A Native Plants Enthusiast Responds to Marianne Willburn

 

I admire Marianne Willburn’s skill as a writer and her devotion to the craft I love.  But I was disappointed by her recent post on this site, “Native Plants as a Moral Issue? Consider Me Chief Heretic.”

The point implicit in the title is that Marianne has apparently been criticized about her preferences regarding native versus introduced plant species.  Who, I wonder, has been attacking her?  I’ve been involved with the native plants movement in the United States since the late 1980’s, and my experience has been that while native plants enthusiasts like to share information about the environmental services their flora can provide, none have suggested to me that it is wrong to enjoy non-invasive introduced plants as well.  I’m sure there are some purists of this sort out there, but in 35 years, I have not encountered them.  I had lunch last week with Larry Weaner, one of the pioneers of ecological gardening, and he said the same. 

Marianne makes a reference to the savaging of reputations online, and maybe that is where she has encountered the orthodoxy she dislikes.  All I can say to that is that anyone who has ever posted any opinion on social media has encountered that sort of treatment – the Internet is a favorite gathering place for people who would have been mailing poison pen letters a generation ago.  I also think that comparing the native plant dissenter to Galileo is a bit over the top, especially as what follows in Marianne’s post is essentially science-free.

Marianne did raise some legitimate questions about such new style gardens.  For instance, why should a staghorn sumac (Rhus typhina) be considered preferable to a tree of heaven (Ailanthus altissima)?  The ailanthus is an aggressive spreader, but then so is the sumac.  From an aesthetic perspective, both are somewhat coarse in appearance.  To be fair, I’ve actually seen ailanthus, an import from China, recommended as an ornamental in British gardening books, and as a Brooklyn friend and passionate gardener asserted to me once, any tree that can flourish in an unfriendly concrete cityscape such as her neighborhood deserves some respect.

If Marianne had consulted an ecologist, however, she would have learned some pertinent facts.  The fruits of the staghorn sumac – a north American native – feed around 300 species of North American songbirds; I have found only two North American bird species cited as feeding on the seeds of the ailanthus.  Most would agree that’s an important distinction in a time when our songbird populations are crashing.

Both trees are also quick to colonize disturbed sites, an important service today when we have ravaged so much of our national landscape.  There’s a difference in their impacts, though.  According to a Forest Service report about the sumac, by shading and reducing groundcovers, the sumac seems to enhance the growth of tree seedlings, serving as a positive step in the natural succession back to a mixed and richer flora. The ailanthus, by contrast, is like many other invasive, non-native plants in that it is allelopathic.  It infiltrates the soil with chemicals that deter the growth of other plants, which has the effect of decreasing local biodiversity.  A loss of plant biodiversity has a drastic effect on the local wildlife by robbing insects of plant species they depend on for food, and a loss of insects (whose populations are also crashing worldwide) deprives animals farther up the food chain of foods on which they depend.

Marianne also suggests that in a time when the natural world is so challenged by climate change and habitat degradation, we should welcome any “adaptive” plants that can flourish in the face of such difficulties no matter where they originated.  That too conflicts with the views of most ecologists.  As is suggested by the contrast between the sumac and the ailanthus, plant and animal relationships are highly specific.  Plants evolve toxins to deter plant-eating animals (herbivores), and the herbivores indigenous to the same area gradually evolve a tolerance for those toxins so that they can continue to eat those particular plants.  Over time, a given species of herbivore typically develops a dependence on a particular species or a group of related species of plants.  This is why so many imported, non-native plants are advertised as “pest resistant” – our North American insects cannot eat the imported plants with their unfamiliar toxins.  The net result is that the pest resistant imports play no or a much-reduced role in supporting the local ecosystem.  When an import proves invasive and jumps the garden fence to invade the neighbors’ properties and adjacent wildlands, every one of its individuals represents a small diminishment of the ecosystem’s ability to support itself.  The cumulative effect can be, and often is, devastating.

