Would you like to dress up your tree with a garden around it? That’s fine — as long as it consists of compatible plants in the ground, and is not a raised planter.
Raised planters are a common sight around the bases of trees. They are often built of pavers, bricks or stones and filled with soil and plants. But they’re not a good idea: “That can do the tree serious harm,” said Stephanie Adams, a plant pathologist in plant health care at The Morton Arboretum in Lisle.
The bed will bury the bottom part of the tree’s trunk in moist soil and mulch, encouraging diseases and infections. “It’s a favorable environment for fungi, which are the primary cause of most plant diseases,” Adams said.
“The bark of a tree is meant to be above ground where it can stay dry,” she said.
A raised planter can also smother the tree’s roots by piling soil too deep on top of them. Tree roots normally grow just a few inches below the soil’s surface, where air and water can readily filter down to reach them. Piling a foot or more of soil on top of the roots can cut them off from the air and water they need.
What can you do around the base of a tree without damaging it? Here are some suggestions from Adams:
Spread mulch. A wide, level area of mulch on the soil around a tree has many benefits. It insulates the soil against extreme temperatures. It prevents moisture from evaporating during hot weather. As the mulch decays, it enriches the soil for plants’ roots. “Just be sure it’s not too deep or heaped against the trunk,” Adams said. Spread your mulch in an even layer 3 to 4 inches deep, and right at the base of the tree, keep it 2 to 3 inches away from the bark of the trunk. “If you have a garden under your tree, those plants can share the mulch patch,” she said.
Grow good companions. Trees can coexist happily with many kinds of plants. After all, they don’t grow alone in the forest. However, it’s important to choose carefully. Tree companions must be shade-tolerant species, because they will be under the tree’s branches in summer. They should be perennials or woody plants that last for years. “Long-lived plants don’t cause as much damage to the tree’s roots because you don’t have to dig all the time,” Adams said.
Adams likes small shrubs, such as compact cultivated varieties of hardy hydrangea. It’s best to plant them early in the life of the tree, so you aren’t digging large holes among established tree roots.
Many shade-garden perennials also work well, such as hostas, brunnera, ferns, columbine, and lungwort. “Perennials will improve the soil, because they contribute organic matter when they die back each winter,” she said. Add spring bloom with long-lived bulbs like early-flowering varieties of daffodils and shade-tolerant squill, puschkinia, snowdrops or glory-of-the-snow. Low-growing plants often used as ground covers include Canadian wild-ginger, sweet woodruff, bishop’s wort (Epimedium) and spotted dead-nettle (Lamium maculatum). Find more inspiration by visiting the Arboretum’s Ground Cover Garden (mortonarb.org/explore).
Use containers for annual flowers. Impatiens and begonias are not good choices around a tree, because digging to plant these annuals in the ground every year damages the tree’s roots again and again. Instead, use these colorful flowers in containers filled with potting mix. Set the pots under the tree on plant stands or elevate them on bricks or sticks so they don’t block water and air from flowing through the soil to the tree’s roots.
Give up on grass. Turf grass grows poorly under trees because it requires sun, and trees create shade. It’s also not good for the tree, because it competes too much with the tree roots. “Perennials and shrubs grow in clumps, so the tree’s roots can find a way in between them,” Adams said. “Grass roots are more like a blanket that blocks everything.” Replace grass under a tree with mulch or a perennial garden.
Avoid edging. Let your garden or mulched area have a soft, natural boundary rather than pounding metal or plastic edging into the soil around a tree. “The edging can interfere with tree roots,” Adams said. If the roots are trapped by the edging, they start to circle inside it rather than stretching out from the tree the way they should. Over time, circling roots can begin to strangle the tree’s trunk. “Skip the edging and let your tree’s roots grow free,” she said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or firstname.lastname@example.org). Beth Botts is a staff writer at the Arboretum.