Who Designed That Plant?
Species vs. Cultivars
The days are getting longer and I can feel it: you’re fired up to go out and get some native plants to add to your gardens, window boxes and flowerpots. If you’re like me (well, the old me of a few months ago), the prevailing plant-purchasing strategy is to swing by the garden center down the street, pick out the plants with the loveliest blossoms, and then bring them home to decorate my yard – an approach not dissimilar to the one I’d use for selecting throw pillows.
If the goal is to create a healthy, balanced landscape capable of providing the ecosystem services that support human life, though, a throw-pillow strategy won’t cut it. There are a few additional variables to take into account.
First, there’s the geographic question of where plants are from. As explained here, it’s important to select plants that are native to your ecoregion and that – to the extent possible – have been grown locally, too.
Second, there’s the horticultural question of how they’re created and grown. Through the process of natural selection, hundreds of thousands of plant varieties have evolved, representing an unimaginable array of shapes, colors, sizes and survival strategies. These plants that nature has designed are called species. In addition, humans have figured out how to tweak plants’ features to suit our needs and whims. You want bigger fruits, redder blossoms, glossier leaves, fewer prickers or shorter branches? There’s probably a horticulturalist somewhere who has already engineered that for you. These plants that humans have designed are called cultivars, which is a contraction of “cultivated varieties.”
Cultivars have their place. They can be valuable additions to gardens, including those dedicated to native plants. Cultivars come with baggage, though, so biodiversity-minded buyers should beware.
Genetic Clones vs. Diversity
Most cultivars are propagated “asexually,” or vegetatively, meaning that a human takes part of a living plant and uses it to grow a copy. Examples of this process include cutting, grafting, layering and creating tissue cultures. So, for example, last week I received a catalog from Spring Hill Nurseries. One of its offerings is a cultivar named “Goldflame Honeysuckle.” I don’t know how many Goldflame Honeysuckles have been planted around the world, but I do know that every single one of them is an exact genetic clone of the others.
This is in contrast to the “sexual” reproduction common in nature, otherwise known as pollination. Pollen travels from male stamens to female pistils – often carried by wind or pollinator animals – and then fertilizes into a fruit with seeds. The valuable thing about sexual reproduction is that seeds produced through fertilization contain genetic material from both parents, making them genetically distinct from either of the parents, as well as from the seeds on the next plant over. This genetic diversity helps species adapt to and survive environmental changes.
Genetic diversity within a species is to that species’ survival what a diversified portfolio is to your retirement plan. You wouldn’t want to invest all your savings into a single stock that might tank. A species that encompasses multiple discrete populations with differing experiences and trajectories is more apt to withstand serious disruptions over time than a species that’s homogenous. Having that built-in variation minimizes the risk of the extinction of the overall species.
As a corollary, an abundance and diversity among species is to ecosystem health what rivets are to an airplane. Lose a couple of rivets and you can probably still fly. Lose more than a few and you’re in deep trouble. At some point, the loss of species points to catastrophic ecosystem collapse. It’s very much in our interest to support genetic diversity within species to ensure survival of species.
(I can’t claim credit for these analogies; I stole them from others. But I found them helpful and thought you might, too.)
In this post I reflected on the view that we should stop approaching landscape design as an exercise in imposing a man-made and static vision – however lovely – onto nature. Similarly, I’d argue that we should resist the temptation to fill our gardens with man-made and static versions of plants. The more we can work with the genetically-diverse species plants that nature designs, the better.
Human vs. Natural Selection
Remember that many plant and animal species have co-evolved together over thousands of years. In fact, about 90 percent of insect herbivores are specialists, meaning they can eat or lay eggs on only one type of plant. Meanwhile, the whole point of cultivars is to change one or more traits of a plant species, whether its growth habit, fruit size, flower characteristics, leaf color or disease resistance. What’s never known in advance is whether those alterations will make the plant unrecognizable or inaccessible to the wildlife specialists that depend on it.
