Understanding What Makes Plants Happy

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Understanding What Makes Plants Happy

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Thomas Rainer and I have both been doing the botanical thing for decades; we know, and use, many of the same plants — and even much of the same horticultural vocabulary. But what he and I see when we look at a butterfly weed or a coneflower, or what we mean when we say familiar words like “layering” or “ground cover,” is surprisingly not synonymous.

It turns out I’ve been missing what the plants were trying to tell me, failing to read botanical body language and behavior that could help me put plants together in combinations that would solve challenges that many of us have: beds that aren’t quite working visually, and garden areas that don’t function without lots of maintenance.

As we gardeners shop the catalogs or the just-opening local garden centers with an eye to finally “fixing” that bed out front that has never quite cooperated, I asked Mr. Rainer, a landscape architect based in Washington, D.C., to lend us his 3-D vision.

In his career, Mr. Rainer has designed landscapes for the United States Capitol grounds, the Martin Luther King Jr. Memorial and the New York Botanical Garden, as well as gardens from Maine to Florida. He is an author with Claudia West of the 2015 book “Planting in a Post-Wild World: Designing Plant Communities for Resilient Landscapes.” He is a principal in the firm Rhodeside & Harwell, but will leave soon to start a new firm with his wife, the landscape architect Melissa Rainer, and Ms. West. He advocates an ecologically expressive aesthetic that interprets rather than imitates nature.

Q. You visit a lot of gardens, and probably hear from gardeners like me with beds that just aren’t working. What’s the most common cause?

A. First, we have to understand that plants are social creatures. Our garden plants evolved as members of diverse social networks. Take a butterfly weed (Asclepias tuberosa, named this year’s Perennial Plant of the Year by the industry group the Perennial Plant Association), for example. The height of its flower is exactly the height of the grasses it grows among. Its narrow leaves hug its stems to efficiently emerge through a crowded mix. It has a taproot that drills through the fibrous roots of grasses. Everything about that plant is a reaction to its social network. And it is these social networks that make plantings so resilient.

So if we think about the way plants grow in the wild, it helps us understand how different our gardens are. In the wild, every square inch of soil is covered with a mosaic of interlocking plants, but in our gardens, we arrange plants as individual objects in a sea of mulch. We place them in solitary confinement.

So if you want to add butterfly weed to your garden, you might drift it in beds several feet apart and tuck some low grasses in as companions, like prairie dropseed, blue grama grass or buffalo grass.

Start by looking for bare soil. It is everywhere in our gardens and landscapes. Even in beds with shrubs in them, there are often large expanses of bare soil underneath. It’s incredibly high-maintenance. It requires multiple applications of bark mulch a year, pre-emergent herbicides and lots and lots of weeding.

The alternative to mulch is green mulch — that is, plants. This includes a wide range of herbaceous plants that cover soil, like clump-forming sedges, rhizomatous strawberries or golden groundsel, and self-seeding columbine or woodland poppies.

Q. If I want to try to do it more as nature does, what am I aiming for? Where do I take my cues?

A. The big shift in horticulture in the next decade will be a shift from thinking about plants as individual objects to communities of interrelated species. We think it’s possible to create designed plant communities: stylized versions of naturally occurring ones, adapted to work in our gardens and landscapes. This is not ecological restoration, it’s a hybrid of ecology and horticulture. We take inspiration from the layered structure in the wild, but combine it with the legibility and design of horticulture. It is the best of both worlds: the functionality and biodiversity of an ecological approach, but also the focus on beauty, order and color that horticulture has given us. It’s possible to balance diversity with legibility, ecology with aesthetics.

And it is a shift in how we take care of our gardens: a focus on management, not maintenance. When you plant in communities, you manage the entire plantings, not each individual plant. This is a pretty radical shift. It’s O.K. if a plant self-seeds around a bit, or if one plant becomes more dominant. As long as it fits the aesthetic and functional goals. We can do much less and get more.

Q. Sort of gives new meaning to the phrase “community garden,” doesn’t it?

A. Yes. And plants each also have particular behavior — whether it wants to hang out with other plants of its own species or not. So many gardening mistakes are a result of not paying attention to this.

Q. You make them sound like social animals, which makes me think I’ve been shallow, objectifying plants — choosing among them for just another pretty face, instead of reading their body language to get at their true nature.

