“The trouble with the word ‘invasive'”

We are republishing an article from the Garden Rant blog with permission of the author, Susan Harris.  Susan is a professional garden writer who lives in the Washington DC area.  Garden Rant is an award-winning garden blog with a huge readership of garden writers, landscape professionals, and home gardeners.  They report over 80,000 readers per day. 

In this article, Garden Rant enters the controversial debate about the arbitrary use of the word “invasive.”  We agree that this word is both over-used and misused.  However, this is more than a semantic debate.  It’s an important debate because the word is being used to justify huge destructive projects that are damaging the environment by needlessly attempting to eradicate non-native plants, using polluting methods such as herbicides and prescribed burns.

If you have never debated with native plant advocates, you might find the comments posted to this article of interest.  They are typical of the many dialogues we have had with native plant advocates in the past.  


This is a long-simmering rant about the many ways the term “invasive” causes confusion, and more.  DO weigh in with alternatives, pushback, and rants of your own.

“Invasive” as synonym for “nonnative”

Google “native versus invasive” and the 5.6 million hits confirms my observation that this is a common usage, and it’s led to a common misperception in the public that the opposite of native is indeed invasive.  QED: nonnatives ARE invasive.  Even regular garden writers sometimes use this juxtaposition, which should more accurately be “native versus nonnative” or I guess, “exotic.”

That great leveler, Wikipedia, confirms this problem about the term “invasive”: “The first definition, the most used, applies to introduced species (also called “non-indigenous” or “non-native”) that adversely affect the habitats and bioregions they invade economically, environmentally, and/or ecologically.”  At least their second definition is more accurate and even includes native species like deer.  No argument there.

Defining away the invasive behavior by natives

Can native plants be invasive?  Sure, as evidenced by the above-mentioned deer or in the plant world, wild grape.  But when native plants are termed invasive someone invariably pipes up to correct the writer because by definition, they’re nonnatives only.  And sure enough, legally, by the official U.S. government definition, only nonnative plants can be deemed invasive – for purposes of qualifying for money to remove them.  The 1999 Federal Executive Order on Invasive Species defines an invasive species as a “species that is not native to a particular ecosystem…”

Invasive plant lists covering large regions – even continents!

We all know that plant behavior depends on the exact conditions the plant is growing in, as well as more broadly, the region.  So some plants that behave well in the North are overly vigorous in the South.  Or some, like the infamous purple loosestrife, are vigorous in wet spots, not in dry ones.  Examples abound.

Spirea and Doublefile Viburnum (L), Lespedeza (R)

Spirea and Doublefile Viburnum (L), Lespedeza (R)

Yet this site by the U. Georgia and many other sources, including the National Park Service, don’t distinguish by region, and the resulting list of “invasives” includes these surprises to gardeners near me: several viburnums, two verbascums, several veronicas, red and white clo0ver, Japanese yew, 3 spireas (MOST on the market), various salvias, willows, nandina, grape hyacinth, Miscanthus sinensis (without specifying that it’s only the early-bloomers that spread), Lespedeza thumbergii, Pee Gee Hydrangea, cotoneaster, and strangely, littlestem bluegrass (Andropogon virginicus).  Yet native thugs like trumpet creeper are encouraged and they’re not invasive?

That designation of Spirea really bothers me because it’s such a self-sustaining, easy, low-maintenance and well behaved shrub, one I’ve grown for 30+ years with no signs of trouble.  And yet another source – the  USDA National Invasive Species Information Center – also targets Spirea Japonica and says this about it: “Spreads rapidly and forms dense stands that crowd out native species.” This and other contradictions between official reports and in-garden experiences growing targeted plants is puzzling to me.

Adding to the overly broad regionality of invasive-plant designations, there’s a new book on invasive plants, written for a national audience.

Shouldn’t invasiveness be designated locally?  And sometimes, for certain conditions?

“Invasive” used instead of “spreading”

I’ve heard garden-club members describing their passalong plants at plant swaps as “invasive” if they spread at all.  Which leads to said garden club being accused of encouraging the use of “invasives,” among other things.

Methods of “invasion” all lumped together

Mature Ivy
Mature Ivy

The  USDA lists these characteristics of invasive plants: “produce large numbers of new plants each season; tolerate many soil types and weather conditions; spread easily and efficiently, usually by wind, water, or animals; grow rapidly, allowing them to displace slower growing plants; spread rampantly when they are free of the natural checks and balances found in their native range.”

Yet some of those qualities are valued in the garden – especially tolerance of many conditions.  And for the gardener on a budget, especially one trying to replace their lawn with another groundcover, spreading is a good thing and it’s usually described more positively as “fills in quickly.”

What if the standard were: Does the plant spread in a way that causes harm to natural areas?  For example, plants that are spread by birds, like English ivy, so that the seeds can go everywhere and harm natural areas.  Unlike Spirea and Nandina that spread by rhizome and produce a couple of offspring every year, if that – just like Itea does?  Or if it’s simply spreading in the garden, is it impossible to control, like running bamboo?

daylily-550x412For example, “Invasive Species of Concern” in Maryland includes mostly plants we’d all agree are thugs and not even considered garden plants, but daylily?  As a sun-lover, it won’t spread into the woods and even out in the sun, how hard is it to dig up?

Or a plant could be harmless in a townhouse garden on Capitol Hill but potentially harmful if planted on the edge of a forest.

I wish there were several terms used to describe spreading behavior by plants, rather than the single term “invasive.”  How’s a gardener to choose between groundcovers like pachysandra, periwinkle and English ivy, when they’re lumped together as equally thuggish when only one of them will grow virtually to strangle trees, set seed and spread indiscriminately?

