Written by: Uli Lorimer, Director of Horticulture, ELA Board Member
Gardeners in the Northeast are fortunate to have a wealth of plant choices available in nurseries, garden shops, catalogs, and big-box stores. As you begin planning your garden for the season, consider probing beyond your initial round of plant choices and ask why do I want to plant this? Is it simply for its beauty, or for some other purpose, like more shade or structure? Perhaps we also should ask for whom do we garden—just for ourselves or for ourselves and the natural world? More and more gardeners are recognizing that their small patch of earth is connected to a local, regional, and global ecosystem, so the decisions you make in your garden can have real impacts on the natural world. Examining planting choices through this lens presents another set of questions: What can I plant that is best for local ecosystems? Which members of the animal kingdom can I support with my choices? And once I decide on those plants, where can I buy them? Finding the answers to these questions can be challenging for even experienced gardeners.
Let me begin with a declarative statement: No one gardens better than Mother Nature. Her designs are exquisite, and the connections between plants, insects, and animals that she has forged result from countless generations of evolution. The consequence of this process is biodiversity, as measured not only in species richness, but also in genetic richness. Inviting that biodiversity into our gardens begins with choosing a majority of native plants for the garden, specifically the straight species of natives, if available. The straight species of the plant evolved through natural selection and thus differs from a variety that humans have cultivated, often selecting for an aesthetic quality, like shorter habit, longer bloom time, or double flowers. These are cultivars, or cultivated varieties of the species, and they make up the majority of natives sold in nurseries. You can recognize a cultivar by the way the name is written, ending in a non-Latin name appearing in single quotes after the genus and species, such as Clethra alnifolia ‘Hummingbird.’
Cultivars are billed as improvements over the species in terms of their aesthetic performance in the landscape. But from an ecological perspective, is a shorter perennial really better because it does not flop over? Perhaps if we planted the unadulterated species closer together, instead of spacing the plants so that they do not touch each other, adrift in a sea of mulch, the plants would hold each other up. Observations of plants in nature tell us that they are social organisms, growing in intimate contact with each other and relying on their neighbors for support. Furthermore, what changes are happening to the invisible traits of plants, such as nectar and pollen quality, when we select for traits such as shorter habit, altered flowering times, or darker foliage? The answer will require more research, but the question is still worth bearing in mind when browsing the season’s touted new cultivars in your favorite garden shop.
Locally collected, seed-grown plants are the gold standard in genetic variety. When cloned cultivars succumb to a new blight or an intense drought, this seed-grown plant could end up being the individual capable of surviving climate change. The way in which we propagate plants also plays a role in their ecological value. For example, cultivars that are patented must conform to patent law, which stipulates that the characteristics that make the plant unique must be stable and reproducible. That can be achieved only by cloning—creating a genetically identical copy—through cuttings, layering, or tissue culture. Patented plants, therefore, have less genetic diversity as a result of clonal propagation, rendering them less adaptable to changes in the environment. Cultivars that are not patented may also be propagated clonally, and it is nearly impossible for consumers to discover whether or not the plant they want to buy is a clone. In addition, the industrial scale of commercial horticulture demands that the large wholesale nurseries that supply the bigger garden centers and big-box stores use cloning techniques so that the crops remain reliable, consistently uniform, and economically profitable. A 2017 study by the Mt. Cuba Center in Delaware revealed that 25 percent of the plants in wholesale nurseries in the mid-Atlantic region are natives; of that portion, only 25 percent are the straight species of natives (George Coombs, Denise Gilchrist). In other words, 75 percent of what wholesale nurseries offer are non-native plants, and of the remaining portion, 75 percent are cultivars. When we consider that many landscape designers, landscape architects, and landscape construction companies turn to wholesalers to get less-expensive prices on high-volume orders, the impact on the managed landscape is significant. Even if these professionals want to use only straight species, the available stock is insufficient. Homeowners face the same challenge.
If cultivars of natives bring less biodiversity to your garden, then locally collected, seed-grown plants are the gold standard in genetic variety. Each seed grown plant is genetically unique and therefore adds genetic diversity to the population of its species. When cloned cultivars of the species succumb to a new blight or an intense drought, this seed-grown plant could end up being the drought- or disease-tolerant individual capable of surviving the age of climate change. We simply do not know, which is an argument for preserving as much genetic diversity as possible. But growing plants by seed on a commercial scale is extremely difficult. First, it is crucial to not over-harvest seed from natural populations. Second, seed germination takes longer than growing from cuttings, and the resulting seedlings are not uniform nor consistent. This is fine, even advantageous in nature, but not in the marketplace, where designers and gardeners demand particular qualities from every individual plant. Your seed-grown plant also arrives in your garden with all its evolutionary relationships in place. When you consider that plants are the basis for the planet’s food web, this is significant. Plant relationships with insects are especially crucial, because insects are food for so many different animal species, including songbirds. Although this is another crucial area where more research is needed, existing studies have shown that to successfully reproduce, 96 percent of all songbirds require insects to feed their young. And plants host those insects, so clear connections exist between native species that support the greatest diversity of insects and the future generations of our songbirds. One study has shown that plants in which the foliage color has been drastically altered—from green to red or purple, for instance—do not support the insect life that their original species do. Insects that rely on the foliage of those plants simply do not recognize the purple or red leaves as food (Emily Baisden, Doug Tallamy, Desiree Narango, Eileen Boyle, 2018). Tallamy and others have shown that yards with a minimum of 70 percent native plants are capable of sustaining greater insect and bird diversity than yards with a smaller percentage of natives (Narango, Tallamy, and Peter P. Marra, 2018). What happens to pollinating insects, in particular, when we make changes to the flowers of native plant species is a question that Annie White, a lecturer in the University of Vermont’s Department of Plant and Soil Science, is investigating. White has compared species and cultivars of those species in their ability to attract pollinators (University of Vermont doctoral thesis, 2016). Although she found that some cultivars are actually more attractive to pollinators, the results broadly show that pollinators prefer species over cultivars; and the more altered the cultivars are from their wild relatives, the less the pollinators prefer them. Double-flowered cultivars are gorgeous for us to look at, but they provide nothing for insects. In selecting for more petals, we have sacrificed pollen and nectar, and the end result is a plant that really serves only one audience. Perhaps plant breeders should begin focusing on making selections that increase pollinator activity rather than on looking at aesthetic qualities like habit, bloom time, and flower characteristics. The more altered the cultivars are from their wild relatives, the less the pollinators prefer them. There is more to the question of whether cultivars are the ecological equivalent to species, and more research needs to take place before we can answer this question definitively. Weighing these concerns is difficult, and I do not intend that readers come away thinking that planting a cultivar of a native is wrong. The cultivar of a native species is far better than a non-native plant or a known invasive one. So far, we believe that the use of straight species, when available, supports the greatest amount of biodiversity and is something to strive for. When armed with more information, gardeners can decide what makes most sense for them and what choices can benefit ourselves and the planet.
If you are interested in reading more of Uli’s work, be sure to check out his recent book The Northeast Native Plant Primer