A. One of the most useful ideas that came out of our research was this German idea of sociability, developed by Richard Hansen and Friedrich Stahl. They rank a plant’s predilection to spread on a scale of 1 to 5. A low-sociability plant is one that in the wild is almost always found by itself (Panicum virgatum, for example, is almost always found by itself in a meadow). A high-sociability plant is one that spreads into large colonies (Epimedium or Tiarella cordifolia are Level 4 plants; Carex pensylvanica and Packera aurea are Level 5). You arrange plants according to their sociability level: Plants of lower levels (1 and 2) are set individually or in small clusters. Plants of higher levels (3 to 5) are set in groups of 10 to 20-plus, arranged loosely around the others.

It sounds geeky, ranking plants on a scale, but it’s useful because it informs which you should mass, and which you should mingle. It’s why a mass of 50 echinacea (Level 2) tends to flop. They’re just not meant to cover ground. But if you scatter a handful of echinacea in a mass of prairie dropseed or sideoats grama, it will look great.

For years, I would pack together large grasses like switchgrass, or flowers like garden phlox (both Level 1 plants) and wonder why they got rust or powdery mildew. But if you find phlox in the wild, it will never have mildew. It’s growing out of a lot of lower plants, so it gets good air circulation. This idea changed the way I look at plants and pay attention to how they behave.

Q. I know that nature doesn’t plop a 50-foot tree in a mowed lawn (or mow its lawn at all, actually), so that’s not the winning design tactic. I also know that more diverse layered designs are richer ecologically — and now you are saying they are easier to manage, too. But how do I figure out how to fit the right plants together?

A. We need to start thinking about how, not what. So many garden books focus on what to plant, but so few focus on how to arrange plants to fit together in ecological combinations. When we fit our plants together like a tight jigsaw puzzle, the maintenance goes way, way down. They start becoming resilient systems rather than random objects.

To do this, we need to pay attention to a plant’s shape. Its shape is often an indication of where it grows in the vertical strata of a plant community. Upright plants with low or minimal basal foliage like Joe Pye weed (Eutrochium) or spiky upright plants like beargrass (Nolina bigelovii) have adapted to growing through other plants. Horizontally spreading rhizomatous plants like Pennsylvania sedge (Carex pensylvanica) or beach strawberry (Fragaria chiloensis) have adapted to grow underneath others. You almost have to look at a plant from the vantage point of a chipmunk to see its shape.

What I love about this layering idea is that it gives gardeners flexibility. Those lower layers should be very biodiverse: lots of different plants covering the ground and providing stability. But diversity in this layer does not really look messy, because most of these plants are growing underneath our taller ones, so you don’t really see them.

In my garden, I have a corner with dry shade where I have a handful of shrubs that screens a busy road. Lately I’ve been adding white wood aster (Eurybia divaricata), Appalachian barren strawberry (Geum fragarioides) and Pennsylvania sedge and watching them fill the gaps. Upper layers, on the other hand, are the ones I consider the “design” layers because they shape your impression of the planting. You can arrange them naturalistically, or in neat clumps — whatever style you like. That’s the flexibility: The order and legibility of the upper layers combines with the diversity and functionality of the lower ones.

The really cool thing is you can combine this layering idea with the sociability idea. Those Level 1 and 2 sociability plants tend to be those taller upright plants you use in the top layers of your garden because they like to grow through others. The Level 3 to 5 plants tend to be your lower spreading ground-hugging species.

Q. So I am not shopping for plants solely as decorative objects, but for plants with a purpose — for instance, as a living mulch or a good companion to others. Of course none of that, neither the “sociability” nor the plant’s layer, is on the plant labels. A tag might say “for containers or landscapes” or that the plant is “trailing” or “upright” or “mounding,” but that’s about it. What should the label say to help me put plants together successfully?

A. My dream label would describe things that are actually useful to understanding how it grows. It would describe its shape, its root system (taprooted, deep fibrous roots, shallow horizontal roots); its life span (a short-lived pioneer like columbine, or a long-lasting lavender); its sociability level; its adaptation to stress (quick-establishing, but short-lived ruderal species like Gaura lindheimeri or Nassella tenuissima; a thuggish, fast-spreading competitor like Monarda didyma; or a slow but steady stress-tolerator like Hosta or Calamintha). These are really the factors that explain how it will grow in our gardens.

Q. Where can we learn more? Being Northeastern, I love studying plants on the Go Botany plant finder from the New England Wildflower Society, for instance.

A. The Mt. Cuba Center, the Lady Bird Johnson Wildflower Center and the California Native Plant Society websites all have excellent information about how a plant grows in the wild and what it grows with. But mostly, I think gardeners can get to know their plants by going outside and getting reacquainted. Take a look at their shape, how they spread and see what they are trying to show you. You can learn a lot.

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