Crossing the Line: When Plants Become Invasive

Ivy, imported from Europe, roots at ever possible leaf node, and is considered an invasive plant.

Do you enjoy leafing through plant catalogs each winter?  I’ve been reading plant catalogs and making wish lists since buying my first home and garden more than 30 years ago.  I remember when glossy photos of autumn olive shrubs and Chinese Wisteria filled their pages, alongside day lilies, tree Peonies and endless varieties of Hosta.

I still enjoy reading the catalogs each season to learn about new plants and new varieties of old favorites.  David Austin Roses is required reading each winter, even though I no longer try to grow roses in my forested garden.

There is a long history of botanists and horticulturalists traveling around the world in search of new, beautiful and useful species of plants.  It is an essential part of our nation’s history to both send native American species to Europe, and to seek out and grow imported species here. 

You’ll hear wonderful stories of early colonists risking their lives and freedom to bring back some rice, or a tea shrub, or some other potentially productive and lucrative plant encountered on their travels, to put into production here in the ‘New World.’  Tony Avent of Plant Delights near Raleigh is one of many contemporary horticulturalists still importing new plants from elsewhere.

One of the trees imported from Asia was the white mulberry tree, Morus alba.  They were supposed to form the beginnings of a silk industry here in Virginia.  Sadly, the silkworm industry never took off in Virginia.  Worse, the white mulberry became an invasive species, even hybridizing with our native red mulberry.  But who knew that would happen in the Eighteenth Century?

Paper mulberry trees were imported to Virginia during the 18th Century to help establish a silkworm industry. This species is not invasive.

Another Asian tree imported during the Colonial era, to potentially support silkworms, is the paper mulberry, Broussonetia papyrifera, formerly known as Morus papyrifera.  You may have noticed these odd-looking trees lining Francis Street near the Colonial Capitol building.  They are not considered invasive, but the silkworms didn’t care for them.  In China, they were used in the production of early paper products.

It may take only a few decades for a wonderful new plant introduction to cause enough problems in its new environment to find itself reclassified as an invasive nuisance plant. The very qualities that make a new introduction exciting and marketable may also make it harmful to its new ecosystem.

Back in the 1980s, online nurseries advertised autumn olive, Elaeagnus umbellata, as tough, wildlife friendly shrubs for hedges, windbreaks and even for foundation plantings.  Their early spring flowers attract pollinators and their autumn fruits feed a variety of birds.  They thrive in a variety of soils, over a wide range in many parts of the country.  They are tough and persistent; too persistent.

Autumn olive blooms in early spring, then produces fruit in autumn. As birds spread the seed, it has become highly invasive.

Autumn olive is an attractive shrub.  It was marketed hard, sold very cheaply, and was planted all over the country.  It didn’t take long for those hungry birds feasting on autumn olives to spread their seeds far and wide.  The seeds germinate easily, and the shrubs grow quickly.  Young shrubs quickly shade out other plants around them, claiming more and more real estate as native species nearby are out competed.

It took a few decades for environmental scientists to realize the harm autumn olive does in crowding out native trees and shrubs.  Now you’ll find Elaeagnus umbellata on the Virginia Department of Conservation and Recreation’s list of highly invasive species, along with Elaeagnus pungens, thorny olive, which has a slightly lower ranking for invasiveness.  

The state of West Virginia once encouraged strip mining companies to plant autumn olive and multiflora rose, another invasive plant, on reclamation sites. Now those areas are mostly impassable, and native species haven’t been able to gain a foothold to actually ‘reclaim’ the land.

To rate a spot on the list of highly invasive species, there must be demonstrable evidence that a species poses a threat to Virginia’s forests, native grasslands, wetlands or waterways.  Autumn olive is considered invasive across all geographical regions of the state.  Other old favorites now on the list include yellow flag Iris and Ligustrum, which was planted all over my current property years before we purchased it, and now forms an important part of the landscape.

