Eric’s Pet Plant: Sweet Orange (Citrus sinensis)

Every year about this time  I get thinking it would be nice to have a citrus tree in our little greenhouse – a Meyer lemon, perhaps, or a kumquat. Not so much for the fruit, of which we would get not so much, but for the long season of powerfully fragrant blossoms. A mature plant can sweeten the air for months on end

The sweet orange in blossom over at Yale’s Marsh Gardens. Flowers are only 1 to 1.5 inches across

No way of knowing if it was the perfume that inspired Eric to choose his sweet orange as a Pet Plant, but I wouldn’t be surprised.

Sweet Orange – Citrus sinensis (syn. C. aurantium var. dulcis)

By Eric Larson

The first oranges came to Europe in the 11th or 12th Century, when the Persian Orange or Bitter Orange (C. aurantium) was brought to Italy. Sweet Oranges (C. sinensis)  arrived much later, in the 15th Century, when the Portuguese started importing them from Southeast Asia.

The Portuguese had a monopoly on trade from Asia that they wrested from the Venetians by dint of cutting out the middle-men: they were able to travel directly to Asia around Cape Horn, instead of relying on long supply lines overland from India. This not only let them charger lower prices, it also permitted the transport of more perishable goods. Until the Spanish occupied their country starting in 1580, the Portuguese used the East India trade winds (based on monsoon events) to arrive in Asia before spring, and leave for home by mid-summer.

Botany and Nomenclature

All of the citrus fruits that we eat today are from one ‘super-genus,’ Citrus, which gives us everything from the oranges in all their varieties to the grapefruits, the lemons, the limes and so many others. The Sweet Orange is an ancient hybrid, probably between the pomelo, C. maxima, and the tangerine, C. reticulata.

The interesting thing about members of this genus is that the species hybridize quite readily between each other. The ability to produce different fruit so easily by crossing the pollen from one species to another is probably one reason why this has been underway for such a long time in human history.

It also means that most of the propagating of any  preferred type is done by grafting clonal material onto rootstock, instead of by seed. The seed produced by hybrids (especially those with such a long convoluted history) will not bear fruit true to the parent. It is most likely to produce one of the many older, less-preferred relatives.

Originally the word citrus comes from the Greek word kedros, for Cedar.  In classical Latin, it was used to describe any aromatic tree or wood, but eventually it got focused onto the Citron, C. medica.  The common name orange goes all the way back to Sanskrit origins: nårangah means orange tree.

Other languages co-opted the word, and its descendant forms have referred to fragrance and color as well as the fruit. The Muslim culture brought the name along with the fruit to Spain and Italy.  The words for the sweet orange in many languages reflect the bearer of good news: in modern Greek for instance, the sour orange is called nerantzi, while the sweet orange is called portokali, for its Portuguese importers.

Growing Oranges

Our orange tree is blooming and setting fruit now. The longer day length has triggered its flower production, and the fruit that is present has been there for some months. As you enter the first bay (or room) of our greenhouse 1c, the aroma is unmistakable and intense.  This combination of sweet fruit, evergreen leaves and aromatic presence has made this one of the most intensively hybridized and cultured plants in existence.

In our climate, the orange is a greenhouse plant. They will live through cold snaps with temperatures in the upper twenties (some say as low as twenty two, but I think that’s pushing it) but when temperatures are below freezing for prolonged periods, the trees will die. Northerners can grow citrus in pots, but must bear in mind that they will need full sun through the winter.

We have the luxury here at Marsh Botanic Garden of good southern exposure for the greenhouses, as well as supplemental lighting that provides extra lumens when the weather is cloudy (as it seems to be most every day in the winter here in New Haven).

The famous Orangerie at Versailles was a long bank of windowed rooms on the south-facing side of the main hill. In the summer, the large pots of oranges stayed outdoors, enjoying the long days of a European growing season. In the fall, the pots were rolled on heavy-duty carts into the glassed-in rooms, and held in the plant form of hibernation. This worked for them perhaps, but I wouldn’t try it at home. (I might, if I had the big greenhouse with terrace and the heavy-duty cart. LL)

Orange trees are pretty heavy feeders, needing a good fertile soil and plenty of organic matter, supplemented with a good source of Nitrogen, Potassium and Phosphorus. Trace minerals are also very important, and so is choosing the right type for your soil and climate.

Navel oranges, grown from a sport discovered in Brazil in 1920, prefer low relative humidity and well drained soils. These are grown in the Southwest and in California. In Florida, other varieties are planted that are better adapted to humid conditions and denser soils. Regardless of  type, dwarf varieties should always be chosen for pot culture (that means growing in a pot, not…)

Good watering technique is also very important for citrus, especially when fruit is developing. Again there are varietal differences, with some types more suited to growing in ‘droughty’ soils and less dependent on abundant water, but all will respond to being well watered. One caveat: keep the water away from the trunk, and do NOT over-water: several fungal organisms are major pests of orange, so managing the water is key.

Actually, the pests of orange and other citrus are too numerous to list, and they can cause mortality, either in the damage they do or as vectors for diseases.  Especially in Florida, where groves count in the thousands of trees, the control of insect pests and diseases is a big-time operation, with resources beyond the average bug-zapper bearing down on the problems.

This small green orange is about 3 inches across right now. Eventually it will grow to 5 or 6 inches across,despite being on a dwarf rootstock. I blush to point out the little white fuzz on the upper right portion. Nothing to be too ashamed of, this critter is a mealy bug, one of the main pests of orange in the greenhouse. We have a very strong IPM program, but we also admit that we can’t get every bug. This one will not be there on Monday morning


The fruit takes from seven to eleven months to set and ripen, depending on variety and conditions. Oranges do not signify ripeness by changing from green to orange overnight. They can be left on the tree for some time and picked some weeks after the orange color has been achieved.

The Marsh Garden Orange tree, getting light and keeping warm

Observant readers will notice this tree is towering above our Pelargonium display, highlighted in the last Pet Plant column. Notice the high-tech sign identifying this plant.

Come visit us and see the orange tree and other culinary giants. In the meantime, enjoy the lengthening days and the strengthening sun, and be well.

Disclaimer: The opinions and views expressed herein are the sole responsibility of the author.  Yale University and Marsh Botanical Garden and Leslie Land are not responsible for the iconoclastic viewpoint or less-than-successful humor attempted in these columns.

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1 Comment »

  • Anne Said,

    Thanks for this! I work in a nursery and one of my favourite jobs is to take customers into the citrus house. It is heavenly with all those citrus plants in there; all in different stages of development depending upon age.

    It has been thought that citrus will take degrees down to about 28F for about 15 minutes. After that, they are in danger of dying, even in California (where I live). Anything close to that low of a temperature is dangerous to them.

    They are heavy feeders and need an amendment about four times per year. A little horticultural oil will help against mealy bug and scale.

    A note on colour: In Florida they add an orange dye to them at ripening and picking time; in California it is against agricultural laws.

    I can almost smell your citrus now!

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