Kumquat

Family

Rutaceae

Habit

Shrub

Height

3 m

Spread

2 m

Growth

slow

Lifespan

50 years

First yield

after 3 years

Full yield

after 9 years

Yield per plant

up to 15 kg a year

Yield in cultivation

up to 10 t/ha

Energy yield

700 kcal per m² a year

Hardiness

zone 9–12

Shade tolerance

full sun

Soil

Sand, Silt, Loam

Soil moisture

dry – very moist

pH

5.5 – 6.5

Times apply to the northern hemisphere in a temperate climate and shift with altitude and cultivar.

Edible: The kumquat is the citrus fruit eaten with its skin — and that reverses the usual arrangement: the thin peel is sweet and aromatic, the flesh beneath distinctly sour, so that chewing blends the two into a peculiarly balanced taste. The fruits are oblong-oval and about the size of a large olive, two to four centimetres long. They are remarkably rich in vitamin C: a hundred grams covers around half the daily requirement, alongside pectin from the peel and the essential oil that carries the scent. The whole fruit is eaten raw, rolled briefly between the fingers first to release the peel oil. Processed, it makes marmalade, jelly, compote and candied fruit, is preserved in syrup or salt, added to tea and steeped for liqueur. The pips are edible but bitter and are better removed when processing.

Other uses: Beyond the fruit it is the peel that is of interest: its essential oil consists of over ninety per cent limonene and is used in perfumery and as a natural solvent and cleaning agent. In the countries where it is grown, the kumquat serves as an evergreen hedge and as a component of agroforestry systems, occupying the shrub layer beneath taller trees. Here its value is ornamental: it is in leaf all year, flowers white and strongly scented in high summer and carries its orange fruit for months into spring — a container plant whose best moment comes in winter quarters. Dwarf forms have long been grown as bonsai in East Asia and given as New Year gifts.

Site: In Central Europe the kumquat is kept in a container, which makes the site question a question of winter quarters — these must be bright and cool, five to ten degrees, or the plant drops its leaves. The summer position outdoors should be in full sun and sheltered from rain, ideally against a south wall that releases warmth at night. Hard tap water is the commonest problem: it raises the pH in the pot and causes chlorosis, so rainwater or softened, stood water is preferable. At one and a half to three metres the shrub stays manageable, but the container gets heavy — plan for a wheeled base. The plant poses no hazard to its surroundings: no thorns, nothing that stains, no spreading. Care: Water evenly and moderately; the rootball must neither dry out nor stand wet — root rot from water left in the saucer is the commonest cause of death. Feed with citrus fertiliser from March to August and stop in September so the wood ripens; in winter quarters water only enough to keep the rootball from drying out completely. Prune little: thin out and shorten overlong shoots in spring before growth starts. The plant goes outside after mid-May and needs one or two weeks to acclimatise to full sun, or the leaves scorch. Scale insects, mealybugs and red spider mite appear in winter quarters; checking leaf undersides regularly and keeping the air humid prevents trouble. It flowers in high summer and is harvested from December to March, once the fruits are evenly orange.

Kumquats raised from seed germinate poorly, grow slowly and do not come true, so propagation is vegetative. The standard route is grafting onto Poncirus trifoliata, the trifoliate orange, which passes on hardiness and a compact habit; in warmer growing regions citrange rootstocks are used as well. Budding is done in summer, whip grafting in late winter under glass. Semi-ripe cuttings taken in June root under cover with bottom heat around 25 degrees and a rooting aid, but need patience and give weaker plants. Air layering of older shoots is the most reliable method without a rootstock. Grafted plants bear from their third or fourth year.

The kumquat's wildlife value in Central Europe is low, and that has less to do with the plant than with how it is kept: for most of the year it stands in winter quarters and is simply out of reach for animals. During the summer months outdoors, however, the white, strongly scented flowers supply genuine nectar and pollen, and they do so in high summer when many spring flowers are long over; honeybees and bumblebees take them, and citrus forage counts as high-quality bee pasture in the growing regions. There is no caterpillar association here — in East Asia, by contrast, the genus is a food plant for several swallowtail species. The fruit means nothing to birds, ripening at a time when the plant is already indoors.

Besonderheiten: klimatisch weit kompatibelste Zitrus-Frucht in Jahresø von 12-40°C & Jahresniederschlag von 700-4000mm tolerant; stellen ihr Wachstum bei <13°C ein; junge Triebe sterben bei 0°C die Pflanze manchmal erst ab -15°C; Je wärmer je süßer; Es gibt Hybride mit anderen Zitrusarten für weitere Klimaanpassung, sowie dornen- & kernlose; Auch als Fortunella (Gattung) zugeordnet;

East Asia: southern China. The oval kumquat originates there and is mentioned in Chinese literature as early as 1178. Over centuries it spread as a crop through East and Southeast Asia. It was brought to Europe in 1846 by the Scottish plant collector Robert Fortune, whose name survives in the long-used genus name Fortunella; today the species is usually placed in Citrus. It is now grown in California, Florida and Texas, in Brazil, southern Europe, South Africa and Australia. North of the Alps it is strictly a container plant — planted out, it survives no ordinary winter here.

The kumquat's home is subtropical southern China with a monsoon climate: warm, very wet summers and cool but barely frosty winters. The species tolerates an unusually wide range, from annual means around twelve degrees to over thirty and from 700 to 4,000 millimetres of rainfall, making it the most climatically adaptable of the citrus fruits. The reason lies in its rhythm: unlike most citrus it stops growing completely in winter and enters true dormancy, which lets it survive brief spells down to about minus eight degrees. It wants free-draining, slightly acid soil and rejects both lime and waterlogging.