“Sure, you can name a tree, categorize it, safely identify it. But that tree exists, living the fullness of its quiet life, even if in its long history no man ever stood before it and labeled it. It knows itself already and mysteriously encounters the sun each day, nameless.” Ivan M Granger
In California, many nurserymen, gardeners, and botanists are awaiting the impending publication of the second edition of The Jepson Manual—a comprehensive guide to the state’s nearly eight thousand native and naturalized plants. Scattered throughout the pages will be hundreds of species with scientific names that have changed since the 1993 first edition. Among the species with changed names is the native California shrub deerweed, which we had grown accustomed to calling Lotus scoparius, a name that rolls off the tongue like a drink order in an Italian café. Its new scientific name is Acmispon glaber, which, when said slowly, sounds as if something were caught in your throat. Is this new name the result of a deeper understanding of deerweed or just nomenclatural wrangling meant to keep botanists busy and tenured?
Deerweed is not an isolated case. Douglas-fir has changed scientific names nearly twenty times since its discovery by Europeans, from Pinus taxifolia, through Abies douglasii, eventually settling on Pseudotsuga menziesii, where it remains today. Monterey cypress has been Callitropsis macrocarpa, Cupressus macrocarpa, Neocupressus macrocarpa, then back to Cupressus macrocarpa; in the new Jepson Manual, it will be Hesperocyparis macrocarpa. As botanists debated the genealogy of the edible apple, the most important fruit in the Northern Hemisphere, its name changed through variations of Malus communis, Malus domestica, Malus sylvestris, Malus pumila, and Pyrus malus. It has been said that if you confine three botanists and one plant to a single room, they will emerge with four new names for the plant.
Regardless if these name changes are necessary or academic caprice, a newly published scientific name can have significant, and sometimes detrimental, repercussions for those who use them, especially for commercially important plants. Everyone fears change, and, in this case, for good reason. Change means work and time: learning new names, changing documents, databases, labels, and tags, updating references, and cross-referencing old names until new ones become widely used. The time needed to stay abreast of new names imposes real economic costs on the nursery industry and causes confusion for gardeners, landscape architects, and designers. Despite these downsides, however, there are usually valid reasons why certain scientific names have been changed. Change in any naming system is inevitable and can be viewed as an opportunity to learn more about the plants we love.
Expanding Our Knowledge
Accurate plant taxonomy is essential to our understanding of plant evolution, relatedness, geography, conservation, and rarity. If we know what group a plant belongs to, then we know who that plant’s relatives are and who its ancestors were. Knowledge of relatedness can also help gardeners and nurserymen grow plants more successfully. For instance, understanding that a new plant is a member of the heath family (Ericaceae) will help a gardener find a suitable location with the acidic soils favored by most in that family. Precise knowledge of plant relationships is also crucial in terms of hybridization, rootstock selection, breeding for pest resistance, germ plasm conservation, and the creation of new cultivars.
However, accurate plant taxonomy is not easily achieved. The names of many plants remain the subject of controversy, and time-honored names of common garden plants are occasionally refuted. It is a wonderful and difficult thing to contemplate the diversity of the world’s plants, both extant and extinct. Nature is not easily understood, defined, or categorized, and experts have varying viewpoints on plant relationships and the names that represent those relationships.
Reasons for Name Changes
Scientific names of plants change for various reasons, most of which fall into three main categories. The first two, involving nomenclature and misidentification, are relatively rare in horticultural plants. As an example of the first (nomenclature), names change based on the rule of priority of publication. This rule, stipulated in the International Code of Botanical Nomenclature (you can find it online in case you’ve lost your copy), states that if a species has been named more than once, the first correctly published name is the one that must be used; all others become unused synonyms. A long-established name can sometimes be replaced by an earlier published name if new details emerge about the order of publication. As an example, London plane tree, the most commonly grown urban tree in the temperate world, has been the subject of its fair share of nomenclatural contention. Most authorities believe these trees to be hybrids between Oriental planes (P. orientalis) from Eurasia, and American sycamores (P. occidentalis) from the eastern United States. The common and widely used name, Platanus xacerifolia (the “x” denotes their hybrid origin), was printed in Species Plantarum in 1805. However, the German botanist Otto von Münchhausen published an earlier name for these hybrids, Platanus xhispanica, in 1770, making his the most accurate name to use.
Sometimes the name of an economically important or otherwise well-known plant is conserved for use even though it is technically incorrect. Conservation is a formal process that requires a decision at an International Botanical Congress (held once every six years). The established use of the generic name Chrysanthemum instead of Dendranthema for the garden chrysanthemum is an example of conservation. An investigation of the relationships of the plants grouped in Chrysanthemum revealed that some species were more closely related to members of other genera than they were to each other. The type species of the genus was garland daisy (C. coronarium), a plant familiar to many Californians as both an ornamental and a weedy escapee. The studies indicated that the economically important garden chrysanthemum should be excluded from Chrysanthemum and placed into the genus Dendranthema. Growers protested, and a proposal to designate the garden chrysanthemum as the type species of Chrysanthemum was accepted at the 1999 Botanical Congress in St Louis. Because of this decision, the three species assigned to Chrysanthemum in The Jepson Manual are now assigned to other genera. Garland daisy and corn daisy have became Glebionis coronaria and G. segetum, respectively, and tricolor daisy is now Ismelia carinata.
