Citrus maxima – Pomelo
Citrus maxima (Burm.) Merr., known as pomelo, pompelmoose, shaddock or pummelo, is an evergreen fruit tree from the rue family (Rutaceae), of exceptional importance in the history of citrus cultivation. It is one of the three original, non-hybrid species of the genus Citrus - alongside the mandarin (Citrus reticulata) and citron (Citrus medica) - from which nearly the entire range of citrus fruits known today arose through natural and deliberate hybridisation. Approximately 63% of the grapefruit genome comes from pomelo, and the contribution of this species has also been demonstrated in sweet orange, bitter orange, lemon and many mandarin varieties.
The scientific name has a rich history: Linnaeus described the plant as Citrus decumana, while many older works refer to it as Citrus grandis (L.) Osbeck. The currently accepted combination, Citrus maxima, was published by Elmer D. Merrill in 1917, based on Burman's name Aurantium maximum from “Herbarium Amboinense”. The epithet “maxima” - “the largest” - refers to the fruits, which are the largest in the entire genus. Plants of the World Online (Kew) lists Assam, the eastern Himalayas, Myanmar, Thailand, Laos, Cambodia, Vietnam and the Nicobar Islands as the species' natural range, while in Malaysia, Indonesia, southern China and the Pacific islands, pomelo has been cultivated for so long that it is difficult to distinguish natural populations from escaped plants.
Botanical description and plant habit
Pomelo is a tree 5-15 m tall, usually branching low down, with a somewhat crooked trunk 10-30 cm thick and a broad, irregular crown. Young shoots are angular and often covered with fine hairs, while the branches and trunk bear thorns up to 5 cm long; in many cultivated varieties, however, the thorns are sparse or almost absent. The leaves are alternate, leathery, ovate to elliptic, 5-20 cm long, dull green on the upper surface and finely hairy underneath, with a crenate margin. The most distinctive feature is the leaf stalks - broadly winged, sometimes almost as wide as the leaf blade itself, which distinguishes pomelo from most other citrus fruits. When crushed, the leaves release a distinct citrus scent thanks to their numerous oil glands.
The flowers, the largest among citrus fruits, appear singly or in clusters of 2–10 in the leaf axils. They have 4–5 fleshy, yellowish-white petals measuring 1.5–3.5 cm in length, sometimes greenish-speckled on the outside, as well as numerous prominent white stamens. Their intense, sweet fragrance is used to flavor tea and in perfumery. The fruit is a hesperidium berry, ranging in shape from spherical through flattened to pear-shaped, measuring 10–30 cm in diameter and usually weighing 0.3–2 kg, although record specimens weigh several kilograms. The thick, spongy rind (albedo) is usually 1–2 cm thick, while the flesh is divided into 11–18 segments ranging in color from yellowish to pink and red. In most varieties, the seeds are few, large and yellowish-white.
Growth rate and development
In a tropical climate, pomelo grows moderately quickly. Vegetatively propagated trees in Thailand and China begin fruiting after 3–5 years, whereas seedlings usually need 5–8 years to flower for the first time. A mature tree produces 70–100 fruits per year, and a yield of around 20 tonnes per hectare is considered very good. The plant is long-lived - well-managed orchards remain productive for many decades, and old trees reach impressive sizes. In the tropics, the tree grows virtually continuously, producing several flushes of growth each year; each one may end in flowering, which explains the simultaneous presence of flowers and fruits at different stages of ripeness on the same tree.
In container cultivation in Europe, the growth rate is significantly slower and depends almost entirely on the amount of light and warmth. A seedling reaches from a dozen to several dozen centimeters in its first season, then grows by 20–40 cm per year in subsequent years. A potted plant usually stops at a height of 1.5–3 m, and its size is easy to regulate through pruning. Pomelo has monoembryonic seeds, which is an exception in the genus Citrus: most species produce polyembryonic seeds with nucellar embryos that are clones of the mother plant. In pomelo, only a sexual embryo develops from the seed, so the offspring are genetically diverse - a characteristic that has made the species such a prolific source of new hybrids.
Flowering and propagation
In its homeland, pomelo flowers 2-4 times a year, most abundantly after the rainy season begins, and successive batches of fruit ripen at different times; the main harvest in Thailand takes place in November. Approximately 6-8 months pass from pollination to fruit maturity. The flowers are insect-pollinated, produce abundant nectar and are eagerly visited by bees, although many varieties also set fruit after self-pollination. Thanks to their thick skin, the fruits keep exceptionally well - for even several months in well-ventilated crates, while the flesh becomes juicier and sweeter over time.
In commercial cultivation, elite varieties are propagated vegetatively: traditionally by air layering, and today by budding onto rootstocks of pomelo, mandarin, rough lemon or Rangpur lime. Nevertheless, in Southeast Asia, a large proportion of trees still grow from seed because the seedlings are healthy, vigorous and relatively true to the characteristics of their parents. In Europe, a plant grown from seed is primarily ornamental and suitable for collectors; if the goal is to obtain fruit of a specific variety quickly, the seedling can be used after 2-3 years as a rootstock for grafting.