But as Marianne asserts, the imported plants can evolve to integrate themselves in the natural food chain.  That is true, but the data collected by most ecologists indicate that this is typically a very slow process, one that commonly takes centuries or millennia.  There are exceptions, of course.  Dr. Arthur Shapiro, a distinguished professor emeritus at the University of California Davis (and an entomologist highly regarded by his colleagues) contacted me after I published an episode on my “Growing Greener” podcast in which I discussed with Dr. Douglas Tallamy the limitations of invasive imports such as butterfly bush (Buddleja davidii) as a support for North American butterflies and moths.  Dr. Tallamy noted that while the buddleja does provide nectar to feed the adults, the bush does not provide a hospitable habitat for their caterpillars. 

Dr. Shapiro informed me via email that he had observed two subspecies of a particular native butterfly raising caterpillars successfully on Buddleja davidii in California.  When I checked I found that Buddleia davidii was introduced into North American gardens in the early 1900’s, so the adaptation of just two butterfly subspecies in over a century reinforces how slow this process of adaptation is.  Our ongoing, massive loss of habitat and the accompanying wave of imminent extinctions is an immediate problem (one we have brought on ourselves) and unless we want to experience the horrors of global ecological collapse, we need to take action now. 

But with climate change already destabilizing ecosystems worldwide, maybe we should just be grateful for any plants that flourish in our landscapes? What is the relevance of “native” anymore? This is an argument I have often heard from those who would deny the special services that locally evolved plants can supply to the local ecosystem, and Marianne referred to it in her post.  Support for this position, however, bespeaks a lack of understanding that plants do not grow naturally as individuals but rather as members of communities. 

We are only beginning to understand the subtleties of how co-evolved plants and animals cooperate in these natural communities.  It’s only in recent decades, for example, that we have glimpsed how the plants in such a context communicate through fungal populations in the soil and chemicals they release into the air.  What we do know is that arbitrarily chosen plant imports typically do not integrate effectively into such systems.  They may flourish individually but do little or nothing for the success of the local flora as a whole.   This is why the mainstream of ecologists endorse trying to support plant communities by maintaining or restoring their natural biodiversity.  This will assist their ability to evolve as the climate changes. 

I strongly believe that gardening is personal and that every one of us should practice the craft as we like, so long as we don’t do something that endangers our neighbors or the natural world on which we depend.  If you choose a traditional approach that focuses only on aesthetics, mingling plants from all over the world in a pre-determined design you have drawn up on paper, that is your right and no one should criticize you so long as you don’t make your garden an entry point for invasive species. 

I wish that Marianne was a bit more open-minded in this respect.  In her post she proposed that the native plant assemblage she was visiting didn’t deserve the title of garden at all.  She refers to what she perceives as a lack of “human vision, knowledge, skill, artistry, and stewardship.”  I’ve not visited that garden, but most native plants enthusiasts I know do include a concern for aesthetics and human comfort in their work.  Larry Weaner’s landscape designs have achieved the great popularity they enjoy because he is alert to those issues as well as to the health of local ecosystems.

Besides, if I allow the plants in my meadow to find their own spots and patterns, that does not mean there is no order or structure to this sort of garden.  There is a fascinating natural order in how the different species relate to the topography, soil, and each other.  It took some re-education for a traditionally trained gardener such as myself to learn how to read this, but I now find it very satisfying to work with Nature rather than simply imposing my tastes on it as I was taught. 

And Marianne to the contrary, the new style of design I am learning doesn’t result in chaos and massive amounts of maintenance.  If you learn to study and respect the dynamics of the local ecosystem in your design, and if you accept that an ecologically inspired garden is going to evolve and change over time, in most cases you will find the results much more sustainable both in terms of inputs such as water, fertilizer, and pesticides, and in terms of labor, than trying to enforce a static planting plan and hosting a cosmopolitan botanical menagerie on your site.

 Is “religiosity” and a desire to sell ourselves really at the heart of the current enthusiasm for native plants as Marianne asserts?  It seems to me to be based rather on a desire to interact in a more positive and sophisticated way with our environment.  I find such satisfaction and excitement in pursuing a better understanding of the ecology of my area and creating a garden that authentically reflects the natural beauties of my area.  There’s still room for the hybrid roses and the heirloom cider apples I love.  There’s a unique thrill, though, in watching all the butterflies, moths, amphibians, snakes, and birds who find food and habitat in what was formerly lawn and now is my meadow.   That has become the heart of my gardening.