Citrus in pots can be put outdoors in summer, in a sheltered sunny position, but only when temperatures increase, from mid-June until late September. Keep some fleece handy in case of sudden cold nights in early summer. Temperatures below 7°C (45°F) may cause damage or even death. Grapefruit become more hardy as they mature – young plants must not be exposed to temperatures below 10°C (50°F). Trees can reach 5m (16ft).
A minimum winter night temperature of 10°C (50°F) is fine for lemons. Citrus limon is less cold tolerant than C. × meyeri, which survives down to 5°C (41°F). Limes and C. limon only tolerate temperatures down to 10°C (50°F). Calamondin oranges need a minimum winter night temperature of 13°C (55°F).
Kumquats are unusual citrus, as the fruits are eaten whole – including the skin. Plants are naturally very bushy and highly productive. They can tolerate winter temperatures down to 7°C (45°F) – among the hardiest of all citrus.
Centrally-heated rooms are too hot, lack humidity and light, and stress citrus.
Citrus are hungry plants and need regular feeding. Use high nitrogen summer feed (such as liquid lawn food which is widely available) from late March to October.
There is no need to feed from late October to late March.
In summer, water freely - ideally with rainwater. In winter, allow the surface to partially dry out before watering, then water thoroughly with tepid rainwater, allowing excess moisture to drain away. Overwatering in winter is one of the commonest problems, so err on the dry side.
Indoors, maintain high humidity by standing the pot on a large saucer or tray filled with ‘Hortag’ or gravel. Keep the waterlevel just below the surface of the gravel, or group plants together. Mist the leaves in early morning in summer and ensure there is good ventilation. Hand mist regularly, in winter, to ensure pollination.
Repot plants annually in March, or replace the top 5cm (2in) of old potting media with fresh compost.
Training and pruning
Citrus require only minimal pruning.
In February, reshape plants by thinning out overcrowded branches. ‘Leggy’ plants can be pruned back by up to two-thirds and the tallest branch can be cut back to encourage bushy growth.
Throughout the summer, pinch back the tips of the most vigorous growth, using the thumb and forefinger.
Mature plants may produce unwanted, fast-growing shoots called ‘water shoots’. Remove these when they appear from the main branches at the bottom or middle of the plant and shorten those arising near the branch tips. Be especially watchful for shoots from below the graft on the main stem, and remove such shoots immediately.
When pruning, take care to avoid vicious thorns.
Citrus need a bright sunny position. Most (except grapefruits) only reach 1–1.5m (3–5ft) tall in a pot but can grow larger in good conditions.
They are better grown in pots in a cool climate. Although any good potting medium will do, a soil-based compost such as John Innes No 2 or No 3 is best. If your pot is on the heavy side add up 20 per cent sharp sand or grit. There are also specially formulated citrus composts available.
Plant in spring so they have a growing season to establish.
Read more advice on growing citrus
Mealybug: Small creatures covered in a white ‘meal’ cluster in inaccessible spaces like leaf joints or under loose bark. They suck sap and secrete ‘honeydew’ which causes black sooty mould on the leaves.
Remedy: Use sprays based on plant oils or soft soap. Use biological controls, Parasitic wasps Leptomastix spp. or the ladybird, Cryptolaemus montrouzieri.
More info on Mealybug
Glasshouse red spider or two spotted mite: Leaves become mottled, pale and covered in webbing, on which the mites can be clearly seen; leaves also drop prematurely.
Remedy: They thrive in hot, dry conditions, so mist plants regularly. Use sprays based on soft soap, plant oils or extracts. Use the biological control Phytoseiulus persimilis in the greenhouse.
More info on Glasshouse red spider or two spotted mite
Scale insects: Small yellow hemispherical scales appear on the leaf underside and along the midrib. They suck sap and secrete honeydew which encourages sooty mould.
Remedy: Use sprays based on soft soap and plant oils or the parasitic wasp Metaphycus helvolus in the greenhouse.
More info on Scale insects
Citrus are self-fertile, so a single plant will produce fruit.
Plants 1m (3ft) tall should bear no more than 20 fruits and may need thinning. Kumquats do not need thinning.
Once fully grown, the fruit develops a rich skin colour, and is ready to pick but can also be 'stored' on the tree.
Nigel Slater recommends serving this runner beans with lemon and garlic crumbs, as a lovely side dish for grilled fish.
Kumquat ‘Nagami’: This bears dozens of small, oval fruits on compact plants. The balance of flavours between the sour flesh and sweet rind makes it tasty either cooked or fresh.
Lime ‘Tahiti’: This tasty, small fruited seedless lime, grows to 1.8m (6ft) tall and is very productive. The fruits ripen to a pale green.
Lemon ‘Garey’s Eureka’: The prolific, juicy fruit have few seeds. It’s in flower and fruit for most of the year.
Grapefruit ‘Star Ruby’: Moderately vigorous, the large fruits have deep red, extremely juicy flesh, thin skin, and a very sweet flavour.
Calamondin ‘Tiger’: This variegated cultivar bears masses of orange fruits, 4cm (1.5in) wide. The plant is vigorous and the main shoots should be ‘pinched out’ regularly to keep them in shape.
Navel Orange ‘Washington’: Large, tasty, seedless fruits are very juicy. The trees are vigorous and fruit ripens in late autumn.