For example, the mining bee Andrena lonicerae specializes on the honeysuckle Lonicera gracilipes. While several other types of bees visit this particular honeysuckle to collect pollen, only the Andrena lonicerae species has a tongue long enough to extend through the flower’s long, narrow corolla tube to reach its nectar. Indeed, that bee seems to get nectar only from that particular flower. The two species have co-evolved together. Now think back to that Spring Hill Nursery “Goldflame Honeysuckle” cultivar, which has been bred for its long blooming cycle and “extraordinary bicolor palette.” It’s an open question whether the Andrena lonicerae would even recognize the cultivar as “its” honeysuckle or fit inside the flower well enough to access its nectar.
Researchers are beginning to assess the ecological function of cultivars compared to their parent species. The Mt. Cuba Center in Delaware has a trial garden program specifically focused on evaluating native plants and their related cultivars for both ecological and horticultural value. The main point I draw from these and other studies is that it’s hard to predict whether a cultivar will support pollinators and other wildlife, not to mention perform any other ecosystem functions, as well as its counterpart species. Some do, some don’t.
If there’s evidence that a cultivar performs well, then we should feel free to plant it. But lacking such data, I’d argue we’re better off going with the species as nature designed it, because then we’ll know that plant is carrying its weight in the local ecosystem; we won’t have to guess.
How Do I Tell a Species from a Cultivar?
In short, beyond serving as decoration, plants have critical ecological roles to play, including contributing to biodiversity and supporting a complex and co-evolved food web. So we may need to put a bit more thought into the plants we choose for our gardens than for the accessories we toss on our sofas. But when you and your best intentions are perusing the catalog or browsing the garden center, how do you know what’s what? The labels rarely, if ever, explicitly say on them, “I’m a species” or “this way for cultivars.”
The easiest way to be sure you’re getting geographically- and horticulturally-sound plants is to shop at a reputable nursery (or from a reputable catalog or online site) dedicated to native plants. I’ll write a post on where to find those in the near future, but suffice it to say that these resources remain few and far between.
Meanwhile, and unfortunately, the vast majority of “native plants” sold in mainstream U.S. nurseries are cultivars. Species are hard to find. (For one thing, cultivars can be patented. Species can’t.) In addition, the labeling on plants is often sparse, confusing, misleading or all of the above.
I won’t bore you with a dissertation on botanical nomenclature, which I don’t fully understand myself. But I’ll share a few tips that may be helpful in deciphering labels.
Species plants usually have a common name (say, “purple coneflower”) as well as a two-word, italicized botanical name in Latin (e.g., Echinacea purpurea).
Cultivars will typically have an additional non-Latin, non-italicized descriptor tacked on to the end of the botanical name in single quotes (as in, Echinacea purpurea ‘Fragrant Angel’).
The names for hybrids, which are a type of cultivar, will typically have an “x” in them (such as Echinacea x purpurea ‘Balscanery’).
So if the botanical name is just two italicized Latin words, it’s probably a species. Jackpot! If it has non-Latin and non-italicized words in quotes or an “x” in the name, it’s probably a cultivar.
What If I Can’t Find Species Plants at My Nursery?
The most ecologically-sound choice will always be a species plant from your ecoregion. If that’s not available, consider a) a cultivar that closely resembles a species from your ecoregion or b) a species plant that’s a “nearby native” – from perhaps outside your ecoregion but still within, say, a 200 mile radius of your garden. Try to resist plants, whether species or cultivars, that are not native within that radius. And whatever you do, stay away from invasives!
It’s complicated, I agree. Throw pillows are simpler. The stakes are high, though, and I know we can do this! Happy spring shopping and gardening.
Resources
Online presentation by Uli Lorimer, Director of Horticulture, “Native Species, Selections, Cultivars,” Native Plant Trust, January 13, 2022.
Akira Shimizu, et al., “Fine-tuned Bee-Flower Coevolutionary State Hidden within Multiple Pollination Interactions,” Scientific Reports, Nature.com, February 5, 2014.
Annie White, “From Nursery to Nature: Evaluating Native Herbaceous Flowering Plants Versus Native Cultivars for Pollinator Habitat Restoration,” doctoral dissertation, University of Vermont, 2016.
“Bees, Bugs and Blooms – A Pollinator Trial,” Penn State College of Agricultural Sciences, Department of Entomology, February 5, 2016.
Abigail Lynch, “Why Is Genetic Diversity Important?” USGS, April 26, 2016.
Julie Shaw, “Why Is Biodiversity Important?” Conservation.org, November 15, 2018.