As an example of the second reason (misidentification), plants are sometimes brought into cultivation, propagated, and widely distributed under an incorrect name. By the time the misidentification can be rectified, the incorrect name may already be in widespread use. Name changes of this type are apparent in two widely cultivated trees from eastern Australia. King palm (Archontophoenix cunninghamiana) was for a long time incorrectly sold as Seaforthia elegans, which is an altogether different species of palm not regularly found in cultivation. Australian brush cherry (Syzygium australe) has for years been sold under the erroneous name of Syzygium paniculatum, which is actually a rare species, also from eastern Australia but only occasionally found in cultivation.
The third and most common type of name change happens when advances in our botanical knowledge lead to reclassifications. All taxonomy (the science, art, and craft of classification of all living things) and the scientific plant names generated by taxonomists are based solely on hypotheses. A taxonomist, often in the distant past, proposed that a newly discovered plant should be classified as a member of a particular genus, based on the similarity of morphological characteristics shared by other members of that genus. This naming was usually based on the hypothesis that the plant in question was related to other similar-looking plants with similar names—those in the same family or genus. Since the publication of Darwin’s On the Origin of Species in 1859, and the revelations that accompanied the ideas therein, taxonomists have classified plants based on the perceived evolutionary relationships among them. All species in a genus must be more closely related to each other than they are to species in other genera. In the best-case scenario, a genus (or any other level of classification) consists of all descendants of a common ancestor—and only species descended from that ancestor.
New taxonomic information has accumulated in an accelerated fashion since the advent of DNA sequencing, and that has resulted in a recent flurry of reclassification. Comparing DNA sequences amongst different species has proven to be an effective way to determine relatedness. As old taxonomies succumb to new understanding, name changes are sometimes required to uphold the principle that names must represent true evolutionary relationships. Sometimes, to achieve the goal of having a taxonomic group represent an evolutionary lineage, a genus or family that does not meet these criteria must be split apart. In other cases, mergers bring about the desired results. Many species have been transferred from one genus to another. The tomato is no longer in the genus Lycopersicon; now it’s a Solanum. Hebe has been subsumed again into Veronica. Some species of Acacia have become Senegalia, others Vachellia. Floss silk tree goes from Chorisia to Ceiba. Many species of trumpet trees are reassigned from Tabebuia to Handroanthus. All bottlebrushes amalgamate in Melaleuca, and Callistemon is no longer. And the list goes on . . .
New DNA sequence information has affected two ubiquitous horticultural trees in California: lemon-scented gum (Corymbia citriodora) and red-flowering gum (C. ficifolia). Before 1995, the hundred or so species of Corymbia were treated as members of the closely related genus Eucalyptus in the myrtle family (Myrtaceae). Taxonomic studies using DNA sequences helped scientists realize that Corymbia trees are more closely related to Angophora trees (a third genus in the same family) than they are to Eucalyptus. A finding like this demands a name change; if the trees recently placed in the new genus Corymbia were, instead, to remain under the name Eucalyptus, then all Angophora trees would have to be renamed Eucalyptus.
Some plant families have been merged or split. The maple family (Aceraceae) is now gone and all maples have been placed in the soapberry family (Sapindaceae). All members of the milkweed family (Asclepiadaceae) are now in the dogbane family (Apocynaceae). Plants that have traditionally been treated as members of the lily family (Liliaceae) are now found in more than ten different plant families, including the agave family (Agavaceae), the asparagus family (Asparagaceae), and the aloe family (Asphodelaceae).
Modern plant taxonomy, with its basis in DNA sequences instead of visible morphological characteristics, can make strange bedfellows. Recent discoveries place the cucumber family (Cucurbitaceae), with its herbaceous, vining pumpkins, squashes, and melons, as a nearby relative of the oak family (Fagaceae), known for its stately oaks, beeches, and chestnuts. And, as it turns out, sycamores (Platanus ssp.) are related to the water lotus (Nelumbo nucifera) and to members of the Protea family (Proteaceae), which includes grevilleas, banksias, and macadamia nuts; together, this group of seemingly unrelated plants makes up the modern order Proteales.
Even modern taxonomy, however, has its pitfalls, and disagreements emerge about interpretations of plant names and relatedness. In the dispassionate world of DNA sequences, the judgment and experience of taxonomists still plays an important part in determining the names assigned to plants. Although it is probable that the worst of the turmoil is over, plant names will continue to change for the three main reasons described above. Evolutionary relationships amongst plants will continue to be the subject of scrutiny and reinterpretation. Plant names as we know them now, and the names we will use in the future, are the achievements of hard-working taxonomists. Feeble as the act may seem, taxonomists are working to make sense of the bewildering world of plant diversity—making it, ultimately, more manageable for everyone else. And although this work results in some inconvenience and confusion in the short term, it is ultimately necessary for a deeper understanding of the plants we work with, grow, sell, and love.