Taxonomy for Gardeners Part I: Why Classifying Plants Matters

By Cynthia B, Lauer, Master Gardener, SCMG

The Basics of Taxonomy

Have you experienced the surprise of discovering that a celebrity has an underworld sibling? What about the shock of learning that you have a long-lost cousin who lives in a distant land? Your surnames might be similar but you’re hard-pressed to find anything else you might share. Your distant relatives may not speak the same language or share the same geography as you. Yet underneath the surface, at the genetic level, you actually have a great deal in common.

When it comes to plants, these secrets are exposed through taxonomy, the science of classifying living organisms. To a taxonomist, order is everything. From the top down, the hierarchy is kingdom, division/phylum, class, order, family, tribe, genus, species. Flowering plants are found in the kingdom Plantae and the phylum Magnoliophyta. From there, classifications become staggering in their complexity. Given the sheer numbers—with over 400 families under the Magnoliophyta phylum—class, order (and an interchangeable category known as clades), family, and genus often break down into sub-categories that are actively debated among taxonomists. To complicate matters further, many of our well-known garden plants are a variety, hybrid, or cultivar of the species. These details are always noted in the scientific name for a plant.

Living things are designated by two Latin words or binomial that consists of the genus followed by the species. The carrot, for example, is Daucus carota, one of about 75 species of Daucus worldwide. The common garden peony is Paeonia lactiflora, one of about 36 species of plants in the Paeonia genus. Protocol matters; the genus is capitalized and the whole name is italicized. To qualify as a species, a group of plants must descend from a common ancestor and must be able to reproduce with one another but generally not with the members of another species. The characteristics unique to the species are reproduced through their seeds from one generation to another.

The Benefits in Using Taxonomy

Taxonomic minutiae may sound tedious but they provide a lot of useful information to gardeners. Not only a who’s who of plants, taxonomy also reveals who is related to who, where and when plants evolved, and what this means in terms of a plant’s needs. Unlike taxonomists, most of us pay little attention to the precise arrangement of stipules (the outgrowth on the base of a leafstalk) between two species of rose. Still, it’s fascinating to learn that the rose family includes many of the fruits we enjoy eating (cherry, peach, nectarine, plum, apricot, apple, pear, raspberries, strawberries), or that petunias and tomatoes are close cousins, taxonomically speaking.

There is another important advantage to the scientific name for plants; it avoids confusion over their common names. Landscapers often use “cedars” for hedges but they are not true cedars since they do not belong to the genus Cedrus. Eastern white “cedars” are typically arborvitae (genus Thuja) that some tree farms also market as Western white cedar or Western red cedar. The Eastern red “cedar” is really Juniper virginiana. Martagon, wood, and oriental lilies are examples of members of the genus Lilium. Yet as a common name, the lily abounds. We have the tiger (Hemerocallis), trout (Erythronium), mariposa (Calochortus), blood (Haemanthus), crinum (Crinum), calla (Zantedeschia), canna (Canna), cobra (Darlingtonia), spider (Hymenocallis), voodoo (Amorphophallus), and pineapple lily (Eucomis), as well as the lily of the Nile (Agapanthus) and the lily of the valley (Convallaria). None are true lilies; most belong to a different taxonomic family altogether.

The problem becomes serious when it comes to the control of invasive plant species. At a recent presentation I attended, gardeners were warned against the aquatic and highly invasive yellow iris. Some feared that they had Iris pseudacorusgrowing in their gardens confusing it with their benign yellow ornamental Iris siberica. We know that the Asian multiflora rose (Rosa multiflora) invades woodlands but it shouldn’t be confused with our native roses like the smooth rose (Rosa blanda), the prairie rose (Rosa setigera) the wild rose (Rosa rugosa), the prickly wild rose (Rosa acicularis), and the swamp rose (Rosa palustris). Creeping bellflower (Campanula rapunculoides) is so bad that it has a Facebook support group for gardeners who find themselves dealing with it. The native harebell (Campanula rotundifolia) is a native look-alike. Both are simply called bellflowers.

“Phylogeny of Angiosperms” poster, Asterids section.

section of poster diagram

Knowing the scientific name prevents the horticultural industry from making errors and retail plant consumers from being misled. For advocates of native plants, names make a difference in public opinion when a plant like helenium is known as sneezeweed. Calling coreopsis a tickseed or balsam groundsel a ragwort or an asclepias a butterfly weed doesn’t help the cause. Common names for the marsh marigold (Caltha palustris) includes such inventions as brave bassinets, bull flower, may blob, meadow cowslip, water buttercup, soldier’s buttons, and so many more that in their book, The Gardener’s Guide to Native Plants of the Southern Great Lakes Region, authors Rick Gray and Shaun Booth leave the list incomplete.

Staying On Top of Changes in Taxonomy

New technologies in DNA sequencing among related plants have opened up knowledge about resistance to disease and insects that may improve food crops, develop new drugs, control trade in endangered species, and manage natural environments. This development has revolutionized the science of taxonomy. It allows organisms to be grouped within a common family tree based on their evolutionary lineage rather than just their outward appearance.

The science of taxonomy is dynamic. Carolus Linnaeus’ 1753 publication, Species Plantarum included more than 6,000 plants under his hierarchy and binomial naming system that are still in use today. Now revisions to the International Code of Botanical Nomenclature are adopted at an international congress held every six years. Decisions are inscribed into the International Plant Names Index (IPNI) authorized by a collaboration between The Royal Botanic Gardens, The Harvard University Herbaria, and The Australian National Herbarium. To give just a few examples, rosemary, the common culinary herb, is no longer known as Rosmarinus officinalis but as Salvia rosmarinus, and Russian sage (Perovskia atriplicifolia) is now Salvia yangii. Joe-pye weed is no longer a Eupatorium but a Eutrochium. North American asters are now in the Symphyotrichum genus to distinguish them from European asters with whom, science has proven, they have little in common. Gardeners often struggle to keep up with the changes!

In the next three articles, we’ll take a look at nine families of food-bearing plants—Apiaceae (carrots), Brassicaceae(mustards), Lamiaceae (mints), Cucurbitacea (squash), Fabaceae (beans), Solanaceae (potatoes), Poaceae (grasses), Rosaceae (roses), and Asteraceae (daisies). These cover many, but not all, of the fruits and vegetables we eat. Missing are avocados, bananas, grapes, beets, and many others. An exhaustive list of plant families is impractical. It is also unnecessary to appreciate the key point: fascinating connections between species are confirmed through scientific observations that began hundreds of years ago and continue today.