Defining “Native” and “Invasive”

Stamford CT

Is any plant that grows out of control in the garden “invasive“? Is any plant from a foreign place “invasive”? How close to home does a plant have to grow to be “native”? I have observed a lot of confusion, even among the so-called “experts”, about these seemly simple words.

To foster our collective conversation about native plants and wildlife gardening, I think that it would help to have as close as we can get to common agreement on what we mean by “native” and “invasive”.  Accordingly, this month I’m raising the issues by offering what I’ve come to believe are (or should be) the commonly accepted definitions.  What’s your view? Please join the conversation.

Lesser celandine: beautiful, native wildflower in England, invasive killer of wildflowers in Connecticut

Invasive Species: Let’s start with the definition of “invasive” as the term is used to describe certain undesirable species. To be “invasive”, as I understand it, a species has to:

(1) originate outside the local area;

(2) have the potential to spread long distances on its own; AND

(3) once established, have the potential to out-compete native species in the wild.

We say “have the potential” to spread and out-compete because we hope to identify and stop the spread of new invasives long before they become overwhelming clean up issues. Unfortunately, this doesn’t happen as often as we’d like and is why we need to be so careful in the plants we choose for the garden. Many foreign plants don’t get recognized as an environmental threat until much too late.

Clearly, any plant advertised as “imported, hardy, fast-growing, pest-free, birds love the seed/fruit” is likely to be a problem as is any similar plant that has seeds that are spread by wind. To purists, like myself, the only way to be sure is to stick to natives. (Thank you, Sue Reed, for your spirited defense of purists!)

Aggressive Plants: Some garden plants spread out of control in the garden but aren’t “invasive” because they are unable to spread long distances on their own. Likewise, some local native plants, under the right circumstances, can become out of balance, particularly those early succession plants that thrive in disturbed soil and full sun and that were historically controlled by, for example, the shade of the forest canopy that once extended from Maine to Florida.

“Aggressive” is a good descriptor for any plant that out-competes its neighbors but that doesn’t otherwise meet the definition of invasive. Many of these plants are (only too cheerfully) growing where unwanted so can also be called “weedy”.

A good example of an aggressive, non-invasive plant is pachysandra; it makes a local mess but can’t “jump” long distances.

sweet crabapple: native in the southeast, naturalized in Connecticut. Friend or foe?

Naturalized Plants: What about foreign plants that spread into uncultivated areas but don’t out-compete the natives? Traditionally, we called such plants “naturalized”. Being “naturalized” does not mean that the plants have become natives; it only means that the harm that they are doing in the wild does not seem to be significant, at least for now.

Being “naturalized” doesn’t make a plant desirable since, as a general matter, it is unlikely to be a functioning part of the local ecology in the way that native plants are. Thus, the plant is taking up space that could be occupied by a productive member of the ecology. In the garden, I’m inclined to root out all purely decorative foreign plants except the “legacy” stuff that we collectively seem unable to give up like mature lilacs. In conservation area restoration work, I usually ignore naturalized plants because we don’t have the resources to go after them as well as the serious invasives.

However, whether a plant is invasive or naturalized in a particular place is not always clear and can change quickly if cultural conditions change. For example, in my area, warmer Connecticut winters are no longer killing off undesirable non-natives like the notorious kudzu, and the hemlock-killing wooly adelgid.

Stamford CT’s dominant dogwood is the silky dogwood (Cornus amomum) – not its cousins the grey dogwood or the red-twig dogwood, that often show up on planting lists as “area natives”.

Native Species: In my view, figuring out what is, or is not, native causes more frustration among those of us trying to do the right thing by our wildlife, than figuring out what is invasive or naturalized. Is “the local area” just a few square miles or is it the whole county, state, region or country? Can a plant loose its native status if it’s been missing for a while (e.g. the deer ate it all two decades ago)? Does it matter?

For example, where I live plants are often promoted as “native to New England” or as “Connecticut natives”; sometimes just as “native”, begging the question as to where.

However, the reality is that the difference between the natives which grow along the Long Island shoreline in my town and what naturally grows in the much cooler northern Connecticut up-lands only 60 to 100 miles away is night and day. Yes, some  plants grow in both places but many don’t.  Further, I suspect that if you go even fifty miles north or south, the coastal plant community will be noticeably different.

