It’s quite possible to grow citrus trees in the UK, although not as easy as in subtropical areas. Provided you can give them a warm, sheltered spot in the summer as well as enough protection through the winter, you will be rewarded by incredibly fragrant flowers in the spring and a delicious crop of fruit later in the year. Lemons, limes, oranges, tangerines, kumquat – the range of possibilities is wonderfully tasty.
1) Avoid heat
Citrus trees don’t like to be too hot in the summer, and are therefore better off in a sheltered spot outside.
2) Avoid cold
Citrus trees like to be cool over winter, but not cold: ideally the temperature should not fall below 5°C. This means it’s just about impossible to grow citrus in the ground, even in a greenhouse or polytunnel. They do much better in terracotta pots, which can be moved to a polytunnel or unheated indoor room for the winter.
In warmer areas an unheated greenhouse or polytunnel may provide enough protection during the coldest months, but it’s also a good idea to cover the plants with a layer of horticultural fleece. However, contact spots can cause localised frostbite, so the fleece should be held off the plants using bamboo poles with upturned plastic bottles on them. Drape the fleece over them so that in sunny weather it can be easily pulled to one side. Light levels in winter are far lower than in summer, and most plants need all the extra light they can get.
Over- and under-watering are the two most likely ways of making citrus trees seriously unhappy. This is the most difficult aspect of growing citrus trees, and something it’s easy to get wrong. If you think you’ve overwatered, leave the plant in a warm, sheltered spot where it can drain thoroughly and cross your fingers. Don’t water it again until the surface of the soil has become really dry, and then give it a good soak. To ensure that you’ve reached all the roots, immerse the pot in water until it stops bubbling and then leave it where the excess water can drain away completely.
All citrus plants go through dormant periods and will drop some leaves when they do. This is perfectly natural, and left to themselves they usually recover perfectly well. They are deciduous, though not in quite the same way as trees indigenous to the UK. Unlike an oak or beech tree, if a citrus plant drops ALL its leaves it’s definitely a sign of trouble.
Stress-induced leaf drop can be caused by both over- and underwatering and sudden changes in temperature or lighting conditions. If this happens, don’t try to remedy the situation by subjecting the plant to yet another sudden change.
As well as causing leaf drop, if plants are introduced to strong sunlight without suitable preparation they may get leaf burn. Generally, citrus prefer dappled shade to loads of direct sunlight.
The Eureka lemon, also known as the Four Seasons lemon, is probably the most commonly grown variety in the UK because it has such good resistance our miserable winters. Self-fertile and highly productive, it will give an almost continuous supply of good-sized lemons (with great juice and few seeds) almost right through the year, although the main crop comes between October and February. If you haven’t yet tried growing citrus trees and are unsure of your growing conditions, start with one of these from a reputable nursery – well worth doing, as plants grown from seed can take a decade or more to start fruiting.
Feed healthy plants once a fortnight during the summer, tapering off to once every four to six weeks in winter. Citrus plants grow continually, and while growth is slow during the winter months it never stops entirely.
As soon as the tree gets to be twice the height of its pot, you can consider repotting (three times the height of the pot means you’ve left it too long). Plant in a free-draining mix of loam and compost with a pH of around 6.5 and make sure there is a layer of gravel at least 5cm thick at the bottom of the pot.
Citrus pruning should only be done to create the desired shape, not to increase fruit production. Prune just above a growing bud with a very sharp knife or secateurs either towards the end of the summer or after the first growth in spring. Trim off any weak branches, as citrus fruit is heavy and branches need to be strong to bear their weight. Never prune in winter, as it takes too long for the plant to recover and disease can set in more easily.
Citrus trees suffer from many pests familiar to the gardener, including red spider mite, scale, greenfly, whitefly, mealy bugs and black vine weevil. Most of these, as adults, suck the sap from the plant and a regular hosing down will keep usually keep their numbers down to manageable levels, although scale is very difficult to remove. However in the case of black vine weevil the young also attack the roots – a much more serious problem. If you see tiny black beetles on the stems (the adults) it may be an indication of weevil infestation. Quarantine the plant immediately and repot as soon as possible, removing as much of the original earth as you can – but don’t do this near other plants or they could be affected. If repotting doesn’t do the trick, you may have to consider using a commercial biocontrol or risk losing the tree.