Gardening on the Planet, Science Says

A New Take on Invasive Plants

I’ve been reading an important book. It’s not new – it was published in 2010 – but it is even more relevant today than when it was fresh off the press. It is Peter Del Tredici’s Wild Urban Plants of the Northeast.

Wild Urban Plants

Conceived of as a field guide to the plants you might encounter in a stroll through vacant lots and other neglected corners of our downtowns and industrial neighborhoods, it is implicitly something far more provocative. By treating these despised plants as a genuine flora, it invites us to take a second look at them, and their artificial settings. Ecologists and plant lovers are much more likely to value relatively pristine rural settings, but the fact is that more than 80% of Americans now live in urban areas (worldwide, the figure is something like 50% and increasing). Del Tredici’s unloved plants, in other words, are the daily point of contact with Nature for most Americans.

Nature lovers who strive to green the city environment routinely root out these plants, disparaging the bulk of them as foreign invaders. Considerable sums have been spent in New York City, for example, to eliminate these “invasives.” Yet, as Del Tredici points out, the landscapes they inhabit are something artificial and new, and the conditions that prevail are grossly unsuitable for the native plants that once flourished there. Tree of heaven (Ailanthus altissima) has over-run the five boroughs of New York because it is far better adapted to their harsh, droughty, alkaline soils than any of the trees that preceded it on those sites. Cities, Del Tredici asserts, are human creations, and the plants that flourish there spontaneously, thriving and spreading without human intervention, are the “de facto native vegetation.”

Tree of Heaven (Ailanthus altissima)

Tree of Heaven (Ailanthus altissima)

This assertion is radical enough by itself. I cannot help wondering though, whether this definition of de facto native will have to be extended far beyond the city limits as humanity fosters ever greater disturbance to the natural environment.  We are entering a period that scientists are labeling as the “anthropocene,” an era in which human activity has become the dominant influence on climate and the environment.   What is native to such an era is anybody’s guess, but I an sure it will look like nothing we have ever seen before. We will, I suspect, come to treasure the plants, of American origin or not, that can withstand its stresses.

Posted by on February 20, 2017 at 10:25 am, in the category Gardening on the Planet, Science Says.
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14 responses to “A New Take on Invasive Plants”

  1. It is interesting to me to read the replies to this post. Yes, it can certainly be fun to be able to identify the plants that just happen to pop up in your yard, and it is also good to not have to feel guilty about loving a plant in your garden that is not native. However, those who disdain native plants and readily jump on the bandwagon of the non-natives that are hardy pioneers in our forgotten, urban spaces under the rationale that these are the survivors and therefore appropriate, are operating under an indifference to the ecological interdependencies that exist in the natural world as a result of a long-term co-evolutionary process between plants, insects and the rest of the food web that depends on them.
    Our historical development and planting design practices are why so much of our wildlife populations are declining. For example, songbird populations in the Mid-Atlantic have declined close to 80%. Habitat loss and degradation are leading causes as is our horticultural practice of promoting introduced species. Accepting our many weedy invaders as those who should now belong does nothing to address this critically important issue.
    We can no longer afford to live with our natural areas only “out there”. Rather, I hope more of us encourage others to join in the work of providing supporting native habitats in our urban spaces.
    It is not necessary to re-create a pre-European settlement native habitat in an abandoned lot. But we must, through our development policies and gardening and landscape practices, work at recognizing and supporting the needs of our critical insects and pollinators and the plants that they depend on. Without them the rest of the dependent food chain collapses.
    Emphasizing native plants in our residential and commercial landscapes, and adopting horticultural practices that encourage their use and success is critical to this effort. If we don’t push forward on this, it is our loss… of more than we currently know.

  2. Susan Harris Susan Harris says:

    Thanks for the recommendation. I just embedded Peter’s talk here on the Rant.

  3. Nell says:

    Re: skr’s discussion of what is and isn’t “urban” —
    Sixty percent of the U.S. population is still a strong majority, so even with the arbitrary cutoff of a million-metro, the book fills a need for a huge number of people.

    But think about what “urban” means from the plant point of view. It’s all on a gradient, but once you get a critical mass of development in any one spot, the tough generalist and pioneer plants have the advantage. Grading and paving that compact soil and alter drainage are accompanied by elimination and separation of existing ecosystems — forest, woodland, scrub, meadow, streamside, marsh.

    Again — all a matter of degree, but the effects can be seen in places no one would call “urban”. I live in a rural area with a few small towns; national forest covers both sides of the county; and the nearest million-metro is more than two hours away in any direction. But there’s also an interstate highway interchange, a pole of development for the last 30 years. In that zone only the toughest local plants make it without human intervention.

    However, there has been some human intervention, including encouragement by local and larger government to plant native trees and shrubs. The results in some cases have been pleasing and fauna-fostering plantings, stable and requiring little upkeep.

    I’d urge those who scorn efforts to increase planting of native trees and shrubs to stop thinking of them as artificial restorations, the planting equivalent of historic preservation. Start thinking of them as if you were a songbird, or a local ground-nesting bee, or a butterfly: what’s there to eat? Canopy trees, understory trees, and large shrubs are the strongest engines of a local food web (compared with herbaceous and ground-layer plants). Value to the region’s fauna should be a real consideration for plants on that scale.

  4. Tom Christopher says:

    I don’t think it’s necessary to take a position for or against in this conversation. I’ve spoken to Peter Del Tredici and know that he does not do so. There are many places and situations where ecological restoration is a good approach, and others where a novel ecosystem is indicated. I think more discussion is needed about the parameters of both

  5. The author of this book, Peter Del Tredici, gave a presentation at the San Francisco Presidio in January 2015. It’s worthwhile to explain who Del Tredici is because his credentials are impeccable. He was, for many years, the director of the Arnold Arboretum, Harvard’s research arboretum and he is still Professor of Landscape and Design at Harvard.

