Do not worry — Botany Photo of the Day has not decided to change its content to science fiction! Today’s photo may look like a creative writer’s vision of unexplored planets, but it in fact shows the fruit and thorny branches of Citrus trifoliata.
The fruit of Citrus trifoliata may look like an orange (the common name for this species is trifoliate orange), but it certainly doesn’t taste like one. The fruits are edible, but are intensely sour and bitter, and are best used for adding pectin to jams that feature tastier fruit. A Citrus trifoliata lemonade recipe advises to “take a barrel of water, a barrel of sugar and add one sour fruit” (UofA Division of Agriculture). Some sources do not consider this species to be a member of Citrus. Many botanists use the synonym Poncirus trifoliata (L.) Raf., considering trifoliate orange to instead be the closest relative to the genus Citrus (and the only species in Poncirus). For this article, I will stick to Linnaeus’ original name, as this is the one accepted by The Plant List.
Whether you think trifoliate orange is a Citrus or not, this species is undeniably important to the lemons and oranges that we love to eat. Citrus trifoliata (pdf) makes an excellent rootstock for other Citrus species. It is very cold hardy (withstanding temperatures well below freezing), so other Citrus species grafted onto the rootstock can produce trees with tasty fruit that survive in cold climates. Arguably, the most significant advantage of a Citrus trifoliata rootstock is that it confers resistance to the citrus tristeza virus, the most economically-damaging Citrus disease. Trifoliate orange also hybridizes freely with other citrus, and has been used to make numerous crosses including: citremons (with lemons), citranges (with sweet oranges) , and citrumquats (with kumquats).
Aside from Citrus trifoliata‘s contribution to our morning orange juice, it is also an interesting ornamental species. Its young green branches are dense and bear stout, 5cm-long stipular spines. The overall effect of the plant’s habit is that it can be considered an evergreen for aesthetic purposes, though it is actually deciduous. Trifoliate orange can be pruned into an impenetrable hedge or low boxwood-type wall. The twisted-stemmed cultivar ‘Flying Dragon’ makes an unusual specimen tree. The young shoots of Citrus trifoliata are stiff and have a triangular cross-section, and the leaves are trifoliate (composed of three leaflets), shiny and dark green. They turn a buttery yellow before dropping in the autumn. The flowers are white with pink stamens, hermaphroditic, and have a typical, citrus smell. These give way to globose (round) fruit that measure 3-5 cm in diameter. Technically, the Citrus trifoliata fruit is a modified berry termed a hesperidium, which has a tough leathery rind encasing a fleshy interior composed of separated sections.
Note: Citrus trifoliata is naturalized in the American southeast, and forms dense thickets that are impenetrable and displace native species. Planting trifoliate orange in these areas is not advised.