Japanese stilt grass grows around a stand of Lycoris radiata bulbs that are ready to bloom.

I recognize lots of old favorites and a few current foes on the long list of invasive plants.  Many of them came to North America accidentally, like Japanese stilt grass.  Others were imported to solve specific problems, like erosion control, or because of their hardiness and beauty.

In their home ecosystems, most often somewhere in Asia, these plants were considered useful.  Because there were animals that ate them, or the people used them, their spread wasn’t a problem.  They were part of the ecosystem.

But when you move a plant from one ecosystem to a novel ecosystem, the controls that kept them in balance suddenly are missing.  The imported plants often out compete the local flora because the local fauna doesn’t control them.  A plant that won’t be eaten by insects, deer or rabbits has a huge advantage in the nursery industry. This is one reason why imported plants often are favored over native species by landscapers and garden centers.

It’s hard to avoid using invasive plants.  You will find invasive plants for sale in local nurseries and in lots of plant catalogs.  Even plants on the official state list of invasive species won’t be labeled as such. Nurseries continue to sell popular plants, like English ivy or Japanese barberry because customers ask for them and buy them. If you don’t happen to know the plant is invasive, you’ll happily buy it, plant it, and then it may be years later before you realize the mistake.

Take the beautiful chocolate vine, Akebia quinata, that I ordered from a well-known nursery catalog and planted 7 or 8 years ago.  I was so pleased that I almost bought two of them for cross-pollination so I could get tasty fruits each summer.  The first few years I delighted in its unusual dark burgundy flowers, delicate leaves and twining habit.  A few years later, I noticed it growing up into some nearby trees.  Now, I invest hours pulling up its long running stems, clipping them out of trees and shrubs, and generally trying to tame it.  It pops up repeatedly in certain parts of my yard, a long way away from where I planted it all those years ago.

Akebia quinata, chocolate vine

I spend about as much time battling the Akebia vines as I do pulling up Japanese stilt grass, which sneaks up and around desirable plants.  Mosquitoes love it, so I don’t pull it as often as I should.  Invasive plants cost us so much, beyond the time to battle them.  They destroy ecosystems, interfere with food production, and push out desirable native plants.

In some cases, the invasive plant thrives in the same ecosystem as a native species from the same genus.  There are native Phragmites americanus in our waterways, growing near invasive foreign Phragmites australis.  These invasive aquatic reeds can expand on their rhizomes up to 16’ per year to choke waterways and shade out native plants.  They create a huge problem in local waterways, trapping sediment so the water grows shallower, with the European species out competing our native North American reeds.  Thus far, the only effective way to control them appears to be misting herbicides from the air, which is wildly expensive.

Phragmites choke local waterways, destroying habitat for native plants and animals.

A more recent mishap occurred when I planted a pot of Miscanthus sinensis ‘Fire Dragon’ into a raised bed at the Williamsburg Botanical Garden.  Someone had donated it to the 2020 plant sale, and I planted it without first doing the research.  Invasive grasses grow quickly and aggressively.  They produce thousands of air-borne seeds.  Another garden steward recently brought it to my attention, and so we are removing the seed heads and making plans to remove the plant.

Miscanthus, Chinese silver grass, has gained popularity in recent years because it is highly ornamental and very easy to grow.  Deer won’t bother it, and it quickly grows into a huge mound of vegetation.  M. ‘Fire Dragon’ turns scarlet and orange in the autumn.  It creates a problem when it takes root in meadows, fields, or other gardeners’ spaces.  It easily escapes cultivation to out compete other species.

Miscanthus seed heads towered over Canna lilies before they were cut away. The plant will be removed before the spring.

You would never know that many species are considered invasive when you find them at a garden center, and most invasive plant species may be sold without any warnings.  It is up to us, as informed gardeners, and responsible consumers, to do our homework and avoid buying, planting, and perhaps even avoid nurturing invasive plants.  This is difficult when the invasive is a plant you happen to like growing. 

