Sweet orange

About

A sweet orange is a hybrid rather than a wild species, a cross between pomelo and mandarin that arose in China and has been propagated by grafting and budding ever since. Every navel orange in the world descends from a single mutation found in Brazil in the 1820s.

Outside the subtropics it is a container plant. Outside from late May to October, indoors or in a conservatory for winter, kept above 5 deg C and in the brightest place available.

The thing that kills most orange trees here is water rather than cold. Citrus roots need air, and compost kept permanently damp rots them from below while the leaves yellow and drop and the owner reaches for the watering can again.

Fruit takes nine to twelve months to ripen, so a tree carries flowers and fruit together for part of the year, which is much of its charm.

Appearance and character

An evergreen tree with a dense rounded crown and slender thorns on young growth, which reduce as the tree matures.

Leaves are oval, 6 to 10 cm, glossy dark green, leathery, with a narrowly winged leaf stalk. Crushing one releases the scent of the fruit, since the leaf is dotted with oil glands held up to the light.

Flowers open mainly in spring, 2 to 3 cm across, waxy white with five petals and a boss of yellow stamens, and the scent is powerful and carries a long way.

Fruit is round, 6 to 10 cm, ripening from green to orange, with a tight peel that clings to the flesh unlike a mandarin’s.

Navel varieties carry a small undeveloped secondary fruit at the base, which is the navel itself.

Where it is from

Citrus sinensis originated in southern China and south-east Asia as a natural hybrid of pomelo and mandarin.

It was cultivated in China for well over two thousand years and reached the Mediterranean through Arab trade in the tenth century, arriving in Europe long after the bitter Seville orange.

Portuguese traders brought sweeter varieties direct from China in the fifteenth century, which is why the fruit is called portokali in Greek and portakal in Turkish.

Wealthy European households built orangeries to overwinter tubs of citrus, a practice that ran from the seventeenth century onwards and is the direct ancestor of how the plant is grown in cool climates today.

Brazil, the United States, China and Spain are the main producers now, and the crop is overwhelmingly grown for juice.

How it grows

Buy a grafted plant. Trees grown from a pip take a decade or more to fruit and give unpredictable results.

Pot into free-draining citrus compost. Ordinary multipurpose compost holds too much water and is the usual cause of failure.

Stand it outside from late May, after the last frost, in the sunniest sheltered spot available. Move it in stages over a week so the leaves adjust.

Bring it in during October, before night temperatures drop below about 5 deg C, into a cool bright room, porch or conservatory.

Water thoroughly and then let the compost dry down before watering again. Use rainwater where possible, since lime in tap water raises the pH.

Repot every two or three years in spring, into a pot one size larger, and prune lightly to keep a good shape.

Through the seasons

Spring brings the main flush of flowers, usually in April and May, with a scent that fills a conservatory.

Move the tree outside in late May and start fortnightly feeding with summer citrus feed.

Through summer the tree grows and the fruit that set in spring begins to swell. Water regularly and let the compost dry between waterings.

Some fruit drops in June, which is the tree shedding what it cannot carry, and is normal.

Bring the tree in during October and switch to winter citrus feed, reducing water sharply.

Fruit colours through winter and is ready from December to March. Oranges hold on the tree for weeks once ripe, so pick as needed.

Care difficulty

Moderate. Nothing about the plant is delicate, and the problems come from the winter indoors.

Dry warm air, low light and overwatering combine to strip a tree of leaves between November and February. A cool bright room suits it far better than a warm dim one.

The watering rule is the one to learn. Thorough soaking, then allow to dry down. Not a little every day.

Pests under glass are the other recurring job, with scale, mealybug and red spider mite all common.

Given the right compost, the right winter position and disciplined watering, an orange tree lives and crops for decades.

Watering

Water thoroughly until it runs from the drainage holes, then let the top few centimetres dry before watering again.

In summer outdoors that usually means twice weekly, and more in hot weather.

In winter indoors it may mean once a month. Reducing water in winter is the single most useful habit for keeping a citrus alive.

Never let the pot stand in a saucer of water. Citrus roots need air and rot quickly when they cannot get it.

Use rainwater where you can. Hard tap water raises the pH over time and causes yellowing between the leaf veins.

Warning signs, diseases and pests

Leaves yellowing and dropping in winter. Usually overwatering, sometimes a cold draught or a sudden move. Check the compost before watering again.

Yellow leaves with green veins. Iron shortage from hard water or high pH. Use a sequestered iron feed and switch to rainwater.

Brown limpet-like bumps along stems and leaf ribs. Scale insects. Scrub off with a soft brush and soapy water.

White waxy fluff in leaf joints. Mealybug. Dab with horticultural soap or use a biological control.

Fine pale speckling with webbing beneath the leaves. Red spider mite in dry indoor air. Raise humidity and mist regularly.

Black sooty coating on leaves. Sooty mould growing on pest honeydew. Deal with the insects and wipe the leaves.

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.

Marginal leaf scorch that does not improve with watering, with stunted growth and fruit that fails to ripen properly. Citrus variegated chlorosis, a bacterial disease spread by sap-feeding insects that has devastated orange groves across Brazil since the 1980s. There is no cure once a tree is infected, and it is not a risk for a tree grown outside an area where the disease and its insect vectors are established.

A pinprick sting mark on ripening fruit that develops into a soft rotting patch, with small maggots inside when the fruit is cut open. Mediterranean fruit fly, established across the Mediterranean, the Middle East and much of Central and South America. Pick up and destroy fallen fruit, and net or bag fruit as it ripens where the pest is present locally.

Growing environment

Full sun outdoors from late May to October, in a sheltered position.

Bright light indoors through winter, in a cool room, conservatory or porch at 5 to 12 deg C. A heated living room is too warm and too dry.

Free-draining citrus compost, which is the single most important growing decision.

Moderate humidity. Central heating dries the air and brings on red spider mite and leaf drop together.

A pot of 40 to 50 cm suits a mature tree, with plenty of drainage material at the base and a saucer that is emptied rather than filled.

Feeding

Use a purpose-made citrus feed, which comes in summer and winter formulations for good reason.

Summer feed is higher in nitrogen and goes on fortnightly from March to October.

Winter feed is lower in nitrogen and higher in trace elements, applied monthly from October to February.

Feeding a summer formulation through winter produces soft pale growth that pests find easy.

Top-dress with fresh compost each spring where the tree is not being repotted.

Troubleshooting

All the leaves dropping in January. Overwatering, cold draught or a warm dry room. Check the compost first, since dry roots and rotted roots look identical from above.

Flowers but no fruit. Poor pollination indoors. Brush between flowers, or move the plant outside during flowering.

Fruit dropping while small. Normal June drop, or irregular watering.

Fruit staying green. Not enough heat or light, or picked too early in its cycle. Oranges need nine to twelve months on the tree.

Leaves yellow all over rather than between veins. Waterlogging or cold, and rarely a nitrogen shortage.

Sticky floor beneath the plant. Honeydew from scale or mealybug. Look under the leaves and along the stems.