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. Low temperatures will inhibit flowering and may cause damage or even death.
A minimum winter night temperature of 10°C (50°F) is needed for lemons and limes. 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 not ideal for citrus as they are generally too hot, lack humidity and light leading to stress (see below).
Repot plants annually in March, or replace the top 5cm (2in) of old potting media with fresh compost.
Feeding and watering
Citrus are hungry plants and need regular feeding. Use high nitrogen citrus summer feed from late March to October. In winter switch to winter feed that is specific to citrus.
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 water level just below the surface of the gravel, or group plants together. Hand mist regularly, in winter, to ensure pollination.
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.
Citrus need a bright sunny position. Most only reach 1–1.5m (3–5ft) tall in a pot but can grow larger in good conditions, depending on the rootstock.
They are better grown in pots in a cool climate so they can be moved indoors. Although any good potting medium will do, a soil-based compost such as John Innes No 2 is best. 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 biological controls and encourage ladybirds.
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 biological control 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 biological controls in the greenhouse.
More info on Scale insects
Citrus are self-fertile, so a single plant is able to 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.
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.
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.
Grapefruit ‘Star Ruby’:Moderately vigorous, the large fruits have deep red, extremely juicy flesh, thin skin, and a very sweet flavour.
Lemon ‘Garey’s Eureka’:The prolific, juicy fruit have few seeds. It’s in flower and fruit for most of the year.
Navel Orange ‘Washington’:Large, tasty, seedless fruits are very juicy. The trees are vigorous and fruit ripens in late autumn.