Bright-colored fruit, packed with vitamins, citrus definitely bring sun and a happy way of life to mind, even at the heart of winter. Today, they’re grown in orchards all over the planet. In temperate climates, only milder regions grow them in fields, but many citrus varieties are grown in potted plants from North to South in other regions!
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Citrus are truly one-of-a-kind. Their shiny evergreen leafage releases delicious citrus scents when you rub the leaves. Branches have more or less spikes, and the pink or white blooming is very fragrant. The pip-containing fruit are technically berries (with pips), the skin of which is inordinately thick and filled with essential-oil-producing glands, delicious and fragrant as well! One thing, though, is true: it’s very difficult to distinguish different varieties because of successive hybridization.
Citrus are, originally, all native to South-East Asia. Their cultivation quickly spread thanks to the Sumerians and, later on, the Arabs. There are three main genus: Citrus, Poncirus and Fortunella which we will still distinguish for clarity, even though since 1998, most botanists tend to group them all under a single genus, Citrus.
Poncirus trifoliata is the only citrus with deciduous foliage. It does have a few advantages, even though its fruits aren’t palatable at all. Very hardy (-4°F/-20°C), its very fragrant white spring blooming is highlighted by its shiny three-lobed leaves, a very ornamental combination. It is the most often-used rootstock on citrus plantations, and sometimes sends shoots out from under the graft point. Though it doesn’t really like limestone, it does however cope very well with drought because it sends its roots out very deep. Its small, bland round lemon-like fruits have a strongly scented rind. It does thus make for a beautiful little ornamental tree, up to 12 feet tall (4 meters), great for planting defensive anti-intruder hedges thanks to its sharp spikes. The ‘Flying dragon’ has corkscrew-like branches.
The three groups behave very differently once winter arrives, and so do the fruits they give. This table sets the main citrus described below side by side, from the toughest ornamental to the one species happy inside a heated house.
| Citrus | Genus or species | Cold it takes | What it is grown for |
|---|---|---|---|
| Trifoliate orange | Poncirus trifoliata | -4°F / -20°C | Ornamental tree and rootstock; spiny defensive hedge, fruits inedible |
| Kumquat | Fortunella margarita, F. japonica | 7 to 14°F / -10 to -14°C | Small fruits eaten whole with the rind, raw, candied or as marmalade |
| ‘Satsuma’ mandarin | Citrus unshiu | 10 to 14°F / -10 to -12°C | Large, nearly pipless early fruit; ripens without a heat wave |
| Bitter orange | Citrus (incl. myrtifolia) | 9 to 15°F / -9 to -12°C | Flowers for distillers, rind for candy; myrtifolia flavours Chinotto |
| Orange, clementine, mandarin, ‘Meyer’ lemon | C. sinensis, C. reticulata, C. deliciosa, C. lemon ‘Meyer’ | About 20°F / -7°C at best | Table fruit; only for sheltered gardens or pots brought under cover |
| Calamondin | x Citrofortunella microcarpa | Not hardy; keep indoors | The one citrus for a dry, heated room |
Read the hardiness column first: it decides whether a citrus stays in the ground or travels into the greenhouse each winter.
Generally, citrus orchards are concentrated in a few locations: around the Mediterranean, the Gulf of Mexico, California. Main varieties include the Meyer lemon, the Valencia orange, and tangerine. In sheltered gardens (15 to 20°F or -6 to -9°C), you can try yourself at growing certain species like the orange tree (C. sinensis), the clementine tree (C. reticulata), the mandarin orange (C. deliciosa) and the ‘Meyer’ lemon tree (C. lemon ‘Meyer’). Most other lemon trees are too fragile (20°F or -6°C is the minimum temperature).
Its very fragrant flowers are a key ingredient for distillers and the bark is used for making candy. The tree tolerates freezing down to 9-15°F (-9 to -12 °C), but the harvest is lost at 26°F (-3°C).
Dwarf bitter orange ‘Bouquet de Fleurs’ has very fragrant flowers, and myrtifolia both are well suited to growing in a pot (6 feet tall instead of 15 to 20 / 2 meters instead of 5 to 7).
This last species, with myrtle-like leaves (pictured here) produces fruits that are both mild and bitter: they’re what make the Maltese drink “Chinotto” so special.
Kumquat is a particularly resistant to the cold: down to 14 and even 7°F (-10 to -14°C). This very ornamental tree reaches 9-12 feet tall (3-4 m), and produces small oval fruits for Fortunella margarita , and round ones (slower growth) for F. japonica. Fruits are eaten whole, with the rind. The peel actually has a milder, softer taste the the flesh inside. Best raw, candied, or in marmalade. However, only very few fruits ripen early enough in areas where summers are short and cool. Moreover, if it freezes down to 24°F (-4°C), again, fruits will drop. Late blooming (sometime between May and July) means they won’t ever lose flowers to a late spring frost. Harvest begins in November and, depending on how fast some ripen, may last until the month of May. The Fortunella ‘Fukushu’ cultivar is the most vigorous, it produces larger fruits.
The ‘Satsuma’ mandarin orange (Citrus unshiu ‘Satsuma‘ is pretty hardy, comparatively. It can cope with cold weather down to 10-14°F (-10 to -12°C), if it isn’t wet. These large fruits with only very few pips are a favorite in Japan. They’re early, highly productive, and don’t require the usual heat wave to ripen well. The tree has a slightly weeping bearing, and also bears a great many fragrant flowers.
‘Owari’ produces larger, firmer fruits, whereas the ‘Okitsu’ mandarin orange is ready for harvest as early as September.
When grown in pots, all the citrus species listed above benefit from winterizing in a cold greenhouse, meaning temperatures should hover between 32 and 46°F (0° to 8°C). If need be, set up a small heating system. If temperatures rise over 55°F / 13°C, the tree doesn’t go dormant: you will need to water once a week and fertilize once a month, make sure it gets a lot of light, and refrain from setting it too close to a heat source (risk of drying out).
Good to know: calamondin x Citrofortunella microcarpa is, and probably always will be, the only citrus that is truly happy in the dry, heated atmosphere of a house!
