Kumquat

About

Kumquat reverses the usual citrus arrangement. The skin is sweet and the flesh is sour, so the fruit is eaten whole and the two balance each other in the mouth.

The plant is small, neat and among the most cold-tolerant citrus, taking brief light frost that would damage a lemon. That combination makes it the easiest citrus to keep in a container.

It also goes dormant more readily than other citrus, which suits our winters. Where a lime keeps trying to grow in poor light, a kumquat rests.

Fruiting is generous for the size of the plant. A shrub 1 m tall can carry a hundred small orange fruit through midwinter, which is a considerable ornament in itself.

Appearance and character

A small dense evergreen shrub with fine twiggy growth, usually thornless or with a few small spines.

Leaves are small, 4 to 8 cm, narrow, glossy dark green above and paler beneath.

Flowers are white, 1.5 cm, scented, opening in mid to late summer, which is later than most citrus.

Fruit is small and oval in ‘Nagami’ or round in ‘Marumi’, 2 to 4 cm long, bright orange, with a thin sweet peel and sour flesh containing a few seeds.

The fruit holds on the plant for months, so a kumquat in fruit is an ornamental plant through the whole winter.

Where it is from

Kumquat originates in southern China, where it has been grown for well over a thousand years.

Botanists have moved it repeatedly between the genus Fortunella and Citrus, and current treatment places it in Citrus, though nurseries still sell it under both names.

The genus name Fortunella honoured Robert Fortune, the plant hunter who introduced it to Europe in 1846 and who is better known for taking tea plants out of China.

It is a fixture of Lunar New Year celebrations in southern China and Vietnam, where potted fruiting plants are given as symbols of prosperity.

Its natural range gives it a cooler tolerance than tropical citrus, which is exactly why it works in a conservatory.

How it grows

Buy a grafted plant. ‘Nagami’ is the oval sour-fleshed standard and ‘Meiwa’ is rounder and sweeter.

Pot into free-draining citrus compost, which matters as much here as for any citrus.

Stand it outside from late May in full sun, and bring it in during October to a cool bright position at 3 to 10 deg C.

Kumquat tolerates a cooler winter than most citrus, and a frost-free porch or unheated conservatory suits it well.

Water thoroughly and let the compost dry down, reducing sharply in winter.

Prune lightly in spring to maintain the neat shape, and repot every two or three years.

Through the seasons

Growth starts in spring, and the plant puts on new shoots through early summer.

Flowering is late, usually July and August, which sets kumquat apart from the spring-flowering citrus.

Fruit sets in late summer and develops through autumn.

Bring the plant under cover in October, switching to winter citrus feed and much less water.

Fruit colours through late autumn and is ripe from December to February, holding on the plant for weeks.

Pick as needed. The fruit stays in good condition on the shrub far longer than it does off it.

Care difficulty

Moderate, and the most forgiving citrus for a temperate grower.

Greater cold tolerance and a genuine winter rest remove most of the problems that affect limes and grapefruit in a dim cool winter.

The plant is small and slow, so it stays manageable in a pot for many years.

The usual citrus watering discipline still applies, and overwatering is still the main cause of loss.

Pests are the same as for any citrus and no worse.

Watering

Water thoroughly until it drains, then wait until the top few centimetres dry.

Twice weekly outdoors in summer, and as little as monthly in a cool winter position.

Reduce water sharply as soon as the plant comes indoors and growth slows.

Never leave the pot standing in water.

Use rainwater where possible to avoid the yellowing that hard water causes.

Warning signs, diseases and pests

Leaf drop in winter. Overwatering or a warm dry room. A cool bright position suits this plant better than a heated one.

Yellow leaves with green veins. Iron shortage from hard water. Use sequestered iron and rainwater.

Brown bumps along stems. Scale insects. Scrub off with soapy water.

White waxy fluff in leaf joints. Mealybug. Dab with horticultural soap.

Speckled leaves with webbing. Red spider mite in dry air. Mist and raise humidity.

Fruit dropping before ripening. Water stress, or a plant carrying more than it can support.

Blotchy yellow leaves with small, lopsided, bitter fruit that stays green at the base. Citrus greening, the world’s most destructive citrus disease and incurable once a tree is infected. Not established in Europe, Australia or New Zealand, where a sighting is reportable. Buy trees only from a certified, disease-free nursery.

Growing environment

Full sun outdoors from late May to October.

A cool bright winter position at 3 to 10 deg C. Kumquat tolerates the cold end of the citrus range and appreciates a genuine rest.

Free-draining citrus compost at pH 6 to 6.5.

Shelter from strong wind outdoors.

A pot of 30 to 40 cm suits a mature plant, which stays smaller than any other citrus here.

Feeding

Feed fortnightly with summer citrus feed from March to October.

Switch to winter citrus feed monthly through the cold months, or stop entirely if the plant is being kept cool enough to rest.

Avoid overfeeding a small slow-growing plant, which produces soft growth and little extra fruit.

Sequestered iron corrects hard-water yellowing.

Top-dress with fresh compost in spring between repottings.

Troubleshooting

Flowers in August with no fruit. Poor pollination indoors. Brush between flowers.

Fruit very sour. ‘Nagami’ has sour flesh by nature and is eaten whole so the sweet skin balances it. ‘Meiwa’ is the sweeter variety.

Plant losing leaves in a heated room. Too warm and too dry. Move it somewhere cooler and brighter.

Slow growth. Normal. Kumquat is naturally slow and compact.

Fruit shrivelling on the plant. Underwatering, or fruit held long past ripeness.

Plant sold as Fortunella. The same thing under an older name.