So what is “native” for my town? Our local flora and fauna evolved together over the last 13,000 or so years since the last Ice Age. They are mutually dependent. For example, many native insects have evolved to eat only certain native plants, and many native amphibians, reptiles, mammals and birds have evolved to rely on these insects. Meanwhile, native plants may need the help of specialized soil organisms to obtain nutrients, pollinating insects to fertilize their flowers, and ants, birds, squirrels and the like to spread their seeds.

When undisturbed, the native populations balance and nurture each other. For example, as Doug Tallamy notes in his must-read book, “Bringing Nature Home“, our tree-nesting birds feed their babies primarily on native insects. Thus, the native birds simultaneously provide for their children and keep these insect populations in check – a control that we do not have over the  invasive insects which cause so much damage.

Nesting Baltimore Oriole at Scalzi Riverwalk Nature Preserve in Stamford CT: needs native insects to feed its children.

So I suggest that the best test for “native” is that which is an evolved part of the local ecology. The best way to determine what is actually native to a particular area is to survey what grows there in undisturbed areas (watching out for garden escapees and plants which were introduced in the wild by well-meaning individuals).

What historical records show once was in the area and what plants are or were in neighboring areas is useful. However, this information may not be determinative due to barriers to re-integration of locally-extinct plant species no longer supported, or controlled, by their companion species (microrhizomes, specialized leaf eaters, pollinators and seed spreaders, etc.) who left the area when their host plant went missing.

Today, for clarity, the term “local genotype” is becoming popular to signify those forms (naturally-selected cultivars) of the species that have evolved in a defined local area, and, thus, are a functioning part of the area’s ecology.

Where do “nativars” or “nearly natives” fit into all this?  They add to the confusion!  Check back next month for a continued discussion.

Please leave a comment and join the discussion. We can’t wait to hear from you.

Credit where due: Much of this, for me, has come from struggling to answer the troublesomely astute questions posed by the Master Gardener Interns who work with me on conservation area restoration projects. Many thanks to Master Gardener Interns, Class of 2011, Cee Moreyn and Lisa Shufro, for their contributions to this post.

© 2011 – 2012, Sue Sweeney. All rights reserved. This article is the property of Native Plants and Wildlife Gardens. We have received many requests to reprint our work. Our policy is that you are free to use a short excerpt which must give proper credit to the author, and must include a link back to the original post on our site. Please use the contact form above if you have any questions.

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  1. says

    When it comes to the crunch I battle with this. We are in a mediterranean climate, which I understood as fynbos in the last garden on the slopes of Table Mountain. BUT we are now in the Swartland, which is renosterveld. Still mediterranean, but hotter and drier. My proteas died sadly, one by one. I learn by looking more carefully, that the fynbos proteas grow up there, on the mountain. Here in the valley, I must learn to find indigenous/native plants again. Delighted to find the Riebeek West nursery and a nurseryman/landscaper who doesn’t look blank when I ask for plants to give me a renosterveld sense of place.
    Elephant’s Eye recently posted..Inspired by Ifafa lilies

    • Sue Sweeney says

      It is amazing that we are a world apart, literally, but our issues are exactly the same. This kind of sharing is one of things that I thank the web for everyday.

  2. says

    Really useful conversation, Sue, about a subject which is confusing for just about everybody. Thank you for clarifying all the terminology as well as the “what ifs” that introduce so many variables. Looking at local ecosystems very carefully is a great way to start figuring out what is native to your area – this is where local nature walks with guides can be so useful for helping ID what is growing. Interesting that you bring up native dogwoods – in an area of less than a mile around where I live, I see areas where Pagoda Dogwood is the dominant dogwood (generally cool, shaded areas) where on just “the other side of the hill” there are thickets of Gray dogwood and not a Pagoda to be seen. It proves that especially in a diverse topological landscape, it’s all about the right plant for the conditions, which is one of the most important rules of ecological garden design :)

  3. says

    It is helpful to define the terms so that people have a better sense of the issue regarding invasives. Too many people think that if you mention “invasives” that you are speaking about all non-native plants and that is just not true.
    Ellen recently posted..Late Summer Yellows


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