The answer lies in genetic diversity.
*This article was originally published in Wild Seed magazine Volume 3, pp. 14-17.
A spring trip to a garden center to seek out new plants is a welcome ritual for avid gardeners. But selecting plants has become more complicated in recent years, with terms such as “wild-type” (a plant grown by horticulturists from seeds of wild plants, to distinguish it from a true wild plant growing untended in nature) and “nativar” (a cultivated variety of a native plant species) muddying our choices.
Both wild-type and nativar are labels that provide hints about the genetic composition of the plants whose surface beauty catch our eye at a nursery. Since wild-type plants are grown from wild seed, they have a representative mix of the genes present in wild populations of the parent plants. Nativars on the other hand, are typically cloned plants, so each one is genetically identical to all nativars of the same name.
Why should we think about genetic diversity when we’re gardening? For one thing, emphasizing diversity in our gardens makes them more resilient to the detrimental effects of climate change, harmful insects, and diseases. For another, a diverse native plant garden is more supportive of native wildlife.
Choosing Between Cultivated and Wild-type Plants
Consider one of my favorite native perennials: foam-flower (Tiarella cordifolia). A versatile little plant that grows in part sun to full shade, foam-flower is a consistent spring bloomer that also has wonderful fall color. It thrives in a range of soil types, and holds up well in severe drought despite its preference for moister soils.
There are two distinct botanical varieties of foam-flower that occur in the wild: T. cordifolia var. cordifolia has a spreading habit, sending runners out from mother plants to form a dense mat over time. It has a broad natural range in eastern North America from Georgia north into Canada. T. cordifolia var. collina, on the other hand, has a clumping habit that lacks runners, and a more limited natural range, extending north from Georgia only as far as Maryland.
Foam-flower has fantastic practical application in a shady garden. Ecologically, it provides forage and critical overwintering habitat for insect pollinators. Aesthetically, plants with a spreading habit work well as living mulch, knitting together to cover the soil. Consider using it to add interest to a “leggy” woody species like mountain laurel (Kalmia latifolia), whose stems look lovely emerging from foam-flower’s green carpet. Foam-flower is also incredible in combination with other spring bloomers, such as creeping phlox (Phlox stolonifera) or gracefully spilling out of shady containers. Its evergreen foliage makes foam-flower a nice choice for gardeners seeking a four-season display.
Indeed, foam-flower is hardly a one-feature wonder. Its stark white flowers are a beautiful sight in May, but its long-lasting and diverse foliage make it an all-around exceptional ornamental plant. In fact, its varied foliage characteristics make it appealing to plant breeders, who have managed over the last few decades to turn this native plant into dozens and dozens of (sometimes garish) nativars with leaves of seemingly every color and myriad shapes and sizes.
The recent explosion in foam-flower breeding is driven partially by the gardening public’s growing interest in native plants. Once a fringe niche in horticulture, native plant gardening is now more mainstream than ever. Unfortunately, by manipulating the genetics of native plant species like foam-flower we often contradict our best efforts at supporting local ecosystems.
A good example of this was illustrated recently in the breeding of another common native plant genus, Lobelia. Annie White, a Ph.D. candidate in ecological landscape design at the University of Vermont, compared the relative pollinator support of native plant species with some of their commonly available cultivars, and her findings demonstrated the potential damage to wildlife caused by genetic manipulation through plant breeding.
A hybrid of cardinal-flower (Lobelia cardinalis) and blue lobelia (L. siphilitica) called “Fan Scarlet” (L. speciosa) to hummingbirds appears identical to the red cardinal-flower. White’s research revealed that ‘Fan Scarlet’ and other similar hybrids, however, provide only a fraction of the nectar energy a hummingbird requires. In other words, attracting hummingbirds to a garden with these particular nativars can be considered unproductive at best and cruel at worst, drawing them in with the promise of a full meal only to offer up a half-empty plate. Unfortunately for hummingbirds and other creatures, most nurseries sell only cultivars (such as the Lobelia hybrid “Fan Scarlet”) rather than wild-type plants.
Nurseries also favor vegetative propagation—that is, making more plants through cuttings, grafting, and/or tissue culture (in which thousands of tiny, identical plants are grown in petri dishes). When plants are propagated through these methods rather than from seed, they often reach commercial size more quickly. Vegetative propagation also yields a uniform crop of clones, which makes it more efficient for nursery employees to move quickly through flats of plants when watering, fertilizing, repotting, or pinching back shoots to encourage fuller growth. This efficiency leads to labor cost savings so it is an important consideration for a nursery that produces thousands of plants of a single species. Vegetative propagation also suits the retail market because customers look for the best possible prices and want all their chrysanthemums to look alike.
