By Jil Swearingen
From the April 2023 Issue
There are thousands of plants to choose from when considering what to install in client landscapes. Unfortunately, many eye-catching plants commonly grown and sold in the U.S. nursery trade are also harmful invasives in our ecosystems and agricultural environments. Currently, a staggering 1,050 non-native invasive plant species are documented as impacting natural areas in the U.S.
While many landscapers know the effects of non-native invasives, others may not realize the ripple effect a suburban planting can have on the larger region. Let’s revisit some of the ways invasive plants impact natural areas. They include:
- Reducing populations of native species, locally and regionally.
- Replacing native plants with non-native species that do not support native insects, which are essential food for birds and other wildlife.
- Forming monocultures.
- Hybridizing with native plants, diluting and altering their qualities.
- Killing native trees and shrubs by girdling (i.e., blocking the movement of water and nutrients through vascular tissue)
- Shrinking urban forests where invasive vines kill trees along edges by smothering and girdling, leading to uprooting and toppling of covered trees.
- Suppressing mycorrhizae (link), the beneficial soil fungi that help native plants take up water and minerals.
- Altering soil acidity (pH) and functions.
- Altering hydrological conditions.
- Increasing frequency and intensity of wildland fires.
Cost & Management
Managing invasive species costs billions of dollars annually, $120 billion by one recent estimate. The problem has become so extensive, that some states have banned landscaping plants known to be harmful invasives. Yet many are still sought after by consumers. As a result, in some cases, sterile varieties of popular landscape choices have been introduced—which brings its own complexities.
The now infamous Callery pear tree, with varieties such as “Bradford,” “Aristocrat,” and “Cleveland Select,” is illegal to sell in Ohio as of this year. Similar bans in South Carolina and Pennsylvania take effect in 2024. In North Carolina a “Bradford Pear Bounty Program” provides a free native tree for every Bradford cut down (up to 5), and the Missouri Invasive Plant Council (MoIP) again offered its Callery Pear “buyback” program to receive a free replacement tree just this April.
Japanese barberry (Berberis thunbergii) is another common landscaping plant on the legally banned list in many states such as New Hampshire, Maine, Massachusetts, Minnesota, and Pennsylvania. In some cases, like New York, which originally banned barberry, sterile versions developed since 2017 are allowed. (Sterile varieties include the WorryFree® Collection including Crimson Cutie® and Lemon Glow®. Lemon Cutie® and Mr. GreenGenes™ will be introduced in 2023. These barberries were bred by Dr. Mark H. Brand at the University of Connecticut, a process that took 15 years.) Another non-native invasive, burning bush, has had a sterile variety since 2011.
Yet even sterile varieties can potentially be problematic. In some cases, they can cross pollinate with fertile species and perhaps even revert to fertility over several years—continuing their invasive characteristics. Even if truly unable to spread, sterile varieties of non-natives don’t offer the same eco-benefits as a comparable native plant.
Plant This, Not That
The species and varieties highlighted below are but a tiny sample of invasive plants commonly grown and sold in the U.S. Because these plants are damaging
our environment and harming wildlife, it’s important to avoid them when designing a landscape or conducting plant installations.
Many hardy, locally adapted native species are available and the survival of native birds and other wildlife is dependent on them. Support your local native plant nurseries by buying plants grown from seed, not collected from the wild, and specify plants native to your ecological region and growing conditions. Avoid cultivars of native species which may have modifications that make them less nutritious for native wildlife. To search if a plant is native in your area, try npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysearch. USDA Plant Hardiness Zones have been provided as a reference, but does not mean a plant is actually native to all included zones.
Editor’s Note: Don’t assume a plant is the best choice simply because it’s native. Even among natives, “right plant, right place” applies. Research the native options mentioned to determine if they are appropriate for the specific site. For instance, Viriginia creeper has many positive qualities. Hardy from zones 3 to 9, it’s drought and salt tolerant and grows in any soil or light conditions. It also feeds birds and rodents, is the larval host for several species of moth, can prevent erosion, and turns beautiful colors in Fall. However, the berries are toxic to humans and the sap can be irritating to the skin. Additionally, as a nearly “bulletproof” vine, it can easily overwhelm an area and choke and kill other plants. Once established, it can be difficult to remove.
I have both Virginia creeper along a fence and American wisteria along the eaves of a free-standing garage. While beautiful and attracting many birds—Northern flickers visit the creeper; robins built a nest in the wisteria—these plants require vigilant cutting back and control.
- Porcelainberry (Ampelopsis glandulosa var. brevipedunculata). Native options: Summer grape (Vitis aestival var. aestivalis), Zones 5-8; California wild grape (Vitis californica), Zones 7-10; and Virginia creeper (Parthenocissus quinquefolia), Zones 3-9.
- Asian bittersweet (Celastrus orbiculatus). Native options: Virginia creeper (Parthenocissus quinquefolia), Zones 3-9.
- English ivy (Hedera helix). Native options: woodland phlox (Phlox divaricata), Zones 3-8; mistflower (Conoclinum coelestinum), Zones 5-9; wild ginger (Asarum canadense), Zones 4-8; and golden ragwort (Packera aurea), Zones 3-8.
