Would you like to decorate your tree with a garden around it? That’s fine – as long as they’re compatible plants in the ground and it’s not an elevated planter.
Raised planters are a common sight around the base of trees. They are often built of cobblestone, brick or stone and filled with soil and plants. But they’re not a good idea: “It can do serious damage to the tree,” said Stephanie Adams, plant pathologist in the Plant Health Division at Morton Arboretum in Lisle.
The bed will bury the base of the tree trunk in moist soil and mulch, encouraging disease and infection. “It’s a favorable environment for fungi, which are the primary cause of most plant diseases,” Adams said.
“A tree’s bark is supposed to be above ground where it can stay dry,” she said.
A raised planter can also smother the tree’s roots by dumping too much soil on them. Tree roots typically grow just a few inches below the surface of the earth, where air and water can easily 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 at the base of a tree without damaging it? Here are some suggestions from Adams:
Spread mulch. A wide, level mulch area on the ground around a tree has many benefits. It insulates the floor against extreme temperatures. It prevents moisture from evaporating in hot weather. As the mulch breaks down, it enriches the soil for the plants’ roots. “Just make 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 keep it right at the base of the tree 2 to 3 inches from the bark of the trunk. “If you have a garden under your tree, these plants can share the patch of mulch,” she said.
Grow good companions. Trees can happily coexist with many types of plants. After all, they don’t grow alone in the forest. However, it is important to choose carefully. Tree companions must be shade-tolerant species as they will be under the tree’s branches in summer. It should be perennials or shrubs that last for years. “Long-lived plants don’t damage tree roots as much because you don’t have to keep digging,” says Adams.
Adams likes small shrubs, such as compactly cultivated varieties of hardy hydrangeas. It’s best to plant them early in the tree’s life so you don’t dig large holes between established tree roots.
Many shade garden perennials also work well, such as hostas, brunnera, ferns, columbines, and lungwort. “Perennials will improve the soil because they contribute organic matter as they die back each winter,” she said. Add spring bloomers with long-lasting bulbs like early blooming varieties of daffodils and shade-tolerant squills, pushkinia, snowdrops, or snowdrops. Low-growing plants often used as ground cover are Canadian wild ginger, sweet woodruff, bishop’s weed (Epimedium) and spotted deadnettle (Lamium maculatum). Get inspired by visiting the Arboretum’s groundcover garden (mortonarb.org/explore).
Use containers for annual flowers. Impatiens and begonias are not good choices near a tree, since digging to plant these annuals in the ground each year repeatedly damages the tree’s roots. Instead, use these colorful flowers in containers filled with potting soil. Place 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.
Avoid weed. Peat grass grows poorly under trees because it needs sun and trees provide shade. It’s also not good for the tree because it competes too much with the tree roots. “Perennials and shrubs grow in clusters, allowing the tree’s roots to find a way between them,” Adams said. “Grass roots are more like a blanket that blocks everything.” Replace the grass under a tree with mulch or a perennial garden.
Avoid edges. Instead of pounding metal or plastic edging into the ground around a tree, let your yard or mulch area have a soft, natural border. “The edging can disturb tree roots,” Adams said. When the roots get pinched by the ledge, they begin to circle in it instead of stretching out of the tree like they should. Over time, circling roots can strangle the tree trunk. “Skip the edging and let your tree’s roots grow freely,” she said.
For tree and plant advice, contact the Plant Clinic at Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or email@example.com). Beth Botts is a staff writer at the Arboretum.