Invasive Plants

Invasive Plants

  • By Cathy Caldwell
  • /
  • February 2015 - Vol. 1 No. 2

by Cathy Caldwell 

You’ve probably heard about invasive species.  I can already hear my husband groaning; he’s heard an earful about invasives already.  These troublesome plants and animals have gotten a lot of press in recent years.  But, like my husband, you might be wondering why you should be concerned.  Even if you are concerned, you might not be fully informed about how you can help.   Before we get to these questions, let’s start with a good definition.

What is an invasive plant?    Invasive plants are  —

  • plants that are NOT native to a given are area AND
  • have the ability to out-compete local plant species
  • causing environmental and economic damage to ecosystems, agricultural fields and home landscapes.

“Invasive Exotic Plant Species Identification and Management,” Pub. No. 420-320, (Yancey 2009).

Sounds fairly straightforward, doesn’t it?  But if you were envisioning foreign invaders, that would be wrong.  Most plants that are now classified as invasive were brought here on purpose for cultivation in our gardens — or else they were brought here accidentally via shipping (as packing material and the like).  And most of the “alien”  (also referred to as “exotic” or “nonindigenous” or “nonnative”)  species that were introduced into American horticulture have NOT become invasive — think tulips and apple trees.

Why Be Concerned about Invasives?  

Because they are fast growers, and because they can tolerate a wide range of conditions and can reproduce easily and rapidly, invasive species make terrible neighbors. They out-compete native species for the same resources, eventually reducing the populations of native species (and sometimes even eliminating species from a community). Some invasive species have more complicated effects which are felt higher up the food chain, resulting in fewer native birds and wildlife. Most important, invasive species have the ability to alter natural functions of biological communities and ecosystems by changing the soil conditions.

Scientists say that Invasive species cause ecological damage by reducing biological diversity  and changing ecosystem functions such as flood and fire regimes.  Biological diversity is a big word, but it simply means having a wide variety of species of plants, animals, microorganisms, etc.  Without a diversity of pollinators, plants, and soils, our supermarkets would have a lot less produce, and we’d have fewer sources for new medicines.

Invasive species also damage and degrade agricultural crops, pasture and forestlands, and clog waterways.  Fixing these problems can be very expensive.     Invasive species (both invasive plants and animals) cost Virginia more than $1 billion annually, while nationally the cost exceeds $120 billion.

But do all invasive plants cause damage on such a grand scale?

If you peruse a list of invasive plants, you might shake your head when you notice that some popular plants — including nandina, periwinkle and Japanese barberry — may be on the list.  They all appear on the list of United States invasive plants at the website of,  At this point, some people  just throw up their hands and dismiss the whole subject of invasive plants.

In fact, there are plenty of relatively harmless plants on some invasives lists.  Take nandina, for example.  Yes, it’s a pretty prolific self-seeder (case in point: my own gardens), but it has so far shown no capacity for occupying vast amounts of territory — like the Japanese stiltgrass that’s filling our forest floors, nor does it have the ability to kill trees, like my nemesis, Oriental bittersweet (more about that later).  Nandina bears watching, however.  In Florida it is creating tight groves that are forcing out other plants.  It’s not unusual for a plant to be invasive in one region and fairly harmless in another region, and this is true sometimes for native plants as well.

All this confusing information is why we gardeners must arm ourselves with knowledge.  When our friends and neighbors ask why we need to bother worrying about invasive plants, we need answers.  And the most important answer we can provide is that some invasive plants — though not all — can do tremendous damage to our forests, waterways and agricultural fields.  “To fully understand the invasive plant issue, one must consider that invasive plant species have varying degrees of impact, ranging from relatively harmless to very environmentally disruptive.”  Alex X. Niemiera and Betsy Von Holle, “Invasive Plants: A Horticultural Perspective,”  Pub. No. 426-080 (This article is well-worth a careful read; it explains HOW invasive plants can cause damage and why it’s not easy to predict whether a new-to-the-trade plant will turn out to be invasive).

Let’s Concentrate on the Worst Invaders

We can probably afford to ignore some mildly invasive plants, but NOT those that are “very environmentally disruptive.”   Fortunately, among all the invasives lists, there are some very helpful lists that rank each plant according to its environmental impact.  These science-based lists include —  NatureServe, a nonprofit conservation organization, which has a large database where you can search for a species and then click on the button for US. Invasive Species Impact Rank (the “IRank”), eServe?init=Species).   Another recommended source is the Southeast Exotic Pest Plant Council,

For a highly informative list of invasive plants in Virginia,  go to the Virginia Department of Conservation and Recreation website,  This list ranks each species according to its degree of impact in Virginia, and it also indicates which regions the plant is impacting — whether coastal, piedmont or mountain area.  Since our area is partly piedmont and partly mountain, we have to pay attention to both of these categories.  Albemarle County’s website has a list that identifies the plants that are HIGHLY INVASIVE and WIDESPREAD in Albemarle County.  Obviously, we need to focus on these major troublemakers.

