Taxonomy for Gardeners

Gardeners learn that scientific names promise stability, but more and more frequently, those names are changing.

A few years back, having just completed the Master Gardener training program, I remember spotting a beautiful leaf on a small sapling in the understory of a nearby woods. Its venation pattern included a series of curving parallel veins, each emanating from the leaf’s base on either side of the midrib, all meeting at the tip. Through an app and field guide, I determined the sapling was Cornus alternifolia, commonly known as alternate-leaf dogwood, pagoda dogwood, blue dogwood, pagoda-cornel, and green osier. Though its bracts aren’t as showy as a relative in the Cornaceae family, Cornus florida or flowering dogwood, it is both attractive and fragrant come spring. The scientific name that took hold in my memory was the genus Cornus, followed by the specific epithet alternifolia. After all, I had recently passed the course and was now a Master Gardener volunteer. Beginning to use the Linnean binomial system was a milestone for me, and I felt satisfaction in deepening my knowledge base. But even back in 2018, change was in the air, and Cornus alternifolia was already becoming known by a new name, the synonym Swida alternifolia, grouping the genus with other blue-fruited dogwoods.

Shifts in the taxonomy and subsequent renaming of many plants have occurred frequently during the last few decades, and more changes are promised. While alternate-leaf dogwood remains most commonly known as Cornus alternifolia, many taxonomists don't think it belongs in the Cornus genus anymore. Instead of one genus, they have revised almost all plants in Cornus into four different genera. For example, Cornus florida is now also known as Benthamidia florida, belonging to a new genus that groups together the larger bracted dogwoods.

While rules of botanical nomenclature can also alter a plant's name, taxonomic change is the focus of this article, especially the voluminous shifts and additions in epithets, genera, and modifications in the rank of family. Here are a few examples. Several species of popular house plants were once classified in the genus Thaumatophyllum, then moved to Philodendron, and are now back in Thaumatophyllum. The family of maples, formerly known as Aceraceae, is now referred to as Sapindaceae, the soapberry family. Once known as part of the Asclepiadaceae family, milkweeds are now ranked in the dogbane family, called Apocynaceae. The chestnut oak, also known as rock oak, was Quercus prinus, but to distinguish it from swamp oak (Quercus michauxii), chestnut oak is now Quercus montana. Like dogwoods, the entire genus of Salvia has been split into many new genera, while other plants have been folded into the salvias, like common rosemary (Salvia rosmarinus), formerly known as Rosmarinus officinalis, and Russian sage (Salvia yangii), formerly known as Perovskia atriplicifolia. If I had paid more attention in my ninth-grade Latin class, perhaps these pronunciations would roll off my tongue.

Deep in jungles, swamps, and volcano shafts around the earth, flora, fauna, and other unusual life forms are still being discovered. While taxonomists find, describe, name, and classify these discoveries, one of the major inroads in taxonomy today is dealing with the mass of data scientists are accumulating, especially about the 300,000-plus species of plants we already know. For one, what are their kinships and genetic connections along evolutionary lines or their phylogeny? Scientists want to understand how species are related to one another and how each plant has evolved differently and adapted to its environment.

The image of Carl Linnaeus, the Swedish taxonomist of yesteryear, wearing a waistcoat and breeches, studying the stamens and pistils of orchids through a simple lens, is not the image of a taxonomist today. Today’s taxonomist is often referred to as a systematist. These specialists delve into many areas but are generally less interested in the reproductive systems of angiosperms and more concerned with the order and characteristics of chromosomes and strands of DNA. Sophisticated technologies allow them to peer at what was once an unseeable world and observe life at a molecular level. Once upon a time, during the nineteenth and much of the twentieth centuries, taxonomists tended to lump organisms into general categories. But especially since the turn into the twenty-first century, with many new tools and insights, the process of "lumping" has given way to "splitting."

Whether to huff in dismay or shrug at scientific progress, so many name changes can confound gardeners, or at least me. It can be helpful to recall the different ranks in the taxonomic hierarchy. I use "Keeping Precious Creatures Organized For Grumpy Scientists" to help me remember Kingdom, Phylum, Class, Order, Family, Genus, and Species, but I’m flummoxed as to whether I should modify my mnemonic device to include subfamily, tribe, subtribe, subgenus, subspecies then variety, etc., and don’t forget clades.

A wee dose of irony also helps to mitigate my confusion. Take the aster as an example. The smooth blue aster that I’ve grown from seed was once called Aster laevis but is now known as Symphyotrichum laeve. In the 1990s, taxonomists determined that the North American taxon known by the genus Aster doesn't belong in that genus. Dozens of plants were reclassified. Do not fret. Horticulturalists, nurseries, botanical institutions, and even scientists still call them asters, even though the Aster genus is now a much smaller group of plants found mostly in Eurasia. Our native asters are grouped among a whole host of new genera. Note the little, lowercase "a," not the big, uppercase "A." After all, asters are still in the family Asteraceae.

The avalanche of data we are living through and the knowledge that comes with it contribute to a broader human understanding of all floras. For instance, environmentalists learn about the resiliency of plant communities and what plants best adapt to climate change; they hone strategies to maintain biodiversity and improve germplasm conservation. Not only scientists but horticulturalists and plant breeders develop a deeper awareness of kinships and the genotypes of plants; they become more adept at conjuring the latest cultivars or implementing the newest methods to improve plant vigor and control pests and disease. The impacts are ongoing.

No authoritative proclamations are sounded about shifts in taxonomic rankings. Agreement among taxonomists is not automatic. For the public, a synonym of the accepted scientific name starts to appear, but is not adopted all at once. While there is general harmony about the reclassifications of asters, controversy still exists around new designations regarding dogwoods and the genus Cornus. Disagreement is only quelled by the parsing of evidence and offerings of incontrovertible proof. Years, if not decades, can pass. Gardeners and other plant people, like botanists and foresters, wait for the dust to settle.

How does all the hubbub among taxonomists affect me? In my gardening, I’m attempting to pay closer attention not only to the genus and species of a plant but also to the rank of family. While writing this article, a tangential question popped into my mind. Is too much of the mint family (Lamiaceae) growing in my garden? Is there a good balance of other families? Cornaceae is quite prevalent; at least several specimens of alternate-leaf dogwoods are growing nearby. In spring, their scent will be lovely. But wait, do I call them Cornus alternifolia or Swida alternifolia? Like Shakespeare’s rose, Cornus alternifolia, by any other name, will smell as sweet. I’ll stick with Cornus for now.

World Flora Online

World Flora Online is one resource helping to retain order and stability despite the many taxonomic changes resulting from our increasing knowledge about the plant kingdom. This open-access site aims to provide pertinent information on all the earth’s known flora. As such, it lists accepted scientific names and every Linnean binomial synonym. Its database catalogs vascular plants, such as flowering plants, ferns, and conifers, along with non-vascular bryophytes, including mosses, liverworts, and hornworts. The website is handy to verify what a plant is being called or has been called, along with other encyclopedic information. World Flora Online is rich with valuable information and relatively easy to explore.

The Global Strategy for Plant Conservation, a group of taxonomists and botanical institutions, formed World Flora Online more than a decade ago. The online database was one initial step in its larger aim to stop the loss and extinction of plant species worldwide and create a more sustainable and biodiverse future in which plants and humans can thrive. Its objectives were adopted by the United Nations Convention on Biological Diversity in 2002.