curve where your neck widens out into your shoulder,” Yiesla said.
The tree should sit in the soil with the roots covered but the flare visible: “Picture a crewneck T-shirt, where your shoulders are covered but the curve of your neck is exposed,” she said.
If you look around for a healthy older tree in your neighborhood, you’ll see that its trunk doesn’t rise straight out of the soil like a fence post or a telephone pole. As it emerges from the soil, it tapers and curves from the roots up into the trunk. “That curve tells you the roots are just beneath the surface,” Yiesla said.
Before you dig the planting hole for your new tree or shrub, use your fingers or a garden knife to probe the soil around the trunk. Find the flare of the major roots and remove all soil above that level.
The roots that matter are the large structural roots that will support and anchor the plant. You may also see some smaller roots that have sprouted above the structural roots because the plant was too deep.
“Trim them away,” Yiesla said.
Once you have located the root flare, measure from that point to the bottom of the root ball to determine how deep to dig the hole. Make the hole wide and shallow, and just deep enough in the middle so the plant will sit with its root flare at the same level as the surrounding soil.
As you refill the hole after you set the plant in place, stop partway and check to be sure the root flare is at the right level. Gently tamp down the soil around the plant. Then fill the hole just to the level of the root flare. If you have excess soil, don’t pile it on top of the root ball or around the trunk; spread it out evenly on the soil around the hole.
Then water the tree or shrub and spread an even layer of mulch 3 to 4 inches wide on the soil around it.
“Don’t pile the mulch against the plant,” Yiesla said. “You don’t want to bury the root flare in mulch any more than you want to bury it in soil.”
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.