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How you plant under trees is as important as what you plant
Tall trees may be the highlight of a landscape, but often it’s what home-owners do in the low light beneath those trees that add to the health and beauty of the scene. Proper landscaping procedures and an understanding of the potential for shade-tolerant turf, plants, bushes, and understory trees are key elements in creating and maintaining a panoramic view.
“The desire to do something under and around existing trees is very common, especially because turf grass often struggles in those areas,” says Gary Johnson, professor of urban forestry at the University of Minnesota. “However, you need to consider the entire landscape and understand the impact that the garden will have on the trees overhead, as well as how those trees will affect the plants underneath their branches.”
Root of the problem
Johnson says gardeners intent on planting under a tree tend to focus only on the garden, often to the detriment of the tree. “Trees are typically injured, and often killed, either because tree roots are damaged while planting the garden, oxygen to tree roots is cut off by creating a raised planting bed for the garden, or herbicides are misused while maintaining the garden,” he says.
Rototilling is a common mistake, according to Johnson. “Even the largest trees depend primarily on water, oxygen, and nutrients taken up by roots in the top foot of soil. Running a rototiller six to eight inches deep destroys those roots,” he says.
“It’s all about the roots,” agrees Kathleen Cue, associate county Extension agent for horticulture in Omaha, Neb. “There’s a misconception that all tree roots grow downward. Several large ones do - to support the tree - but 90% of a tree’s roots spread out pancake-like only 12 to 18 inches deep.” Cue says damaging those roots with tillage, by cutting them out to make room for plants, or by cutting a trench more than six inches deep to install edging materials can place the tree under stress. “Trees are slow to react to stress and symptoms may not appear for two to three years,” she says.
“Insects and diseases are quick to attack a tree under stress,” says Johnson. “Bark beetles attack weakened conifers, and wood boring insects, like flat-headed borers, exploit stressed deciduous trees like the ash, oak, and maple. If the tree dies it is blamed on insects, but it was caused by root damage.” Instead of tilling to remove grass and weeds under trees, Johnson recommends an alternative. “Put down a layer of newspapers, wet them down and then add several inches of mulch. This kills the vegetation, yet dissolves to allow moisture and air through. Then, use a shovel or tiling spade to place the plants and simply move over if you hit a tree root.”
Cue suggests that gardeners use perennials or ground covers, rather than annuals, when planting under trees. “Annuals are planted yearly so you increase the risk of damaging tree roots,” she notes. Many gardeners are surprised to learn that adding soil around the base of a tree to create a planting bed is also a mistake. “Adding even a few inches of soil over the tree roots can kill the tree because it puts the shallow feeder roots further away from needed moisture and oxygen,” says Cue.
“Instead of soil, use an organic mulch containing composted leaves, wood chips or decomposed manure,” says Johnson. “These mulches allow water and air through. Add the mulch in two to four inch layers over time, but don’t get it too thick. It needs to dry out so tree roots don’t grow up and into it as this could cause stem girdling - a process where roots grow back towards the tree and wrap around the trunk like a tourniquet.”
Experts advise against using products such as landscaping fabric or plastic under the mulch. A layer of rock is also a poor choice because it increases soil temperature and holds less soil moisture. When adding mulch to create a raised bed and cover exposed tree roots, Johnson cautions against piling it against the tree trunk. “This keeps the bark wet and can create a haven for bacteria and fungi,” he says.
Protected root zone
Tree roots typically spread out two to three times the radius of the tree canopy. However, for practical purposes landscape experts recommend that home owners establish a “protected root zone” under the drip line, or shadow, of the tree. Within this zone, soil disturbance and grade changes should be minimal and tree roots protected. “If trenching is necessary, avoid this protected zone,” says Cue. “Any paving, such as a sidewalk, should also avoid this area unless pervious pavers are used.”
Johnson says another approach is to size the “protected root zone” using a formula related to the diameter of the tree trunk. “For every inch in trunk diameter at your chest height (roughly 4 1/2 feet) allow one foot for the radius of the protected zone around the tree. For a tree with a 10-inch trunk diameter, protect roots within 10 feet of the trunk, or in a circle with a diameter of 20 feet,” he says.
Selecting plants
Chris Starbuck, Extension plant specialist at the University of Missouri, says the first choice is whether to attempt growing grass. “Stiff competition for moisture from tree roots and shading from the tree canopy make this difficult,” he says. “This is especially true with trees like maples, which have dense foliage and shallow root systems.” Contrary to common belief, shading is only a minor problem under most trees. “They can be trimmed up to about 12 feet to let in more light, but don’t trim from more than one-third of the trunk height,” adds Starbuck.
“Trees with compound leaves and coarse root systems, like ash, honey locust, and Kentucky coffee trees are friendlier to grass and plants growing underneath,” says Johnson. “However, it’s difficult to grow anything under maple, conifer, and poplar trees.”
Starbuck says there’s a wide choice of shade-tolerant plants to choose from for under-tree gardening. “Ground covers like pachysandra, vinca, and liriope are good choices. Be aware that in southern regions, vines like wintercreeper and English ivy can choke out native vegetation. Any of these eliminate mowing near trees which could damage exposed roots or the tree trunk.”
Spring flowering bulbs are a good choice under deciduous trees because they grow and bloom before the tree’s leaf canopy forms. Some that perform well are grape hyacinths, crocus, bluebells, and daffodils. “Plant them in a ground cover so their brown foliage will be hidden later in the season,” says Starbuck.
Other shade-loving plants include hosta, ginger, astilbe, and ferns, Cue adds. “Shrubs you may want to try are azalea, rhododendron, chokeberry, hollies, and leatherwood. Just remember to choose plants that do well in the dry shade that exists under trees.”
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