More science-based info, please

In researching the topic of “native versus exotic” I came across one example of the type of reporting on invasive species I’d like to see more of – based on research, not scare tactics.  Just one quote from this article by Cornell will piss off some readers, but here goes:  “A small percentage of plants exhibit invasive tendencies, while the majority of plant introductions are benign or beneficial.”

Solutions for “invasiveness” coming?

Plant breeders are hard at work breeding out invasiveness in popular garden plants, as reported on the Native Plants and Wildlife  Gardens blog.  Though controversial, especially among native-plant advocates, this type of breeding is recognized by pragmatists as a step in the right direction.

Daylily photo credit.

7 thoughts on ““The trouble with the word ‘invasive'””

  1. For once I agree with your post. Great care should be taken in using terminology. A plant that merely spreads in the garden and causes work for the gardener (which is of interest only to other gardeners) is of a different order of magnitude from a plant that invades natural areas or farmed land and has the power to change ecosystems and/or cost millions in economic damage. In the witch-hunt to eradicate prohibited noxious and noxious weeds (terminology used in regulations in Canada) we should be careful not to go after those plants that really cause no harm. Also, with the exception of plants that are of horticultural origin, all plants are native to somewhere. Hence we should be using terminology like “native to” or “non-native in our area.” Thus a plant may be native to one (American) state, but non-native in another. If a plant is invasive that statement should be qualified with more information, e.g., “invasive in wetlands in eastern North America.” Incidentally, I don’t like the use of the word “thug” for any plant – far too anthropocentric!

  2. I love your blog, but want to add that in the Bay Area, and I’m guessing other areas with similar climate, the biggest problem plant that humans brought here is ivy. Hedera Helix/English ivy is less invasive than Heder Canariensis, which is the ivy pictured. I sometimes comfort myself that if humans die out, the plants and animals will survive, but not with that ivy which would eventually completely covers and kill all trees, shrubs, rocks, etc. It doesn’t need water, and containing it in yards doesn’t work because birds eat the fruit and drop the seeds. I saw it recently in the middle of a wilderness trail on Mt. Tamalpais. Heat, cold, and drought doesn’s stop it. No animals here seem to eat it.

    If you look at our freeways which used to use the magnificent glowing ice plant as ground cover (which hurts no one and is one of our most beautiful blooming plants from South Africa), the dominant plant is now Hedera Canariensis, and it has almost completely covered the plums and other small trees on freeway 580. On 13, it is killing redwoods. And the road maintenance crews who seem to continue cutting down beautiful healthy trees will rarely cut the ivy. If the ivy grows on buildings, it destroys them. Seriously, it’s just a matter of time before it covers everything.

    I absolutely love most of the non-native plants and trees, and agree with this article about the silly contradictory classifications. I just want us and the wild animals to have as many trees and plant variety as possible, which is good for the environment when many native trees are dying here and elsewhere. I just don’t want plants or humans to kill them.

    So to answer the question here, about what groundcover to choose, only ivy is a problem in the Bay Area, as far as I know. Periwinkle is the sign of an urban rather than wilderness park, but it is beautiful and will not kill trees or shrubs. I also love the hated broom, acacias, pines, eucalyptus, etc.

    1. I would like to add one reassuring bit of information. I have been told by two professional arborists that ivy is not capable of killing tall trees. One of the arborists was quoting a scientific study. The other was speaking from personal and professional experience in the Bay Area. The latter is a professor of forestry at UC Berkeley. This was new information for me at the time. Up to that point I had shared your anxiety about ivy growing on trees and I had volunteered in my local park to pull ivy down from the trees.

      One of the most important things I have learned in the many years I have been studying this issue is that nature is remarkably resilient. In early December of 2013 we published a post about a walk we took in a native woodland in the Central Valley. The huge valley oaks were covered in native wild grape and blackberry. Yet, the trees were very much alive. So, here is yet another example of native plants behaving much like some non-native plants and neither seem to be doing any irreparable harm.

      1. 1) Let’s get real. Humans are the invasive species.
        “Non-native” plants and animals are just tag-alongs or conscriptees to the invasions.

        2) I’d be curious to read your arguments that Kudzu and Himylayan blackberries do no “irreparable harm”. I say that would be a value judgment, not a matter of science

        1. I can’t evaluate kudzu because it’s not in the Bay Area so I have not had the opportunity to evaluate it. I try to confine my blog to issues of which I have first-hand knowledge unless there is a scientific study available.

          I wouldn’t plant Himalayan blackberry. Clearly it spreads rapidly. However, here in the Bay Area it is one of the most important sources of food for birds. It produces more berries than its native counterpart.

          But the primary reason why I defend blackberry is that it requires a lot of pesticides to kill it. The herbicides used to kill it must be applied while it is actively growing. That’s also when the plant is producing berries. So, in addition to losing their food source, the birds are also being poisoned by the fruit they eat. When these plants are poisoned in public parks, I am also concerned about the potential for children to eat the berries.

          Yes, some plants are invasive. The questions for me are: What is the ecological cost of trying to destroy the plant? Is it realistic to eradicate the plant? Is the cost of trying to eradicate it greater than the cost of leaving it alone? Will the plant eventually fit in and not dominate its neighbors?

    2. Periwinkle in my VT woods crowds out native species and offers no benefit. It is not nearly as choking as the bittersweet. But the invasives are the only things that will grow in that dead area of my woods. In other parts of the woods partridge berry covers the ground, offering food.

      I think it’s pretty easy to see that some invasives are truly menaces (phragmites, kudzu, bittersweet, purple loostrife) and some things that are ok in your area are truly menacing in other areas. When someone like Floret tells pll to plant bittersweet to make a fall wreath it’s a problem because we are all buying online and there is often very little info about what might be invasive.

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