Many homeowners wonder how to get grass to grow beneath a tree, where it often is bare or patchy.
“It’s better for both the grass and the tree if you don’t try,” said Spencer Campbell, Plant Clinic manager at The Morton Arboretum in Lisle.
Since nearly all the grasses we use for lawns are plants that need full sun, they won’t grow well in a tree’s shade. At the same time, their mat of shallow, dense roots can cause problems for the tree, preventing water, air and nutrients from getting down to the tree’s roots.
Caring for grass under trees creates another danger: Lawnmowers and string trimmers can easily damage the tree’s bark. Just beneath that bark lies critically important, very vulnerable tissue that transports water and nutrients between the leaves and the roots. “If the bark is chewed up by power tools, the damage can weaken or kill the tree,” Campbell said.
The best approach is to place some distance between the tree’s trunk and the lawn. “Spread a wide, even layer of a nice organic mulch around the tree,” he said. “That will push the grass out closer to sunlight, and the mulch will create a buffer zone that keeps power tools away from the bark.”
Unlike grass, the mulch will allow air and water to easily seep down to the tree’s roots, which spread out horizontally just below the surface of the soil.
Research has shown that trees whose roots are covered with mulch grow better and are healthier than trees that have grass growing up to the trunk. The mulch also will insulate the tree’s roots against extremes of heat and cold. It holds moisture in the soil in dry weather and, as it decays, it will enrich the soil.
If you prefer the look of plants beneath a tree, you can add shade-tolerant plants, such as ground covers or hostas. “There is a wide choice of plants that are better adapted to shade than grass is, and many of them won’t compete as fiercely with tree roots as turfgrass does,” he said.
The Arboretum’s Search Trees and Plants tool (mortonarb.org/search-trees-and-plants) can help you find shade-tolerant alternatives to turf, or consult the Plant Clinic (mortonarb.org/plant-clinic).
It’s best to install these plants when the tree is young and has not yet spread out a wide root system, so you aren’t cutting too many tree roots when you dig holes. If you must plant below a mature tree, use small plants so you can dig small holes. Spread mulch around these plants too; they will benefit as much as the tree does.
Grasses don’t mix well with trees because they did not evolve to live together. Nearly all the grass species we use for lawns come from wide-open, mostly treeless grasslands where they live under the open sky and all-day sun.
Most lawn grass species will only thrive if they get at least six to eight hours of direct sunlight every day. Some grass seed mixes that include fescue grasses and are labeled for shade may be able to get along with somewhat fewer hours of sunlight, but even they will not perform well in dense shade.
“Plants grow best when they are in the right place,” Campbell said, “and underneath a tree’s branches is not the right place for grass to grow.”
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.