I’m as guilty as anyone of tolerating some invasive plants.  I happen to like ivy, Hedera species.  I sometimes plant it in pots on the patio, rationalizing that I’ll keep it in control. There is a long tradition of growing English Ivy in our area.  Just take a stroll through Colonial Williamsburg, and you’ll see it used as a ground cover and in many garden spaces.  Yet, you have probably seen ivy pull over a mature tree when allowed to grow rampantly through its branches, shading the tree and adding too much weight.

I also have beautiful shrubs which turn scarlet in the autumn, known as ‘burning bush,’ that grow throughout my neighborhood.  You may know them as Euonymous alatus.  This is another ornamental Asian, heavily marketed mail order shrub, gone wild across the landscape.  Just like autumn olive, it produces highly desirable seeds that birds spread around.  Our native Euonymous americanus, known as hearts a’busting or strawberry bush, is a better alternative, although it may be browsed by deer.

Euonymous alatus, burning bush, as it begins to turn scarlet in early December.

Other plants on the list, that may or may not surprise you, include creeping Jenny, Vinca minor, Japanese barberry, Chinese Wisteria, many types of honeysuckle, Norway maple and silver poplar.  Nandina domestica, known as heavenly bamboo, may make the list at some point in the future. Like burning bush and autumn olive, birds freely spread the seeds of this suckering shrub. Still a favorite of landscapers for its red winter berries, you’ll frequently find it used in holiday decorations. But it creates problems in many landscapes as it seeds itself around freely.

The best solution for stopping invasive plants from spreading is to refrain from buying or planting them in the first place.  The second is to remove them once we know they are invasive.  That may be easier said than done.  As many times as I cut back our autumn olive hard to the ground, it continues to send up new shoots.  An herbicide may be the only way to eradicate it.  

This green ground cover vine, Vince minor, has naturalized throughout our neighborhood. It is on the Invasive Species list and spreads by rooting at its nodes and by spreading seeds. It soon appears on any disturbed soil. The Italian Arum isn’t on the statewide list and isn’t invasive in our area. It has grown invasive in some areas of Northern Virginia, however. Each plant produces many seeds, spread by birds and small mammals.

These are sometimes tough calls to make.  Most of us already live in ‘novel’ landscapes composed of a mix of native and non-native plants.  Many imported plants don’t pose a threat and add to the beauty of our gardens, supporting wildlife and providing other benefits. 

Plants on the Invasive Species list may be plants we grew up admiring on roadsides, like Albizia julibrissin, mimosa, royal Paulownia, and Pyrus calleryana, or Bradford pear.  We may not even be aware of which plants are natives and which have been imported, until we do a bit of homework.

This is not to say that native plants won’t out compete their neighbors and spread aggressively, too.  If you have had goldenrod, Solidago species, colonize in your own yard you know what I mean.  But we don’t call our native species ‘invasive,’ no matter how exuberantly they may grow. 

The yellow ground cover is creeping Jenny, which very quickly grows dense and can out compete other plants. While all of these plants are imported from elsewhere, only the creeping Jenny is considered invasive.

Native species of plants function as part of our ecosystem, supporting insects and other animals specific to our local environment.  When we lose native species in an area, we are at risk of losing native animal species too.  The local food webs are broken, and the problem grows worse as more species are lost.

So, it is wise to plant and protect our native plant and animal species.  We should use native plants when possible, and at minimum, make sure that new plants we purchase aren’t on the list of invasive plants.  Our native plants are beautiful and highly adapted to our soils and climate.   When we use plants that have come from elsewhere, let’s make sure that they remain contained, and that we ‘do no harm’ when making our gardens.

We all share the responsibility to model mindful gardening practices.  The first step is to educate oneself on which plants are considered invasive in our area.  The next is to make wise choices.

We can all do that today:   See Virginia’s Invasive Plant List. 

Native Jack in the Pulpit leaves grow among ferns at the Williamsburg Botanical Garden

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