Citrus

What is citrus?

Citrus is a genus of evergreen shrubs and small trees in the rue family, Rutaceae. The fruit we buy as oranges, mandarins, lemons or grapefruit comes from only a handful of wild ancestors. A genome study published in Nature in 2018 identified three of them as the main sources of today's cultivated fruit: the citron, the mandarin and the pummelo. The sweet orange is a mixture of mandarin and pummelo, the lemon descends from sour orange and citron, and the grapefruit is a cross of pummelo and sweet orange. The finger lime belongs to a separate Australian branch of the genus.

All of them share glossy, aromatic leaves, fragrant white flowers and a fruit whose segments are filled with juice vesicles. Most cultivated trees are grafted onto a rootstock, which decides much about vigour, soil tolerance and frost hardiness.

Origin and growth

The same study places the origin of citrus in the south-eastern foothills of the Himalayas. The Mediterranean came to citrus late and in stages, as the archaeobotanist Dafna Langgut has traced: first the citron, with remains from a Persian-period garden near Jerusalem, then the lemon, found in the Forum Romanum and dated to around the turn of the era. Sour orange, lime and pummelo followed from the 10th century, carried by Muslim traders by way of Sicily and Iberia. The sweet orange arrived in the 15th century on Genoese and Portuguese trade routes, and the mandarin only in the early 19th century.

Oranges form rounded trees. Navel oranges such as Washington Navel are seedless; blood oranges such as Tarocco owe their colour to cold. Mandarins are the most frost-tolerant of the common kinds. Lemons grow more upright and flower several times a year, but they are the most cold-sensitive. Grapefruit forms large trees with heavy fruit. The finger lime stays a thorny shrub or small tree and grows naturally beneath the rainforest canopy.

Requirements

Citrus wants warmth, sun and an even water supply. University of California extension advisers give approximate limits for mature trees: about -6.7 °C for satsuma mandarins, -4.4 °C for oranges and -3.3 °C for grapefruit; true lemons are more sensitive still. How long the cold lasts matters as much as the lowest reading, and the fruit is damaged earlier than the wood. For the finger lime there is no reliable threshold.

The Sardinian description of the Muravera lemon names the best soils as fertile, deep and well drained, with organic matter usually not below 2 %, clay not above 35 %, lime not above 30 % and a pH near neutral. Waterlogging harms the roots. In a dry summer citrus needs regular water; in Muravera the groves are irrigated with groundwater, mostly by sprinklers beneath the canopy.

Fruit, use and harvest

Most oranges and mandarins in the Mediterranean ripen from late autumn to spring. The Washington Navel of Muravera is harvested from mid-December to March, while the late local orange of Muravera can hang on the tree until July. Lemons flower and fruit almost all year.

One caution belongs here: grapefruit and its juice can change how certain medicines act in the body, and the U.S. Food and Drug Administration advises reading the leaflet or asking a pharmacist. The thorns of the finger lime and of many lemon trees are sharp.

In the food forest

In a food forest citrus belongs to the small-tree layer. It is evergreen, so it shades the ground all year, and its fine feeder roots lie close to the surface. That makes a permanent mulch layer especially useful: it keeps the upper soil cool and moist where the roots feed. Citrus profits from shelter against wind, and in Muravera windbreaks are part of the traditional groves. Fast-growing nitrogen-fixing plants can serve as nurse and windbreak in the first years and be cut back for mulch as the trees grow, following the logic of syntropic agriculture.

In the Mediterranean climate

Sardinia has a citrus tradition of its own. In the Sarrabus, around Muravera at the mouth of the Flumendosa, the Washington Navel has been grown since the early 1900s on bitter-orange rootstock, pruned and picked by hand, on alluvial soils sheltered by the Sette Fratelli massif and cooled by sea breezes. The region lists the low-acid sweet lemon of Muravera and the late blonde orange of San Vito, harvested in April and May and now being propagated again because it was close to disappearing. Muravera celebrates its Sagra degli agrumi every spring, in 2026 for the 52nd time. Further north, around Siniscola and Orosei, grows the pompia, a Sardinian citrus that is inedible raw and eaten only candied.

Sicily shows what cold does for colour: blood oranges colour reliably mainly around Etna. Spain and Portugal show a new risk. The African citrus psyllid, a vector of the bacterial disease huanglongbing, was found in Galicia in 2014 and in Portugal in 2015; the disease itself has not been recorded there. Citrus is not on the EU list of invasive alien species and does not spread on its own.

Propagation and care

Citrus is usually grafted; seedlings are possible but variable and slow to fruit. Mulch, shelter from wind and deep, regular watering in summer matter more than heavy fertilising. Potted trees need a large container and a frost-free spot or protection in the coldest nights.

How Terra Humana understands it

Citrus links Sa Medada to the island's own history. In the Sarrabus, further south along the same coast, people have shown for more than a century what patient, hand-tended groves can give. Terra Humana would like to see citrus trees in the small-tree layer of a mixed planting, sheltered by nurse trees and covered by mulch, rather than in a monoculture. Whether the more tender kinds, the lemon and the finger lime above all, will settle on the slope is something the coming winters are meant to show.