In recent years there has been a surge of interest in growing citrus fruit. Some gardeners grow the trees solely for their decorative value, while others hope to produce edible fruit. Nurserymen have responded to this and citrus plants are now commonly available in cooler regions. Oranges, tangerines, mandarins, lemons, limes and kumquats are the most commonly available to the average gardener.
Citrus are trees of sub-tropical or tropical origin and are generally suited only to hotter climates, but as long as they are kept to a small size they can easily be grown in containers in a warm conservatory. They can be moved outside for the summer, or indeed left inside under glass if you prefer. Generally they will not tolerate frosts and so are unsuitable for growing outside except in warmer areas where these rarely occur. Below about 13ºC (55ºF) the trees become dormant, so it is essential that they are placed where they will have long periods of sustained warmth. Even when moved outside in summer, they should be given a sheltered position, away from any cold winds. There are some varieties that are more tolerant of cold conditions. Check before buying whether they are best kept under glass or can be moved outside.
Where citrus fruit is grown commercially in areas with potential frost, soil is heaped up around the trunks to the level of the lower branches in the winter as protection. Even if the topgrowth is killed by frost, new growth regenerates from the trunk once the soil is cleared away in spring. This is a major task and is normally carried out only for the first four years, but you may feel it worth attempting in the garden in order to grow trees outside. If potted trees are left outside they are far more prone to frost damage as the rootball can easily freeze through the sides of the container. Always move these inside whenever frost is forecast.
If citrus trees are grown in pots, they are small enough not to need staking in any way. However, outdoors they can easily blow over, so it may be necessary to anchor the container in some way. If you grow citrus as standards then you will need a cane in the pot to support the tall stem.
Generally, citrus trees are sold in containers and have already been shaped by the nursery or grower, leaving little to do. However, it is possible to buy citrus as unbranched trees, which are much cheaper than mature plants and possibly essential if you intend to grow them outside (container plants are usually too small). These will need some preliminary training to establish the desired habit. Generally, training is as for other bush trees. Allow the trunk to develop to the desired height, then stop it above a bud. In the case of container plants, this may be 30–60cm (12–24in); it will be higher for planted trees. Standard trees grown in containers can be treated in the same way except that they must be allowed to reach their final height before the tip is removed. Choose the top three or four branches during the following winter and shorten these to 25cm (10in) in the case of container trees and 30–35cm (12–14in) in the case of outdoor trees. Remove any other side shoots that appear below this. The following winter shorten all leaders of all main shoots back by about one-third and remove any side shoots that are growing towards the centre of the plant and causing congestion. Continue to do this until the tree matures.
Little pruning is usually necessary on mature plants. However, frosted wood will die back, and must be removed. If containers are kept in inadequate light levels, growth will become drawn. This can be redressed by removing the top by up to one-third, preferably in mid-winter when the plant is dormant. Remove some of the resulting shoots if the tree becomes too dense and overgrown, thus reducing the light reaching the lower branches. Aim to avoid legginess by improving the light conditions. Sometimes a container-grown tree is too large, and difficult, to move in and out of its winter home. In this case it may be necessary to remove some of the outer branches to reduce its size, a job best tackled in mid-winter. Finally, remove any crossing branches and any other shoots that crowd the bush. If the top growth of a tree is killed by frost, the rootstock may throw up new shoots. But these will be rootstock, not the upper grafted part that bears the fruit. If the tree is purely ornamental, this may be acceptable. Select one main shoot, then proceed as for initial training. For a fruiting tree, however, you will need a replacement.
Training a citrus standard
In cooler countries the only way that citrus fruit can be overwintered is by growing them in containers and keeping them under glass. Most trees sold in these countries are suitable for just that, but it is worth checking with your supplier that they are intended for that purpose. Citrus plants can often be purchased in the container in which they will grow.
Year one, winter: If not already trained by the nursery, allow the leader to develop to the desired height and then remove the tip at a bud. Remove any unneeded laterals.
Year two, winter: In the following winter shorten the branches to about 25cm (10in) and reduce the new leader to about two leaves.
Subsequent years, winter: Continue to cut back the new growth on the shoots to make them branch into a bush shape. Once established little pruning is necessary, other than to pinch back the leader growth to a couple of buds and remove any misplaced shoots.