A Gardener’s Guide to Plant Nomenclature, Part II
In the May 2016 issue we talked about Linnaeus, who developed the two-name or binomial classification system for plants, which was codified in his treatises, Species Plantarum (1753) and Systema Naturae (1758). Of course, the two names that are the foundation of the classification system — genus and species — are the key to a plant’s identity. But there’s more to it. This month we will clarify the terms on those tags you find stuck in the pot of that plant you’re about to purchase.
If you spot an intriguing plant in a nursery, you’ll likely take a look at its tag. Usually, the tag will identify the plant by its genus and species (or species epithet, a more technical term for species that you’ll sometimes see in books and articles; not something we’ll worry about here). Let’s suppose you’re looking at a boxwood; it might be labeled as Buxus (genus) microphylla (species) , and the tag may also include its common name, littleleaf boxwood. In fact, the Latin term “microphylla” means small-leaved. But nothing is ever simple, right? So you’re often going to see a third name, and maybe even more following the genus and species name. For example, you might take a shine to a plant whose tag says: Buxus microphylla var. insularis. Now you’re dealing with a variety.
What exactly is a variety? A variety is a subdivision of a species — meaning that some minor variation has happened — perhaps a difference in color or size — from the original species. A variety is a naturally-occurring variation of individual plants within a species. An example of a variety is the thornless honeylocust tree, Gleditsia triacanthos var. inermis. It is a naturally-occurring thornless honeylocust. Sometimes the difference between a variety and a species is a difference in flower color. The typical white-flowered dogwood we see in Virginia is the species, Cornus florida. There’s a naturally-occurring pink-flowered version, cornus florida var. rubra.
Most varieties will produce seeds that are true to type, which means that the seedlings grown from a variety will also have the same unique characteristics of the parent plant. For example, there is a white-flowering redbud that was found in nature. Its scientific name is Cercis canadensis var. alba. The varietal term “alba” means white. If you were to germinate seed from this variety, most, if not all, would also be white-flowering.
Oh, and just to make things a little more complicated, sometimes scientists decide to re-classify a plant based on a newer understanding of its relationship to other plants. That has happened to B. microphylla var. insularis, which has been reclassified by some authorities as B. sinica var. insularis.
Want to go a little deeper? If so, you might want to look at another subdivision below the species level: subspecies. This term is not often found in catalogs or plant tags, but just so you know, the term subspecies is also used to describe a subgroup of a species. Usually, the word subspecies is used to describe a natural population of plants with a variation from the species that is found in a particular geographical distribution or ecological range. A subspecies that you’ll often find in the nursery trade is Hydrangea anomala ssp. petiolaris.
Suppose you’re wandering the aisles of a nursery and you find a rose with this name on the label: Rosa ‘Blaze Improved ‘ (‘Paul’s Scarlet Climber’ x ‘Gruss an Teplitz’). Now that’s a mouthful. But the key point is that little multiplication sign telling you that this rose is a hybrid.
Hybrid: is a genetic cross between two different species and can be the result of a series of crosses between parents. Hybrids may also occur between different genera, subspecies, varieties and cultivars. The offspring of a cross can be fertile, partly fertile or completely sterile (if a horse breeds with a donkey, the result is a mule which is almost always sterile). Usually, the offspring of a hybrid will not be true to type.
A plant that results from cross breeding — a hybrid — is identified with an x in its name. So that rose with the mouthful of a name is a cross between ‘Paul’s Scarlet Climber’ and ‘Gruss an Teplitz’. So just by reading the label, you may see the family history behind your rose, though sometimes you’ll just see something simpler, like Rosa x ‘Blaze Improved’.
Hybrids sometimes occur naturally, but more commonly, they are the result of human efforts. The aim of a human hybridizer is to produce plants with desirable characteristics that can be sold commercially. If the hybridization yields successful results, the plant is usually given a cultivar name and you’ll begin to see it in nurseries and plant catalogs. So, a successful hybrid is also a cultivar. For example, Meserve Holly hybrids were originally bred by Mrs. Leighton Meserve of New York by using two species, Ilex rugosa (prostrate holly, a low and spreading shrub holly, for cold hardiness) and Ilex aquifolium (English holly, a large tree holly, for foliage and berry beauty). You’ll find a number of popular Meserve hybrids in most nurseries, including Ilex x meserveae ‘Blue Boy’ & ‘Blue Girl’.
If you’re a vegetable gardener interested in seed-saving, it’s important to know whether you’re dealing with a hybrid. That’s because a hybrid plant is NOT a good candidate for seed-saving: when you plant the seeds you saved from a hybrid, you end up with a random mix of traits from the grandparent plants and earlier generations. In other words, the seeds are not true to type. Now there are plenty of benefits to the newer hybrid vegetable seeds: they’ve often been bred for disease-resistance, and often have so-called “hybrid vigor” — stronger growth, higher yields, and even higher survival from the seedling stage. But if you want to save seeds from the vegetables you grow, you’ll want open-pollinated and heirloom varieties, which will allow you to save true-to-type seed.
