Intermingling and the Aesthetics of Ecology

Is intermingling really more ecological? Or just the stylized look of ecology?

This summer the Highline had its four year anniversary. Perhaps the greatest testament to its massive success is the extent to which the strategy of “intermingling” plants—as opposed to solid massings of single species—has been accepted as a new ecological best practice. The traditional horticultural practice of massing plants together in solid blocks is now seen as static and old school. Mixing plants into carefully woven tapestries is the expression of the ecological zeitgeist.  Almost all of the world’s planting avant-garde (Oudolf, Kingsbury, Sarah Price, James Hitchmough and Nigel Dunnett, Cassian Schmidt, Dan Pearson, Roy Diblik) have projects that celebrate this mixed style.

Noel Kingsbury and Piet Oudolf’s latest book, Planting: A New Perspective, is a celebration of rise of a more intermingled style. Kingsbury has long been an advocate of this mixed planting style, but this latest book positions intermingling as a part of a new international movement. Intermingling is seen not only as a new design trend, but as a way of creating better ecological function. In a recent article in the journal Topos, Kingsbury writes:

Creating intermingling plant combinations, whether aesthetically driven or strictly functional, creates an ecology. In a conventional horticultural planting, plants are discouraged from interacting, but when they do, ecology starts to take over. "Trends in Planting Design." Topos, 83, 2013

Statements like these raise several questions in my mind: is intermingling really more ecological? Or is it just an aesthetic that imitates ecology? And what about function? Does intermingling plants result in more stable, lower maintenance plantings? Or does it require more intensive gardening to maintain it? 

My own experiments with intermingling have been eye-opening. I wanted to record a few of my own thoughts about intermingling and also hear your reactions to this rising trend.

Does intermingling create an ecology?

Mixed planting schemes most certainly evoke the look of a natural plant community. For me, this aesthetic is one of the main artistic benefits of intermingling plants. It is more visually dynamic, and when done well, can be incredibly evocative of natural plant communities. But the question of whether mixing plants actually creates better ecological function is a claim that I’m not sure can be definitively made.
  

Upland forest; photo by Nicholas Tonelli via.

I’d first like to point out that plants growing in solid masses occurs all the time in nature. I recently came back from a hike through the Shenandoah National Forest. What struck me was the strength of the vegetative patterns: large sweeps of Hay Scented Ferns ran along slopes; entire rooms of Hamamelis were carpeted with Pennsylvania Sedge. And these examples are not outliers.  In fact, I’d even argue that more established plant communities exhibit a higher degree of easily discernible vegetative patterns—more legibility—than recently disturbed communities. In established plant communities, plants have learned to co-exist in separate layers.  Each layer has one or a small handful of dominant plants. Intermingling often exhibits itself as a result of disturbance, which restarts the process of succession.  

So at least for me, highly intermingled planting schemes are often more reminiscent of plant communities in transition (disturbance) than plant communities in climax. Of course, there are many examples of highly mixed plant communities and many examples of highly massed plant communities. The fact that both can be found in the wild raises questions to me of why intermingling is the more natural aesthetic, whereas massing is less so. Why can't both compositional strategies can be used to evoke moments in nature? 

So how could we test the claim that intermingled plantings create better ecological function? Here is an experiment I would love to see done: take two plots of open land, each ten-square meters in size. In one plot, plant thirty species of perennials in clean clumps (masses); in the other, mix the exact same thirty species randomly. Then measure their performance. Did one attract more wildlife? Were pollinators more active on one than another? Was one more stable (did it resist weed invasions and persist) than the other?

Here’s my hypothesis: both would perform similarly on all ecological measures for the first few years. Whether an Echinacea is rubbing shoulders with a Sporobolus as opposed to another Echinacea would likely not affect ecological quality. The species selection itself is the primary influence of ecological function.

But would composition matter? Here is where we head into a gray area. Yes, I absolutely think the way plants are arranged affects ecological quality; however, I do not think intermingling necessarily makes for more ecological quality.  

A poorly composed intermingled planting (I’ve had more than my share) can be a functional and aesthetic mess. More aggressive species overtake more demure ones. Slow to establish (but often longer living) species can be eaten by quicker establishing plants, reducing diversity. Taller plants can shade out shorter plants; clumping rhizomatous species can choke out ephemerals. When humans attempt to create artificial ecosystems, any number of unforeseen variables can introduce competition, often at the expense of the original composition. Intermingling plants requires a high level of skill and knowledge. It requires an almost intuitive understanding of how plant morphology is related to its competitive strategy—a skill that so few teach, and even fewer understand. As more designers want to imitate the Highline or other celebrated projects, it opens the door to chaotic plantings.  A bunch of bad imitiators could turn the public off from a wilder aesthetic.

However, well-designed mix planting can actually replicate much of the function of wild plant communities. A robust ground-covering layer holds the ground and resists weed invasion; medium-sized clumping plants can spread slowly and regenerate themselves over time. Taller plants can provide structure and stability through time. If a planting actually replicates the functionality of a plant community, then I do think the claim that it is more ecological could be valid.

So for me, I wish the emphasis were less on intermingling and more on plant community-based design.  Plant community-based design may use masses of plants in certain layers and mixed plantings in others.  The degree of mixing or massing can be determined as a result of the designer’s aesthetic and functional goals as well as the degree to which it exists in wild communities. The important point is not so much whether plants are mixed, but whether each functional layer of a community is present in a planting. This is precisely what Claudia West and I will be exploring in our book.  

Oudolf's Boon garden balances intricacy with legibility

What I’m actually more interested in are plantings that balance legibility with intricacy. I wish the discussion would shift away from intermingling and more toward how we can create plantings in designed landscapes that are both attractive and ecologically dynamic.  I’m imagining something like a carpet of grasses that from a distance appears as a single species, but upon closer inspection, reveals a multi-layered matrix. Or balancing a highly interplanted scheme with a large sweep of a single species to create contrast and visual relief from the complexity. The work of English landscape architects James Hitchmough and Nigel Dunnet does exactly this: focuses on how to make functional communities that the public can accept (often using annuals as way of increasing public tolerance for wild plantings).

Whether a planting is mixed or massed should not be the standard by which we evaluate ecological quality. People’s tolerance for simplicity versus visual cacophony varies greatly. Let’s not strap down ecological quality to a mixed style. After all, a mixed planting can be just as poorly executed as a conventional massed planting, and often, even more visually chaotic. And by all means, let’s not eliminate massings of single species from our toolbox.  Massing may be “conventional,” but in order for designers to create dynamic plantings that can withstand the pressures of urbanization and climate change, we need ever compositional tool available.