Just a sample of the wide variety of citrus fruits. Photo: Bernhard Voß, Wikimedia Commons
Collective nouns are names for a collection of things: A clump of grass, a herd of cattle, a school of fish, a murder of crows … and I think we should call a group of citrus trees a confusion of citrus. Because that’s what they are, at least botanically.
Ever since I began growing citrus plants (starting with the usual orange pip harvested from a store-bought fruit, then sown in a pot that I tried as a kid), I’ve been struggling to put a proper botanical name on the plants I grow. But the names keep changing. The more taxonomists look at the case of citrus fruits (genus Citrus, of the Rutaceae family), they more they discover about the plants’ complex background … and the more the names are likely to change.
You see, pretty much any fertile citrus plant will cross with any other, even of a different species. A mandarin, for example, will cross with a lemon, a grapefruit or even a kumquat (Citrus japonica recently moved into the genus Citrus from its former genus, Fortunella).
Not only are most of the citrus we use daily complex hybrids, but many wild species are themselves hybrids.
That’s why farmers raising citrus fruits generally root cuttings or graft their plants: it’s the only sure way of obtaining a fruit that is true to type. I mean, would you want to plant field with mandarin pips only to discover, 15 years or so later when they start to produce, that they’re actually a something quite different … and probably quite unsaleable?
Just to confuse things further, though, many citrus are also capable of apomixis: producing seeds clonally, without any exchange of pollen. In that case, the plants grown from seed will be 100% identical to the mother plant. The sweet orange (Citrus ×sinensis), that is, the orange we usually eat, is a good example. It’s sterile, but produces seeds through apomixis, so generally will come true to type from seed.
You can’t classify citrus by looks alone. Because of thousands of years of crossing and backcrossing, similar-looking citrus fruits may have quite different ancestries. The two “lemons” I grow as houseplants, for example, the Meyer lemon (C. ×meyeri) and the Ponderosa lemon (C. medica × C. maxima) × C. medica), are considered only distantly related to the true lemon (C. × limon).
The genus Citrus appears to have evolved some 8 million years ago in the southeast foothills of the Himalayas, in a triangle between Assam (northeastern India), northern Myanmar and northwestern Yunnan (China). This is based on a fossil species (C. linczangensis) found there. Then it spread on its own, mutating and hybridizing as it went, throughout southeast Asia, eventually reaching many islands (Indonesia, Taiwan, Japan, the Philippines, etc.) and even crossing the Wallace line to Northern Australia.
Three ancestral species, all still grown, citrons (C. medica), mandarins (C. reticulata) and pomelos (Citrus maxima), were the main species involved in creating the citrus fruits we know best (oranges, limes, lemons and grapefruits) with various admixtures of other species for lesser-known fruit.
More recently, humans became involved in moving citrus plants around. The various peoples of Micronesia and Polynesia, for example, carried the fruit to many of the Pacific islands around 3000–1500 BCE.
The citron (C. medica), whose large fruit sometimes weighs 3.5 kg, was apparently the first species to reach Europe from India, brought overland via ancient incense trade routes, possibly around 1200 BCE, and was known to the Romans from various parts of their empire.
Arab traders are believed to have brought lemons, pomelos (also spelled pummelo), and sour oranges overland to southern Europe around the 10th century CE while sweet oranges arrived some 5 centuries later, carried around the Cape of Good Hope (Africa) by Genoese and Portuguese traders. Then citrus crossed the Atlantic with Spanish conquistadors to reach Florida and Latin America. By 1663, citrus were being grown at Versailles (near Paris, France) in special greenhouses known as orangeries.
And as they traveled, the different species crossed further … and continue to do so to this day
What Wikipedia Says
You’ll find all sorts of opinions on which citrus fruit belongs to which species, so I’m not even going to pretend to have all the answers. Here is what I found on Wikipedia at the end of 2019. I can guarantee that this information will change: citrus have not yet finished confusing us!
In the following list, the plants with multiplication signs in their botanical names (×) are known to be hybrids (although it’s not always clear as to exactly which species are the parents) while those without multiplication signs are ancestral species.
Bergamot orange – C. ×bergamia (C. ×limon × C. ×aurantium)
Bitter orange – C. ×aurantium (C. maxima × C. reticulata)
Blood orange – Citrus ×sinensis cultivars
Buddha’s hand – C. medica sarcodactylus
Calamondin – C. ×microcarpa (C. reticulata × C. japonica)
Citron – C. medica
Clementine – C. ×clementina (C. ×deliciosa × C. ×sinensis)
Grapefruit – C. ×paradisi (C. maxima × C. ×sinensis)
Ichang papeda – C. ichangensis
Kaffir lime – C. hystrix
Key lime (Mexican lime) – C. × aurantiifolia (C. medica × C. micrantha)
Kumquat – C. japonica
Lemon – C. × limon (C. medica × C. ×aurantium)
Lime or Persian lime – C. × latifolia (C. ×aurantiifolia × C. ×limon)
Mandarin orange* – C. reticulata and C. reticulata × C. maxima*
Mediterranean mandarin – Citrus ×deliciosa
Meyer lemon – C. ×meyeri (C. medica × C. ×sinensis)
Navel orange – C. ×sinensis cultivars
Papeda – C. micrantha
Pomelo – C. maxima
Ponderosa lemon – (probably (C. medica × C. maxima) × C. medica)
Sweet orange – C. × sinensis (C. maxima × C. reticulata)
Tangelo – C. ×tangelo (C. reticulata × C. maxima or C. ×paradisi)
Tangerine – C. × tangerina
Trifoliate orange – C. trifoliata, often treated as Poncirus trifoliata
*The original mandarin orange is a true species (C. reticulata), but most commercially produced mandarins have some pomela (C. maxima) “blood,” so are considered C. reticulata × C. maxima.
And there are well over 100 other citrus fruits less known to Western culture.
Confused? You should be. After all, that’s why I’m suggesting the collective name “a confusion of citrus.”