Environmental requirements and tolerance
Pomelo is a tropical and subtropical species and the most warmth-loving of the popular citrus fruits. The optimum growth temperature is 25-30°C; below 13°C and above 38°C, growth stops, while the average temperature of the coldest month in commercial-growing regions does not fall below 15°C. Short-term drops to around -2°C damage the leaves, while frost below -3…-4°C threatens the entire plant; on the USDA scale, the species is classified as zone 10. Cultivation produces the best results in lowlands, up to 400 m above sea level, with annual rainfall of 1500-1800 mm; irrigation is essential during a prolonged dry season.
The soil should be deep, fertile and well-drained yet moisture-retentive, with a slightly acidic reaction (pH 5-6.5). Among citrus fruits, pomelo is notable for its salt tolerance: in the Menam Delta, it grows on ridges flooded with brackish water, and Thai growers claim that salt improves the flavor and juiciness of the fruit. The tree also grows on the oolitic limestone of Florida and the Bahamas and on mine spoil heaps in Malaysia, demonstrating the great adaptability of its root system. However, it does not tolerate stagnant water or compacted, clayey soils, where root rot caused by Phytophthora quickly develops.
Cultivation in a temperate climate
In Europe’s temperate climate, pomelo is grown exclusively in containers. In summer, the plant should be placed in full sun - on a terrace, balcony or in the garden, in a spot sheltered from strong winds, which can break its large leaves. The growing medium is prepared from citrus soil or a universal mix with the addition of 20–30% perlite, coarse-grained sand or fine bark, always over a drainage layer. Water generously when the top layer has dried out, preferably with water that has been left to stand or rainwater, and never leave water in the saucer. From spring to the end of August, fertilize every 2–3 weeks with a citrus fertilizer containing micronutrients; characteristic yellowing of the leaves between the veins signals an iron or magnesium deficiency, common with hard water.
Overwintering is the most difficult stage. Move the plant indoors when nighttime temperatures fall below 8–10°C, preferably to a bright, cool place with a temperature of 10–15°C: an unheated greenhouse, conservatory or cool stairwell with a large window. In such conditions, pomelo enters dormancy and needs only sparse watering once every two to four weeks. Overwintering in a warm, dry home with too little light usually results in leaf drop - in that case, supplementary lighting with an LED lamp for 10–12 hours a day and higher air humidity are necessary. In spring, acclimatize the plant gradually by first placing it in partial shade to avoid leaf scorch. Repot every 2–3 years, in early spring, into a pot several centimeters larger. The most common pests are spider mites, scale insects, mealybugs and aphids, which are easy to spot on the undersides of the large leaves.
Uses in ornamental gardening
Pomelo is a valued conservatory and collector’s plant. Its enormous, glossy leaves with winged petioles make even a young seedling look exotic and stand out among other citrus plants, while a mature tree in a large pot becomes a specimen plant in a winter garden. Fragrant flowers appearing on older plants are an additional attraction, and each fruit that sets is an event because of its size. The species tolerates pruning well, so it can be trained as a classic standard orange-tree form or as a dense, rounded shrub. In warmer regions of Southern Europe, pomelo is also planted in the ground in sheltered gardens, where it forms impressive, shade-providing trees.
Cultural significance, history and name etymology
Pomelo has been cultivated in Asia for at least two thousand years; it probably reached China around 100 BC, where it became a ceremonial fruit - during the Mid-Autumn Festival and Chinese New Year, the large, round fruits symbolize prosperity and family unity, while in Vietnam they form part of the traditional tray of five fruits placed on ancestral altars. In Thailand, the Nakhon Chai Si region along the Chao Phraya River is famous, in China the provinces of Fujian and Guangxi, and in Japan the Banpeiyu variety from Kumamoto, known for its record-sized fruits. The fruit reached Europe with Portuguese and Dutch sailors in the 16th-17th centuries, and the Caribbean at the end of the 17th century thanks to a captain named Shaddock, who stopped in Barbados on his way to England; hence the English name "shaddock". Grapefruit was developed in Barbados in the 18th century from pomelo and sweet orange. The Polish name "pomelo" most likely comes via English and Dutch (pompelmoes) from the Tamil "pampa limāsu" - "large citrus". Pomelo peel is candied, fried and stewed in Chinese and Thai cuisine, while fragrant oil is extracted from the flowers in northern Vietnam.
Who this species is for
Pomelo is a good choice for anyone who wants to start growing citrus in a container and has a sunny spot available in summer and a bright, cool room in winter. Seedlings are vigorous and hardy, and the large leaves quickly give the plant an impressive appearance. The species will appeal to citrus collectors who want one of the three ancestral species of the entire genus in their collection, as well as botany enthusiasts interested in the history of plant domestication. It is not, however, a plant for people looking for a species to grow outdoors in cooler regions of Europe, or for those who want a quick harvest of fruit from a specific variety.
Summary
Citrus maxima is an exceptional species in many respects: it has the largest fruits and flowers in the genus, winged leaf stalks, monoembryonic seeds, and the status of one of the three original ancestors of all cultivated citrus. It requires warmth, sunlight and cool, bright wintering conditions, but in return provides an exotically beautiful plant and, with a little patience, fragrant flowers as well. Pomelo is not an endangered species - as a crop cultivated for millennia, it has a vast secondary range and is not listed in any threatened category on the IUCN Red List, although its wild populations in Southeast Asia are poorly understood and deserve protection as a reservoir of genetic diversity in citrus.