    Del Tredici’s presentation at the Presidio was a revolutionary event because the San Francisco Bay Area is a hotbed of nativism and the Presidio has spent boobilion bucks trying to replace the existing landscape with the landscape they imagine existed here 250 years ago, before Europeans arrived.
    There is a video and transcript of his presentation available at million trees dot me (Put “Del Tredici” in the search box). As the camera pans the audience you will see a mix of the horrified faces of native plant advocates who are devoted to their radical “restorations” and people who are amused by Del Tredici’s challenge to the delusional fantasy of native plant advocates.

    I recommend this video to those who are still sitting on the fence about the controversial attempts to eradicate all non-native plants in urban settings. I climbed off that fence long ago. My feet are firmly planted in the real world where the urban landscape is an eclectic mix of the past, the present, and the future.

  6. Tom Christopher says:

    I was replying to skr (below) making the point that just because those communities don’t include a million people in their metropolitan areas, that doesn’t make them non-urban.

  7. Juice Box says:

    In what way do Des Moines, Omaha, and New Haven not qualify as urban areas? i read the OP several times trying to understand your statement.

  8. Jodie Cook says:

    I have mixed feelings about this idea of not judging a plant by its origins. On one hand I think that we often fail to see plants as critical living parts of an interconnected system which includes other species that we really don’t want to lose, like native songbirds. These other species depend on native plants supporting soil micro-organisms, micro-arthropods, insects, etc. they also offer a pest control service so we can keep toxics out of our gardens.

    Many creatures that co-evolved with native plants are declining in numbers. If we cultivate an ‘anything goes’ planting philosophy based on our own whims or ignore the decline in native species of all kinds because somebody a long time ago mistakenly brought an adaptable non-native to our regions, then we have to be willing to let go of all the other creatures that depend upon, and co-evolved with, our native plants. I’m not yet willing to do that. Many native plants are tough competitors that can compete with non-natives in urban environments. Sometimes we like exotics because they are exotic – they don’t look like the fit here. We like the novel and the new. Many plant fashionistas see native plants as boring. A regionally appropriate planting as dull because it isn’t a riot of variegated, red and yellow leaves, big blowsy tropical flowers.

    On the other hand, a lot of toxics are put into our environment in the name of eradicating invasive plants. This is not acceptable. I would rather keep the invasive and forgo the toxics.

    So the choice I make, garden by garden, is to choose tough, adaptable natives as my ‘go to’ plants. Use non-natives when they support some part of the native system, like fixing nitrogen or supporting a native butterfly. I don’t use plants that I know will spread uncontrollably, or are detrimental to native ecosystems, or harm wildlife in some way. I think the more information we have about plants as part of our larger ecosystem the better choices we can make in the moment about what to plant, remove or ignore.

  9. Martha says:

    Thank you the post, I look forward to delving into this book. As a western horticulturist transplanted to the northern midwest I have been slowly learning the local garden plants in my 100+ year old neighborhood’s largely laissez faire ( I also call them volunteer) front yards and terraces, aka parking strips. It’s taken a combination of native, garden, and weed guides to do this. Interestingly, just last week I had a conversation with the extension weed scientist about the relationship between the state invasive list and urban core gardens. (Might sterile cultivars be a solution in some cases? We reached no conclusions .) On my neighborhood list serve there have been heated discussions about rooting out decades old drifts of say, “invasive” but naturalized orange daylilies, vs enjoying their summer color. In my little yard I removed a small lawn and replanted with native perennials, which increased the species diversity many fold without introducing invasives. Such an important topic, especially on the urban fringes.

  10. lwc says:

    Who are we, after all, the plant nazis? Watching the red oaks take over our aspen/ birch forests, and finding out that the academics are now recommending red oaks to replace…. nature knows best, i guess. Still trying to reconcile buckthorn and mulberry, but maybe they will become the only food source for the few remaining birds?

  11. Maryk says:

    Just because these foreign plants have proliferated doesn’t change the relatively low value they have. Invasives provide little or no food & shelter for insects, directly impacting the amount of food available for birds. Songbirds in particular migrate thousands of miles, only to find no food or shelter.

    Were the amount of space we are changing not so great, this wouldn’t matter. At this point, I’d say, integrate foreign plants with natives. Learn what an oak tree needs and when possible, plant one.

    At the end of the day, isn’t it worth a little more effort to continue to hear birds singing in our public spaces and home gardens?

  12. Thomas Christopher Thomas Christopher says:

    You certainly have a point, though I think you set a very high bar in your re-definition of urban areas. Omaha, Des Moines, and New Haven, for example, wouldn’t qualify as urban areas by your definition, and I think that would come as a surprise to their inhabitants.

  13. Mary Gray says:

    I love this book. My copy is packed with sticky notes, marking the species that I have recognized growing wild in my yard or at the roadside. Frankly, if you live in any kind of built-up area in the NE or MidAtlantic, this book is a much more relevant field guide than say, one published by Audubon or something. Will I happen upon a Post Oak here in suburban Northern VA? Probably not. Black Locust? Yup. Trilliums? Nope. Henbit? Yes.

    I like how the book doesn’t distinguish between native or non-, just includes the plants that spring up of their own accord, regardless of origin.

  14. skr says:

    The only reason 80% of Americans live in an ‘urban area’ is because the definition of urban area has been expanded to include ‘urban areas’ of 2500 people. IDK about you but a town of 2500 is not urban and it is a little ridiculous to equate living in a small town to living in a metropolis.

    I cruched the numbers a few months ago and about 60% of Americans live in a metropolitan service area of 1 million or more people. I think that is a more appropriate demarcation line.