Highly Invasive Alien Plant Species Widespread Across Albemarle County:

Asiatic or Oriental bittersweet   Celastrus orbiculatus

Japanese stiltgrass   Microstegium vimineum

Garlic mustard     Alliaria  petiolata

Tree of heaven     Ailanthus  altissima

Multiflora rose    Rosa multiflora

Japanese honeysuckle   Lonicera japonica

Autumn olive   Elaeagnus  umbellata

Bristled knotweed Polygonum cespitosum

Chinese privet   Ligustrum  sinense

Depending upon where you live, you may be dealing with highly invasive plants that are, at least so far, confined to a particular part of the county.  The following list identifies those plants.

Highly Invasive Alien Plants In Isolated Locations of Albemarle County :

Kudzu vine                     Pueraria lobata (P. montana)

English Ivy                     Hedera helix

Japanese knotweed    Polygonum cuspidatum

Purple loosestrife       Lythrum salicaria  &  L. virgatum

Spotted knapweed      Centaurea  maculosa

Parrot’s feather             Myriophyllum aquaticum

Chinese lespedeza      Lespedeza cuneata

Johnsongrass             Sorghym halapense

Canada thistle               Cirsium  arvense

Fiveleaf akebia              Akebia quinata

Japanese hops             Humulus japonicus

Mile-a-minute                 Persicaria perfoliata

Japanese knotweed    Polygonum cuspidatum

How We Can Help

  • Be able to identify the highly invasive plants in our area.  It’s especially helpful if you can identify seedlings, so you can remove them BEFORE they grow up and do harm.
  • Be on the look-out for the worst invaders in woodlands adjacent to your home.  Small wooded areas are scattered among our rural subdivisions.  These areas can be a point of entry to larger forested areas.

Start by becoming familiar with each of the plants on the Albemarle County list above.  You’ll soon be able to warn a friend or neighbor about the innocent-looking little bittersweet vine growing in their garden.  I’ve been known to do this myself on a number of occasions, and my zealotry is well-known among my friends.

Each of the plants on the lists above can be studied at the website of the Center for Invasive Species and Ecosystem Health, (Mid-Atlantic Exotic Pest Plant Council Plant List).  You, too, might find you’ve  become a zealous fighter against our most damaging invasives.  Here’s a brief summary for each, plus links to photos and more information:

Oriental or Asiatic bittersweet  Celastrus orbiculatus

Oriental bittersweet vine climbs toward the canopy. Photo by Chris Evans, River to River CWMA,

Oriental bittersweet vine climbs toward the canopy. Photo by Chris Evans, River to River CWMA,


Did I already mention that Oriental Bittersweet is my nemesis?  Once you become familiar with it — and you’ve probably seen its orange and red berries in winter  — you’ll notice it just everywhere.  It seems poised to take over the world!  Right now it is busily taking territory in the Ivy Creek Natural Area and in Shenandoah Park.

Oriental bittersweet is an incredibly strong vine that is capable of smothering tall trees, even large forested areas.  Be on the lookout for small plants and seedlings, so you can remove them before they do harm.  Just be sure to  pull out all parts of the roots, as it can regrow from a tiny bit of root.    If you find a larger plant, don’t just cut the vines!  If you do, you’ll simply stimulate even more vigorous regrowth.  The only reliable method for eradicating an established vine is the cut–stem–paint-with-herbicide method — a/k/a the “cut stump” method — which should only be embarked upon knowledgeably and carefully.

The “cut stump” method does not involve spraying an herbicide.   Instead you “paint” the herbicide onto the cut stem or stump.   Following directions is essential for both effectiveness and safety.  You’ll feel like an expert after reading “Managing Invasive Plants:  Methods of Control” at

Before you start, be sure to read up on using herbicides safely.


Japanese stiltgrass   Microstegium vimineum (Also known as Nepalese Browntop)

Japanese stiltgrass, Chuck Bargeron University of Georgia

Japanese stiltgrass,
Chuck Bargeron
University of Georgia


Japanese stiltgrass is an annual that looks like a small (2-3 ft.) lime-green bamboo.  It tolerates sun or shade and quickly invades areas left bare or disturbed by tilling or flooding. It’s easy to dig it up before it grows into large patches. To help prevent spread into forests, mow it or pull it before the seeds have a chance to mature.  