- Chinese wisteria (Wisteria sinensis) and Japanese wisteria (Wisteria floribunda). Native options: American Wisteria (Wisteria frutescens) within its native range (Zones 5-9), Canada moonseed (Menispermum canadense), Zones 5-8, highly toxic if eaten; and virgin’s bower (Clematis virginiana), Zones 3-9.
- Japanese angelica tree (Aralia elata). Native options: devil’s walking stick (Aralia spinosa), Zones 4-9.
- Leatherleaf mahonia (Berberis bealei). Native options: smooth winterberry holly (Ilex laevigata), Zones 5-9; winterberry (Ilex verticillata), Zones 3-9; and arrowwood viburnum (Viburnum dentatum), Zones 2-8.
- Japanese barberry (Berberis thunbergii). Native options: spicebush (Lindera benzoin), Zones 4-9.
- Glossy, Chinese, and other privets (Ligustrum). The berries of these non-native shrubs are consumed and spread by birds. Privets displace many native shrubs that provide crucial food for wildlife. Native options: inkberry holly (Ilex nigra), Zones 4-11; and yaupon hedge (Ilex vomitoria), Zones 7a-9b.
- Sacred bamboo/nandina (Nandina domestica). The berries contain cyanide and can kill birds like cedar waxwings after consuming large amounts of fruit. Native options: red chokeberry (Aronia arbutifolia), Zones 3-8; smooth winterberry holly (Ilex laevigata), Zones 5-9; and winterberry (Ilex verticillata), Zones 3-9.
- Autumn-olive, Russian-olive, and thorny-olive (Elaeagnus spp.) Native options: red chokeberry (Aronia arbutifolia), Zones 3-8; and black chokeberry (Aronia melanocarpa), Zones 3-8.
- Butterflybush (Buddleja davidii). While it provides plentiful nectar, butterflybush spreads from plantings and is not a host plant for any native butterflies or other insects. Native options: sweet pepperbush (Clethra alnifolia), Zones 3-9; silky dogwood (Cornus amomum), Zones 5-8; buttonbush (Cephalanthus occidentalis), Zones 5-11; and sweetbay magnolia (Magnolia virginiana), Zones 5-9.
- Mimosa/Silk tree (Albizia julibrissin). Native options: honey locust (Gleditzia triacanthos), Zones 4-9; and black locust (Robinia pseudoacacia), Zones 3-8.
- Bradford pear (Pyrus calleryana). This tree has spread widely since the 1960s when it was heavily planted as an ornamental in residential developments. It now dominates fields and roadsides, which is obvious in Spring when it has white flowers. Native options: serviceberry (Amelanchier arborea), Zones 4-9; and southern crab apple (Malus angustifolia), Zones 6-8.
- Norway maple (Acer platanoides). Native options: red maple (Acer rubrum), Zones 3-9; and sugar maple (Acer saccha- rum), Zones 4-8.
- Princess tree (Paulownia tomentosa). Native options: southern catalpa (Catalpa bignonioides), bigleaf magnolia (Magnolia macrophylla), Zones 5-9; and umbrella magnolia (Magnolia tripetala), Zones 5-9.
- Italian arum/lords-and-ladies (Arum italicum). Native options: Jack-in-the-pulpit (Arisaema triphylla), Zones 4-9; and green dragon (Arisaema draconitum), Zones 4-9.
- Fig buttercup/lesser celandine (Ficaria verna). Native options: Any Spring ephemerals native to your area. Examples include: Dutchman’s breeches (Dicentra cucullaria), Zones 3-7; rue anemone (Thalictrum thalictroides), Zones 4-8; white wake-robin (Trillium grandiflorum); Zones 4-8; Virginia bluebells (Mertensia virginica) Zones 3-9; trout lily (Erythronium americanum), Zones 3-9; and spring beauty (Claytonia virginica), Zones 5-9.
- Nodding and common Star-of-Bethlehem (Ornithogalum nutans, O. umbellatum). Native options: Any Spring ephemerals native to your area. See above for examples.
- Japanese knotweed (Reynoutria japonica). Native options: Virginia knotweed (Persicaria virginica), Zones 4-8.
- Crimson fountain grass (Cenchrus setaceus). Native options: broomsedge (Andropogon virginicus), Zones 5-10; little bluestem (Schizachyrium scoparium) Zones 3-9.
- Pampas grass (Cortaderia selloana). Native options: Tall grass species native to the area.
- Chinese silver grass (Miscanthus sinensis). Native options: Grasses native to area.
While hardly exhaustive, this list is a good place to start in avoiding invasive plants.
Swearingen spent most of her career working on invasive species, mainly in the mid-Atlantic region, and is lead author of Plant Invaders of Mid-Atlantic Natural Areas, Field Guide, published in November 2022. She worked for the National Park Service for 22 years, where she served as Chair for the Plant Conservation Alliance’s Alien Plant Working Group, created the Weeds Gone Wild website, developed the Weed US database of invasive plants impacting natural areas in the U.S., co-developed the Invasive Plant Atlas of the U.S., and founded the Mid-Atlantic Invasive Plant Council.
Swearingen would like to acknowledge Rebekah Wallace and Triston Hansford of the University of Georgia, Center for Invasive Species and Ecosystem Health, for their suggestions of invasive plants in Southeastern landscapes; and Bill Johnson, for donating images.
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