And vegetable gardeners will also want to know about the term “F1 hybrid” — a term you’ll often find in seed catalogs and on seed packets. In fact, the terms “hybrid” and “F1” are strictly defined in the seed industry. Hybrid seeds are produced through controlled pollination, which can be a long, expensive process. A seed company chooses parent varieties that will produce first generation offspring — F1 hybrids — with the special characteristics they desire. But the seeds of the next generation will usually not be true to type.
Now let’s go back to the nursery. Or maybe you’re spending a snowy day poring over plant catalogs. You come to a boxwood identified as Buxus microphylla var. japonica ‘Green Beauty.” Well, you’ve happened upon a cultivar (and by the way, it’s a pretty special cultivar because it’s somewhat resistant to the dreaded boxwood blight that’s running amok in our gardens these days; see Sources below).
Cultivar: A cultivar is a sub-grouping of a species and it is usually (but not always) the result of selective breeding by humans, The term cultivar is a combination of the words cultivated and variety. Most cultivars are deliberate hybrids of two plants, as we talked about above. To propagate true-to-type clones, many cultivars must be propagated vegetatively through cuttings, grafting, and even tissue culture. Propagation by seed usually produces something different than the parent plant, and this is an important difference between a variety and a cultivar.
Some cultivars originated as “sports” or mutations that were discovered in the wild. A “sport” is a naturally occurring genetic mutation that causes a sudden change in the appearance of a plant. Imagine that you’re walking in the woods and you find a native Pinxterbloom azalea, Azalea periclymenoides — the one with those beautiful pink flowers. But the blooms on the one you encounter are an exceptionally large size. In fact, it’s so unique that you just might decide to take a cutting, and you take that cutting home and plant it. Voila! Your cutting takes hold and produces those uniquely large blossoms. The next thing you know, you’re propagating more of these special plants via cuttings. Finally, you decide to give it a name — perhaps A. periclymenoides ‘Macroflora’. Why those single quotation marks around the name? Well, that’s the convention that’s followed in the plant nomenclature world to indicate a cultivar.
The cultivar name is always written with single quotation marks. In technical writing, the cultivar name follows the genus and species and is always capitalized and written inside single quotation marks, but not italicized or underlined. For example, October Glory Red Maple is officially known as Acer rubrum ‘October Glory’. And in case you were wondering, it is possible to have a cultivar of a variety. For example, Cornus florida var. rubra ‘Cherokee Chief’.
Right now there seems to be a push for natives with different characteristics. You might wonder if some of these plants are truly native! You might even feel that some of these new cultivars of purple coneflowers are beginning to look a bit Frankenstein-ish.
But it just might have occurred naturally. You might wander into a meadow and find one purple coneflower out of thousands that has triple rows of petals. You could call it Echinacea purpurea ‘Frankenflora’. You now have a cultivar, but you could also call it a selection as well, which is often used with natives; you can divide it, but if you grow it from seed, chances are very high that the seedlings will be the typical purple coneflower with one row of petals.
These days cultivars are planted and used much more than varieties. But the terms are often confused. You’re likely to hear a plant referred to as a variety instead of what it actually is: a cultivar. I’ll let you mull this over. A good source for clarifying this variety/cultivar mish-mash is Botany for Gardeners by Brian Capon. Several more excellent sources are listed below.
Key Things to Remember:
- Seeds taken from species, varieties and subspecies are usually “true to type” — which means that the seeds produce a plant with the same characteristics as the parent.
- Seeds from hybrids (identified with the multiplication sign) almost never produce a plant true to type.
- In order to propagate a cultivar or a hybrid and come up with a plant true to its parent, most gardeners must either root or graft a cutting from the desired plant.
- Sometimes even plant people confuse varieties and cultivars!
Botany for Gardeners (Brian Capon, Timber Press, 3rd ed. 2010)
“Cultivar versus Variety,” Iowa State Hort. News (ISU Entomology, Horticulture and Home Pest News 2008)
“Plant Names: A Guide for Horticulturists, Nurserymen, Gardeners and Students,” www.hortax.org.uk (Horticultural Taxonomy Group, Version 1, March 2007)
“Cultivar versus Variety,” www.ipm.iastate.edu/ipm/hortnews/2008/2-6/CultivarOrVariety.html
“Ilex x meserveae,” Ohio State University, hvp.osu.edu/pocketgardener
“Frequently Asked Questions: What’s the difference between “hybrids” and “cultivars”?
http://anpsa.org.au/faq-9.html (Australian Native Plants Society).
“How are hybrid and open-pollinated vegetables different?” Oregon St.Extension, extension.oregonstate.edu/gardening
“Variety, cultivar, hybrid, heirloom… what terms mean,” Univ. Nebraska Extension, https://huskerhort.com/2014/04/06/variety-cultivar-hybrid-heirloom-what-terms-mean/
“Selecting Landscape Plants: Boxwoods,” https://pubs.ext.vt.edu/426/426-603/426-603_pdf.pdf
“Best Management Practices for Boxwood Blight in the Virginia Home Landscape,” https://pubs.ext.vt.edu/PPWS/PPWS-29/PPWS-29-pdf.pdf. (some boxwood cultivars are much less susceptible to boxwood blight disease than others. This article contains a helpful chart showing the degree of susceptibility of a number of popular cultivars).