Invasive or native? Telling tree of heaven, white mulberry, Norway maple, mimosa and Callery pear from the natives they resemble

Seven invasive trees pass for natives, and one sign on a leaf, a twig or a flower tells each from its look-alike. Tree of heaven leaflets are smooth-edged except for a few glandular teeth at the base, where sumac and black walnut leaflets are toothed all along. A snapped Norway maple leaf stalk bleeds milky sap, and a native maple’s runs clear. The table gives each pair, its sign and the native we grow in its place.

The sign in the third column settles most trees, and the fourth checks a doubtful one. We grow every native in the last column, though not sugar maple or honeylocust.

Tree of heaven, sumac or black walnut

Tree of heaven is told from sumac and black walnut by its leaflet edges, smooth except for a few glandular teeth at the base. Sumac and walnut leaflets are toothed all along, and young saplings and root suckers are the ones most often taken for sumac. A crushed leaf stalk or a snapped twig gives off a strong, offensive smell that reminds some people of peanut butter. Black walnut smells strong too, but few people call it unpleasant. In winter, look for the large, heart-shaped leaf scar.

One female tree can bear more than 300,000 seeds a year, and the roots send up suckers as far as 50 feet away. Its roots, leaves and bark hold chemicals that can keep other plants from taking hold. It is also the preferred host of the spotted lanternfly, an insect from Asia that feeds on native trees too.

Pull a seedling from moist soil with all of its root, since root fragments sprout. A sucker joined to an old root is nearly impossible to pull, and cutting the tree only makes more:

“Mechanical methods, such as cutting or mowing, are ineffective, as the tree responds by producing large numbers of stump sprouts and root suckers.” (Penn State Extension, Tree-of-Heaven)

An established tree is treated with a systemic herbicide from July to the first fall color, when it moves sugar to its roots. If it must come down, it is cut about 30 days after treatment, once the symptoms show.

White mulberry or red mulberry

White mulberry leaves are glossy and smooth on top, with hairs beneath only along the veins. Red mulberry leaves are dull and rough on top, hairy over the whole underside and somewhat larger. Both can be lobed or unlobed, so the lobes tell nothing, but red mulberry’s bud scales have dark brown edges. The place helps too:

“Perhaps the best indicator is the growing location. If the tree is in a high-quality soil environment or inside a woodland, it’s likely a native red.” (University of Maryland Extension, Invasives in Your Woodland: White Mulberry)

Birds spread white mulberry seed, and the tree crosses with red mulberry, making hybrids that lean toward the white parent. Blue Whale Trees grows its red mulberry from seed, and seed source matters, since any white mulberry within pollen range crosses with a red one. Pull white mulberry seedlings as soon as they appear.

Norway maple or sugar maple

Norway maple is told from sugar maple by its sap: pull a leaf from the twig, and a drop of milky white sap beads at the base of the stalk. Native maples have clear sap. The two wings of a Norway maple seed spread wide like a coat hanger, where sugar maple’s curve like a horseshoe.

A Norway maple leaf has five sharply pointed lobes, with deeper notches between them than a sugar maple leaf. Its buds are large, turban-shaped and green to purple, where sugar and black maple buds are brown and sharp-pointed. Norway maple casts a shade so dense that it holds back the wildflowers, shrubs and tree seedlings beneath it. We do not grow sugar maple, and black maple is its close relative that we do grow.

Mimosa or honeylocust

Mimosa is told from thornless honeylocust, the native it is often confused with, by its fluffy pink flower heads in summer. Honeylocust bears small, greenish clusters in late spring. Out of flower, a mimosa leaflet is about 3/8 inch long and a honeylocust leaflet 1/2 to 1 1/2 inches. Mimosa leaflets also fold up at night and when touched.

Mimosa is invasive in North Carolina and other parts of the Southeast, where it seeds into roadsides, clearings and floodplains. It sprouts again when cut. We do not grow honeylocust. In its place, eastern redbud carries magenta flowers on bare branches in early spring, and bristly locust, a native locust, flowers rose-pink in May.

Callery pear, serviceberry or wild plum

Callery pear is told from serviceberry and wild plum by smell: its white blossoms give off a strong, rancid odor, and neither native is known for foul-smelling flowers. Closer in, the anthers at the center of each flower are purple, where serviceberry and plum anthers are yellow.

The fruit settles any doubt. Callery pear bears hard, round fruit a third to half an inch across, light brown with pale spots, in fall. Downy serviceberry ripens blue-black berries in June, and American plum ripens red to yellow plums in August. Wild Callery pears carry thorn-like spurs up to 3 inches long, but American plum is thorny too.

Bradford and the other named Callery pears were bred to bear sterile fruit, yet different ones cross and set fertile seed. Birds spread it, and the seedlings grow into thorny thickets along roadsides and in fallow fields. Downy serviceberry, which rarely passes 25 feet, is a fine substitute.

Princess tree and Siberian elm

Princess tree is often confused with southern catalpa, and both carry big leaves and upright clusters of flowers. It blooms violet before its leaves open, then bears egg-shaped capsules holding thousands of tiny winged seeds. Southern catalpa blooms white, spotted purple and yellow, in May and June, and hangs long, slender pods through winter. A cut princess tree sends up shoots from its roots many feet away.