The Ecological Deficiencies of Cloned Plants
Genetically identical plants may be less adaptable to climate change and may support a narrower range of beneficial insects and other wildlife. For example, it’s nearly impossible to find natural species river birch (Betula nigra) in the trade. Nurseries typically sell only the ever-popular cultivar, ‘Heritage.’ As a result, no matter if you’re in Springfield, Illinois, or Springfield, Maine, the plant you find in the nursery or see growing in a parking lot island is genetically identical to every other river birch across the country.
To understand why local adaptation is important, consider cold hardiness. Cold hardiness in a plant population develops over millennia. Research shows that this trait is complex and controlled by multiple genes, which means that plants of an individual species can vary based on parentage.
Take, for example, red maple (Acer rubrum). The natural range of this native tree extends from Texas to Maine, and populations experience different environmental pressures based on where they occur naturally. If the climate grows colder, seedlings in a population that are less cold tolerant will die out, and those that are more cold hardy will survive. The result of this natural selection is that plants from a population in southeastern Texas are likely to be less cold tolerant—and perhaps more heat tolerant—than one located in southern Maine.
Genetically-driven differences in cold hardiness among members of the same species are difficult to observe in plants because there may be no discernible physical differences. A cold-hardy maple from Maine would look just like a heat-tolerant specimen from Texas. Let’s instead consider a species whose adaptation to cold is a little more obvious: Canis lupus familiaris, the pet dog. Selective breeding by humans has led to most of the differences among the 180 or so breeds recognized by the American Kennel Club, but the process is the same as natural selection: individuals are selectively bred to promote certain traits. For example, we can see that the thick double coat of the Alaskan malamute protects it from harsh winter temperatures. To extend the analogy to plants, the red maple from Maine has its own version of a “fur coat” that helps it survive harsh winter temperatures. We just can’t see it.
From past (painful) experience, we have learned that genetic diversity is never more important than when an exotic insect pest or disease threatens to wipe out an entire species. Most people are familiar with the economic and ecological disasters caused by potato blight, chestnut blight, and Dutch elm disease. In the early 1900s, when the epidemic blight affecting American chestnut (Castanea dentata) began, people raced to cut down the chestnuts before they were damaged by the blight. We now realize that such “salvage logging” may have destroyed trees that had some resistance to the fungus that causes the disease. And so rather than witnessing a slow but natural recovery from the blight, scientists have been scrambling to cross-breed the American chestnut with the Chinese chestnut (Castanea mollissima) to create a hybrid with the attributes of the American chestnut, but armed with the disease-resistance of its Chinese cousin.
Landscaping with Wild-type Plants
At Garden in the Woods, Native Plant Trust’s Botanic Garden in Framingham, Massachusetts, staff planted thousands of foam-flowers in the Curtis Woodland Garden as part of a large-scale renovation project. These plants were all grown at the nursery, Nasami Farm, from seed collected from wild foam-flower. Nasami Farm specializes in growing native plants from locally-collected seed, and many of its plants can be found on display at Garden in the Woods. While they lack flashy names like ‘Spring Symphony’ or ‘Pink Skyrocket,’ the natural species foam-flower planted in the Garden is locally adapted so staff do not have to worry about whether the plants are cold hardy or tolerant of our acid soils.
The foam-flower display is gorgeous, with large sweeps that show off the plant’s variable foliage that makes each plant among the thousands slightly different from its neighbor. This individuality helps to create a more interesting show even after peak bloom time has come and gone, and it helps ensure we are doing as much as we can to support local ecosystems.
Well-behaved Plants Seldom Support Ecosystems
Uniformity is often valued in American landscapes, where perfectly clipped hedges and manicured lawns are more common than meadows. From a professional grower’s or landscaper’s perspective, uniformity often means efficiency and, as a result, for decades well-behaved cultivated forms have been favored over wild-type plants. From a pollinator’s perspective, though, uniformity may mean scarcity, and it’s important that we plant wild forms to ensure our gardens are ecosystems that support these critical creatures. As difficult as it can be to find wild-type plants for sale in commercial garden centers, they are available for the gardener willing to do the extra work to locate the growing number of nurseries (regionally or online) that specialize in growing wild-type plants. Another option that many gardeners find particularly rewarding is to grow their own native plants from nature’s pre-packaged bundles of genetic diversity: wild seeds.
Mark Richardson is Director of Horticulture for Tower Hill Botanic Garden in Boylston, MA, where he oversees the horticulture and horticulture outreach staff and serves as chair of the garden’s sustainability committee. Prior to joining the staff at Tower Hill, Mark served as Botanic Garden Director for Native Plant Trust, where he oversaw Garden in the Woods and Nasami Farm native plant nursery. He has a passion for ecological horticulture and native plants and is co-author of Native Plants for New England Gardens (Globe Pequot, 2018).