Citrus are frost tender plants and temperatures below 7°C (45°F) may cause damage or even death. For this reason, in most parts of the UK, it is best to grow citrus in containers that can be moved to a warmer position over winter.
They are quite hungry plants, so use nutrient-rich compost such as John Innes No 2, with the addition of 20 percent by volume of sharp sand or grit. Use ericaceous John Innes compost in areas where hard water has to be used for irrigation. There are also specially formulated citrus composts available. Repotting is best done in spring, using only the next size pot. In years when repotting is not necessary, remove the top 5cm (2in) of old compost in late spring and replace with fresh compost.
Plants can be placed outdoors in summer, from mid-June to late September, if sheltered from cold winds. At other times, keep them in a cool greenhouse or conservatory. Citrus are not ideal houseplants, as the heat and dry atmosphere can cause scorch.
Citrus like cool, but not cold, conditions in winter, as a little growth does take place. A minimum winter night temperature of 10°C (50°F) is fine for lemons. Calamondin oranges need at least a minimum winter night temperature of 13°C (55°F). Other species will tolerate cooler conditions, not falling below 7°C (45°F).
Citrus originate from regions of free-draining soil but high air humidity, and it is important to keep the air humidity up year round. The best way to do this, is to stand plants on large saucers or trays filled with ‘Hortag’ or gravel and keep the water level just below the surface of the gravel. Mist the leaves in early morning in summer and make sure that there is good circulation of fresh air.
In summer, water freely but do not allow the base of the container to stand in water.
In winter, allow the surface to partially dry out before watering, then water thoroughly with rainwater, allowing excess moisture to drain away. Overwatering in winter is one of the commonest causes of stress in citrus, so keep them on the dry side.
Citrus require feeding throughout the year. From late March to late October, use a summer feed high in nitrogen. Use a more balanced winter feed from late October to late March. These are available from nurseries and garden centres, including fertilisers specifically formulated for citrus.