Siberian elm leaves are singly toothed, where the native elms it resembles have double-toothed leaves, with small teeth on the large ones. Rock elm, the native closest to it in leaf size and shape, has ridged, corky older bark, hairy seeds and a narrower crown. Siberian elm seeds into old fields, pastures, stream banks and prairies. It can shade out the native plants there.

Telling them apart as seedlings

A seedling shows the leaf signs in the table once its first true leaves open, and the flower signs wait until the tree blooms, at least 8 years for a princess tree. The seed leaves that come up first, and the native seedlings worth keeping, are in what a young tree looks like.

Pull seedlings from moist soil with the whole root, since root pieces of tree of heaven, Callery pear and princess tree sprout again. Norway maple and Siberian elm seedlings pull easily too. A tree of heaven that will not pull is likely a root sucker from an older tree, and it needs the summer treatment above.

Why pull them, and what to plant instead

Each of these trees takes ground the natives would hold. Tree of heaven and Callery pear form dense thickets, Norway maple shades out what grows beneath it, and white mulberry is breeding red mulberry away. The loss runs up the food chain, because nonnative plants feed fewer caterpillars. In a 2018 study of 159 yards, Desirée Narango, Douglas Tallamy and Peter Marra found that Carolina chickadees sustained their numbers only where nonnative plants made up less than 30 percent of the plant mass.

Match the native to the job the old tree did:

• A small flowering tree, where a Callery pear, mimosa or princess tree stood: the tree finder lists the natives we grow for it.
• A shade tree in place of a Norway maple or Siberian elm: large natives for fall color.
• A screen along a fence or a property line: natives for screening.
• Bradford pear, burning bush or privet in the yard: native replacements for each.
• A tree you cannot name: send a photograph of a leaf, a twig and the whole tree through the support page.

Common questions

Is the catalpa tree invasive?

No: northern catalpa and southern catalpa are native trees, though planting has spread both across most of the East. The invasive look-alike is princess tree, which blooms violet before its leaves open.

Is white mulberry invasive?

Yes: birds spread its seed into forest edges, old fields and rights of way. It also crosses with the native red mulberry, and the hybrids lean toward the white parent.

Is a mimosa tree invasive?

Yes, in North Carolina and other parts of the Southeast, where it seeds into roadsides and floodplains and sprouts again when cut. Eastern redbud and bristly locust are natives with magenta and rose-pink flowers to plant in its place.

Is the princess tree invasive?

Yes: it seeds into roadsides, stream banks and burned or insect-stripped woods. It is best controlled young, by pulling the seedlings with the whole root.

Sources

• Tree-of-Heaven: Penn State Extension (David R. Jackson, Art Gover and Sarah Wurzbacher, updated 2020), with the page Callery Pear: tree of heaven’s smooth leaflets with glandular teeth and its look-alikes, the offensive smell, the heart-shaped leaf scar, more than 300,000 seeds a year, root suckers up to 50 feet out, the chemicals in its leaves, roots and bark, pulling seedlings and the root suckers that will not pull, the quotation on cutting, and the summer herbicide window; Callery pear’s rancid blossoms, its fertile seed from crossed cultivars and its hard fruit.
• Invasives in Your Woodland: White Mulberry: University of Maryland Extension (Andrew Kling), with the series’ pages on tree of heaven, Norway maple, Callery pear and princess tree: white and red mulberry leaves, the quotation on growing location, hybrids closer to the white parent, pulling seedlings early, the spotted lanternfly’s preferred host, the deeper notches of Norway maple leaves, the coat-hanger and horseshoe seeds, Callery pear thickets, and princess tree’s flowers, capsules, sites, first flowering and shallow-rooted seedlings.
• Tree-of-heaven: Woody Invasives of the Great Lakes Collaborative, with its pages on white mulberry, Norway maple, Callery pear and Siberian elm: the look-alikes of each, saplings and suckers taken for sumac, the walnut smell, red mulberry’s hairy underside, the clear sap of native maples, Norway maple’s shade, Callery pear’s purple anthers, thorns and fruit, Siberian elm’s single teeth and spread, rock elm’s bark, seeds and crown, and the seedlings that pull easily.
• Tree-of-Heaven, Ailanthus altissima: Virginia Tech Dendrology, with the fact sheets for staghorn sumac, black walnut, white and red mulberry, Norway, sugar and black maple, mimosa, honeylocust, royal paulownia and southern catalpa: the peanut-butter smell, the leaf scars, toothed sumac and walnut leaflets, the rough red mulberry leaf, the mulberry and maple buds, the leaflet sizes and flowers of mimosa and honeylocust, and the leaves and flower clusters of princess tree and catalpa.
• Albizia julibrissin: NC State Extension Gardener Plant Toolbox, with the page Paulownia tomentosa: mimosa confused with thornless honeylocust, its leaflets that close at night and when touched, its invasive spread and resprouting, eastern redbud and bristly locust among its native alternatives, princess tree confused with southern catalpa, and its root shoots after cutting.
• Nonnative plants reduce population growth of an insectivorous bird: D.L. Narango, D.W. Tallamy and P.P. Marra, Proceedings of the National Academy of Sciences 115 (2018): fewer caterpillars as nonnative plants increased, and chickadee numbers sustained only where nonnative plants made up less than 30 percent of the plant mass, in 159 yards.