Trees trunks come in all shapes and sizes

What shape is a tree? For most people, it’s like a lollipop or a feather duster: a single trunk topped by a green bunch of branches and leaves.

“Not all trees are like that,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. Some kinds of trees, such as spruces and firs, start branching just above the ground, and other species have more than one stem.

“It’s perfectly normal for a hedge maple or a serviceberry to have three or more main trunks,” she said. “Other species, such as river birches and redbuds, can have either one trunk or several.”

Typically, the stems of a multitrunk tree will all branch from one central trunk down at the base. When you plant the tree, make sure that the root flare of that main trunk — the place where the trunk flares down and out into the roots — is at or just above soil level. “If multiple trunks are coming up out of the soil, the tree is planted too deep,” she said.

When trees are planted too deep, the moist soil that surrounds the bark can cause decay and disease. “If all the bark around the base of the tree rots, that will destroy the ability of the tree to move water between its roots and its leaves,” she said. If a tree is too deep in the soil, it also means that the tree’s roots are buried too far down, so they can’t get air through the soil.

Like all trees and shrubs, multitrunk trees benefit from having a wide, even layer of mulch over their roots. It’s important to make sure that the mulch is not heaped up against the bark, no matter how many trunks or stems a tree or shrub has. “Mulch piled against the bark will hold moisture that can lead to decay,” Yiesla said. “It can also provide cover for animals that chew the bark.”

If a tree has several trunks, is it still a tree? or is it a large shrub? “It’s a gray area,” Yiesla said. “The line between trees and shrubs is very murky.”

Biologically, there’s no difference: They are all woody plants, with stems and branches made of stiff, woody cells that don’t die off in winter. They all have bark. “Mostly, we distinguish trees and shrubs by size,” Yiesla said. Woody plants that naturally grow more than 15 feet tall are conventionally called trees.

It’s not a hard-and-fast rule. “We think of lilacs as shrubs, but if you let a lilac go in full sun it can easily get to be 20 feet tall,” she said. In Midwestern landscapes, yews are usually kept small by pruning, but if you stop pruning a hedge of English yew, it will eventually grow 30 or 40 feet high. In England, there are towering yew trees hundreds of years old.

While Midwesterners picture oaks as huge trees, such as our native white oak and bur oak, many species of oak grow just a few feet high.

Some plants, such as common witch-hazel, are conventionally described as “small tree or shrub” because they can be thought of either way. Usually, the things we call shrubs have more stems than the things we call multitrunk or multistemmed trees.

It’s possible to train a multistemmed shrub, such as a rose bush or some kinds of hydrangea, into a single-trunk form, like that lollipop-shaped tree. You need to start when the plant is very young and consistently prune away all but one stem. This technique is often used in formal gardens; the result is called a “standard.”

Since the plant naturally wants to be multistemmed, a standard requires a lot of pruning to keep its shape. Any plant is much easier to care for if it is allowed to grow in its natural form.

“In your garden, it makes sense not to worry about the tree or shrub labels and just think about what you want a plant to do,” Yiesla said. A river birch, with its pretty peeling bark, might be an eye-catching accent, regardless of whether it has one trunk or five. If what you’re looking for is screening or a green backdrop, a more dense multistemmed shrub may be what is called for.

“It’s most important to make sure that whatever kind of plant you choose, it is suited to the conditions of your yard, such as the available sunlight and soil conditions,” Yiesla said. “Just as each species of plant has its natural form, each species of plant has particular needs. It will only be healthy if those needs are met.”

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.