Garlic mustard Alliaria  petiolata

Garlic mustard is one of the worst invaders in Shenandoah National Park.  It’s a fairly small plant with white flowers.

Tree-of-heaven Ailanthus altissima

Ailanthus is a highly invasive tree which can easily take over an area, replacing native plants.   There is some evidence that tree of heaven has allelopathic properties  — it secretes chemicals that can inhibit the growth of nearby plants.

CAUTION: It’s easy to confuse native shrubs and trees with ailanthus. Ailanthus resembles desirable native sumacs and ash, hickory, and black walnut trees.  How to tell if you’re dealing with ailanthus?  Let your nose be your guide:  if the leaves are stinky, like a burned nut, it’s ailanthus you’re dealing with.

Multiflora rose Rosa multiflora

Japanese honeysuckle Lonicera japonica

Introduced into the United States as an ornamental vine more than 100 years ago, Japanese honeysuckle smothers and girdles trees and other native plants in woodlands throughout the eastern United States.

Autumn olive Elaeagnus umbellata

Autumn olive is a shrub or small tree that has a silvery look, especially from a distance.  Autumn Olive resembles another invasive non-native, Russian Olive.  Cutting either of these down only stimulates new growth.  To eradicate this plant, use the “cut-stump” method (described above for bittersweet).

Bristled knotweed Polygonum cespitosum

Bristled knotweed is also known as Oriental Lady’s Thumb and it’s a small plant that resembles Japanese stiltgrass.  Bristled knotweed forms dark pink flowers, however, late in the season.

Chinese privet   Ligustrum  sinense

There’s an APP for that!  

That’s right.  You can download some terrific apps that will help you to identify potential invasives, and even be able to report sightings to organizations that are working to keep track of invasive infestations.  Take a look at the apps at the bugwood website, which are available for either iPhone or Android.

With one of these apps on your smartphone, you’ll be armed to battle invasives wherever you go.

What Else Can We Do to Help?

We gardeners need to be careful about buying newly-introduced plants and cultivars (called “new-to-the-trade”) — a small percentage of these will become invasive, though it’s difficult to predict which.  Suppose you’d like to add a plant to your garden, but you want to check on its invasive potential.  You can search its invasiveness rank on NatureServe Explorer,

Since my yard now contains several crape myrtles that I’m quite sure I never planted, I thought I’d look up that species.  Crape myrtle has an Invasiveness Rank of Medium/Insignificant (apparently crape myrtle has a medium environmental impact rank in parts of the deep South, but an insignificant impact elsewhere).  For now, I’m enjoying my “free” crape myrtles, but I’m on guard for signs of thugishness.

Be sure to look carefully at  your garden catalogs and at the inventory of your local garden center; you’ll notice that a few problematic invasives are for sale. (lists invasive plants that are sold in the trade by nurseries and garden centers)

If you’re eager to protect our local natural areas, watch for volunteer opportunities to help the Forest Service, the Master Naturalists, and other groups which host work days to control invasives in public parks and wildlife preserves.

If you have more suggestions for ways to help, please let me hear from you.  I’m eager to see comments about your experiences with invasive plants.  But wait a day, please; I just promised my husband that I would not utter the word bittersweet for the next 24 hours.

RESOURCES and LINKS: (“Invasive Exotic Plant Species Identification and Management,” Va. Coop. Ext. Pub. No. 420-320) (“Invasive Plants — A Horticultural Perspective,”  Va. Coop. Ext. Pub. No. 426-080)                (“Control of Invasive Non-Native Plants: A Guide for Gardeners and Homeowners in the Mid-Atlantic Region” )  (distribution maps, how to report invasive plant sightings) (downloadable field guide)  ( “Albemarle County Biodiversity” ; Albemarle County Biodiversity Work Group October 2004) (The worst invaders in the Ivy Creek Natural Area are oriental bittersweet, tree of heaven, and multiflora rose.  In Shenandoah National Park, which occupies a significant portion of the mountainous western region of Albemarle County, the National Park Service is working to curb the worst invaders: oriental bittersweet, Japanese knotweed, garlic mustard, tree of heaven, and princess tree)., Va. Coop. Ext. Pub. No 426-609, “Selecting Landscape Plants:  Groundcovers” (“The nandina species has been labeled an invasive species, especially in the Southeast U.S.; it has a NatureServe invasive plant impact rank of high/low. However, the dwarf types tend to produce fewer flowers, and hence, less fruit than the species or taller cultivars. ‘Firepower’, ‘Gulf Stream’, Harbour Belle, and ‘Habour Dwarf’  . . . are non-fruiting cultivars that do not pose an invasive plant risk.”)