A tree surrounded by green lawn is a pretty picture for many people, but it’s hard to achieve — and tough on the tree.
“Turf grass and trees don’t go together well,” said Stephanie Adams, a plant pathologist at The Morton Arboretum in Lisle. “It’s not a good situation for either of them.”
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In the shade of a tree, where it doesn’t get enough sunlight, grass will usually struggle, becoming patchy and bare. “Grasses are full-sun plants,” Adams said. They are native to open grasslands, such as the steppes of Central Asia, where they evolved to grow in full sunlight with no shade.
Most lawn grass needs six to eight hours of sunlight every day. Although some fescue grass species included in seed mixtures labeled for deep shade can get along on as little as two hours, the grasses typically used in American lawns will not thrive in the shade of a mature tree. Where grass is planted under trees, it can also cause problems for the tree. “Turf grass has a mat of shallow, dense roots that intercepts water, nutrients and air,” Adams said.
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Tree roots grow horizontally, spreading out widely in the top foot or so of soil. Rainwater and air should be able to percolate through the soil to be absorbed by the roots and used to support the growth and health of the tree. But if the tree roots are topped with grass, the turf gets first dibs on water and other resources, and less is available to the tree.
Research has shown tree roots grow much better under a wide, even layer of mulch than beneath grass. The mulch imitates the layer of leaf litter that naturally falls onto the soil around the tree and covers the soil containing its roots. Unlike turf, it allows water and air to seep through easily.
What if you don’t want a wide circle of bare mulch around every tree? “You can still have plants under trees,” Adams said, “but other plants are better than grass.”
Rethinking the area around your tree is an opportunity to extend your garden and make it more diverse, said Julie Janoski, manager of the Arboretum’s Plant Clinic.
The best choices, according to Adams and Janoski, are shade-tolerant perennials or small shrubs. “Woody shrubs grow similarly to trees, with the same kind of roots,” Adams said. “They won’t compete with a tree’s roots nearly as much as grass roots do.”
The roots of shrubs and perennials grow in clumps, not in a mat, leaving gaps for the tree’s roots. “All the roots will intertwine, and they will be able to share the soil and get what they need,” Adams said.
Conventionally, grass is replaced with a wide area of a single ground cover, such as pachysandra, myrtle, or liriope. “That’s better for the tree than grass,” Janoski said, “but it would be even better to be creative and mix it up.”
A mixed planting that includes flowering perennials and shrubs will support pollinating insects, birds and other wildlife, as well as being more interesting took at.
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Be sure to choose plants that can tolerate dry shade. “Because the tree acts like an umbrella and blocks some rain, plants under a tree need to be drought tolerant,” she said. Among the plants to consider are ferns, Solomon’s seal, bleeding heart, lungwort, Virginia bluebells, hostas, hellebores, bishop’s hat, St. John’s wort, fragrant sumac and bottlebrush buckeye. The Plant Clinic can make more recommendations for plants that grow well under trees.
The best time to add plants under a tree is when the tree is young, before its roots have grown out wide. When adding plants under a mature tree, use small plants and try to damage as few tree roots as possible.
“Put the health of the tree first,” Adams said. “Mature, healthy trees add to your property value, and that’s because they have decades invested in them. Turf grass can be replaced quickly and easily, but it should only grow where it won’t harm your trees.”
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.
