Mandarin

About

Mandarin is one of the three ancestral citrus species, alongside pomelo and citron, and almost every other citrus in the world is a hybrid descended from some combination of them. Sweet orange, grapefruit and lemon all carry mandarin in their ancestry.

The loose skin is the family trait. A mandarin peels with a thumbnail where an orange has to be cut, and the segments separate cleanly.

Outside the subtropics this is the group to start with. Satsumas, which belong to the mandarin complex, are the hardiest common citrus and shrug off a light frost that would damage a lemon or a lime.

The plant is also naturally compact, which makes it a better container subject than most citrus.

Appearance and character

A compact evergreen shrub or small tree with a spreading habit, and few or no thorns in most cultivated forms.

Leaves are smaller and narrower than a sweet orange’s, 4 to 8 cm, glossy dark green, with a barely winged leaf stalk.

Flowers are white, 2 cm across, opening mainly in spring, strongly scented and often carried in great numbers.

Fruit is flattened rather than round, 4 to 8 cm across, orange to deep red-orange, with a loose skin that lifts away from the segments.

Satsuma types are seedless and early, clementines are sweeter and rounder, and tangerines are the deeper-coloured group.

Where it is from

Citrus reticulata originates in southern China and north-east India, and it is one of the small number of true wild citrus species.

Chinese cultivation goes back several thousand years, and the name mandarin refers to the orange robes of imperial officials rather than to any place.

Satsumas were selected in Japan and reached the West in the nineteenth century, named after the Satsuma province.

Clementines arose in Algeria around 1900, credited to Brother Clement Rodier as a chance seedling in a mission garden.

Its position as an ancestral species was confirmed by genome sequencing, which rewrote a great deal of assumed citrus history in the process.

How it grows

Buy a grafted plant, and choose a satsuma if the plant will spend time outdoors in a cold garden.

Pot into free-draining citrus compost, never ordinary multipurpose, which holds too much water.

Stand the plant outside from late May in full sun, moving it out in stages so the foliage adjusts.

Bring it in during October to a cool bright room or conservatory at 5 to 12 deg C.

Water thoroughly and then let the compost dry down. Reduce water sharply in winter.

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

Through the seasons

Flowering is mainly in spring, and a mature plant can be covered in scented white flowers in April and May.

Move outside in late May and begin fortnightly summer citrus feed.

Fruit sets in spring and develops through summer, with some natural drop in June.

Bring the plant in during October and switch to winter feed with much less water.

Fruit ripens from November through January, earlier than a sweet orange, which is one reason satsumas suit cool climates.

Pick when fully coloured and slightly soft. Mandarins do not hold on the tree as well as oranges and go puffy if left too long.

Care difficulty

Moderate, and the easiest of the citrus for a beginner.

Greater cold tolerance removes some of the winter anxiety, and satsumas survive brief frost without damage.

The compact habit suits a container and needs little pruning.

The same watering discipline applies as for any citrus, and overwatering remains the commonest cause of death.

Winter indoors is where problems arise, with dry warm air producing leaf drop and red spider mite together.

Watering

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

Twice weekly outdoors in summer, and as little as monthly indoors in winter.

Never leave the pot standing in water.

Use rainwater where possible, since hard water pushes the pH up and causes yellowing between the veins.

Reduce water as soon as the plant comes indoors, since growth slows and the compost dries far more slowly.

Warning signs, diseases and pests

Leaves dropping through winter. Overwatering, a cold draught, or a warm dry room. Check the compost before watering.

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

Brown bumps along stems. Scale insects. Scrub off with soapy water and a soft brush.

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

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

Fruit puffy and hollow under the skin. Left on the tree too long, which mandarins tolerate less well than oranges.

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.

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.

Bright cool conditions indoors through winter, at 5 to 12 deg C. Satsumas tolerate the colder end of that range.

Free-draining citrus compost, which matters more than any other growing decision.

Shelter from wind outdoors, since the fruit is heavy relative to the branch.

A pot of 35 to 45 cm suits a mature plant, with grit in the compost and a saucer that is emptied after watering.

Feeding

Use a summer citrus feed fortnightly from March to October.

Switch to a winter citrus feed monthly from October to February, which is lower in nitrogen and richer in trace elements.

Continuing summer feed through winter produces soft growth that pests colonise easily.

Top-dress with fresh compost each spring in years when the plant is not repotted.

Sequestered iron corrects the yellowing that hard water causes, and is worth keeping to hand.

Troubleshooting

Heavy flowering, little fruit. Poor pollination indoors. Brush between flowers, or have the plant outside while it flowers.

Fruit dropping small in June. Natural thinning, and normal.

Fruit sour. Picked too early. Mandarins need to colour fully before they sweeten.

Leaves curling and dropping after coming indoors. The move itself, plus dry air. Bring the plant in before the heating goes on and mist regularly.

Slow growth and pale leaves in summer. Underfeeding, or a pot-bound root system. Repot in spring and feed fortnightly.

Fruit splitting. Irregular watering during ripening. Water steadily.