Growing citrus in the UK
This is a cool, wet island. Citrus is usually grown in pots and given glass for the winter, not left out year-round.
Citrus plants won't reliably survive winters outdoors, even in the warmest parts of the UK. RHS When grown in containers they can be kept in a greenhouse, conservatory or glazed porch over winter, then moved outdoors in summer into a sunny, sheltered spot. RHS
Growing citrus as houseplants is not ideal long term, as most prefer a cool spell in winter and a more humid atmosphere than centrally heated homes usually provide. RHS
Most citrus form attractive small trees or shrubs less than 1.5m (5ft) tall, so are ideal for growing in containers. RHS If you are new to growing citrus fruit, the RHS says it is best to start with one of the easier types, such as lemons. RHS Certain varieties of kumquats, limes and calamondin oranges can also be fairly straightforward. RHS Look for varieties with an RHS Award of Garden Merit (AGM), which shows they performed well in trials. RHS
Most commercially sold citrus plants are grafted to ensure they fruit at an early age. RHS Ungrafted plants may take up to ten years to fruit. RHS Citrus plants are self-fertile, meaning you only need one plant to produce fruit. RHS
Winter temperatures
Some lemons, for example, will be fine as low as 5 degrees C (42 degrees F). RHS Kumquats often need 7 degrees C (45 degrees F) or more. RHS Limes and grapefruits should be kept above 10 degrees C (50 degrees F). RHS Calamondin oranges need at least 13 degrees C (55 degrees F). RHS
Water and feed
In summer, water citrus plants regularly, ideally with rainwater. RHS Overwatering in winter is one of the commonest problems, so err on the dry side, and never leave pots standing in water, as this can cause the roots to rot. RHS
From late March to October, apply summer citrus feed, which is high in nitrogen. Then from November to mid-March, switch to a more balanced winter citrus feed. RHS
The RHS source writes "From Iate March to October" for the summer feed season: the month is March, and the initial capital I is a typo on that page. We follow the months as written and do not invent a different calendar.
Cropping
The fruit ripens slowly, for up to a year, so plants often carry flowers and fruit at the same time. RHS The RHS says a plant about 1m (3ft) tall should be allowed to carry no more than 20 fruits at one time. RHS Only kumquats, with their much smaller fruits and bushy style of growth, can successfully carry a heavy crop without any need for thinning. RHS
How big is a lemon tree?
Britannica gives 3-6 metres (10-20 feet) if a lemon plant is not pruned, for orchard trees. The RHS, writing for UK gardeners growing citrus in pots, says most citrus form attractive small trees or shrubs less than 1.5m (5ft) tall. Those are different growing systems, so both figures stand.
More on individual fruits: lemons, limes, kumquats and calamondins.