Indoor Lemon Tree Care

Notes on indoor lemon tree care from the GardenAdvice team 

Lemon and orange trees make interesting and attractive indoor plants. They provide year-round interest, with fragrant white flowers, followed by small fruits which take 4-6 months to reach full size. The fruits can be picked at any time after ripening and will retain their flavour on the tree for several months.

All Citrus plants enjoy high light levels, so smaller plants will do well on a windowsill in the house. Larger plants need a conservatory, greenhouse or sheltered summer garden, but should be given supplementary light if they are to be kept away from windows indoors. Be careful not to allow leaf scorch in direct summer sun through glass.

When there is no danger of frost, Citrus trees like to be outside in the summer months, the spring rains remove all the house dust and quite often trigger spring growth. However, they must be acclimated gradually to the new light level, by being moved first to a slightly shaded area outside for 2-3 weeks before being put in their sheltered, sunny spot for the summer. Equally, when being brought indoors in winter, they should be kept in the shade for 2-3 weeks before coming inside. This reduces any stress on the leaves that the sudden light change could cause.

Most Citrus trees can tolerate temperatures down to 4 deg C(even 2 deg C for short periods). They must not be frosted, but a cool period is useful over the winter to rest. They are also tolerant of high temperatures but prefer to be at neither extreme for too long. Owners are often unaware of just how hot their conservatory can become on sunny days and of the stress, this can cause to their plants and trees. A Min/Max thermometer is a must for measuring temperatures when no one is around. Central heating can put the trees under stress in Winter, and ideally, they do better in a cool room or conservatory, rather than a warm living room.

Water less often in winter without letting the pot dry out completely, and increase the amount of water once growth starts in the spring. In a hot conservatory, the trees will dry out more quickly than outside and may need watering as often as once a day in sunny weather, and possibly as seldom as once a fortnight in cloudy winter weather in a cool conservatory. The big pots take a lot of water, so enough must be given to reach the lower part of the pot (maybe several gallons). Be flexible about watering – judge the need by the look of the compost rather than the day of the week! Over-watering can cause problems too, by drowning the roots and creating stagnant soil conditions which encourage root diseases.

The trees should be fed weekly when in growth with a citrus fertiliser (high in trace elements). Use the Summer formulation from March to September inclusive, and the Winter formulation fortnightly throughout the rest of the year. If citrus food is not available, then a seaweed-based fertiliser is adequate if supplemented with the occasional dose of sequestrated iron and trace elements when any yellowing of the leaves occurs. Foliar feeding can also help correct deficiencies. Too much feed will lead to scorching of the leaf tips, and it is a good idea to wash the compost through with lots of water once in Summer and again in Autumn to avoid a build-up of fertiliser salts.

The ideal humidity is about 50%.In hot weather and in central heating, humidity can drop dramatically. If the leaves show signs of stress, the humidity can be raised by a fine spray or standing pots on a tray of wet gravel. In centrally heated conditions, a humidifier can be helpful. Increased humidity will also discourage red spider mite attacks.

Flowers and Fruit: Generally, flowering takes place in May, but may occur several times in the year with fruit setting each time. An enormous number of fragrant flowers appear, but only about 1% will be set on the large trees(more than this would overload the branches when the fruit reaches full size). Calamondins and kumquats and some lemons set a higher percentage of fruit and may even have to be thinned to avoid weighing the branches down too much. Dry, hot conditions will not favour fruit set, which can be improved by misting the flowers.
The fruit gradually develops and turns colour around Christmas time. (The colder weather tends to act as a trigger for colouring). It will then stay on the tree for several months after ripening. Calamondins and Valencia Late Orange trees are noted particularly for holding the ripe fruit on the branches for 6-10 months.
Most Citrus trees are self-pollinating, with the exception of some Mandarins where cross-pollination with an orange improves the yield.

Leaves: Citrus trees are evergreen and will naturally drop an old leaf from time to time. If, however, there is a lot of leaf drop, then the first thing to look at is whether the tree is too dry. This is generally the cause, particularly in the lower half of a big pot. The second most common reason is poor light, so moving the tree to a lighter position may solve the problem. Over watering can also cause problems – do not give more water until the surface of the compost is looking dry. Inadequate feeding may also cause leaf drop, this can be rectified particularly effectively with special citrus food.
Should the leaves drop for any reason, do not be immediately discouraged, as the tree will most likely grow a new crop of beautiful glossy leaves in a month or two, and flower soon after. Often heavy flowering and new leaf growth trigger some leaf drop in Spring. This is quite normal and new leaves will fill gaps, but more diligent feeding may avoid this leaf drop in the first place.
To keep the trees in shape, pinch out the growing tip once a new branch is 10-15cm (4-6 Inches)long. Regular pinching out of branches will encourage bushy growth nearer the middle of the tree, and this can be carried out at any time of year. Pruning of large branches is best done in February, just before growth starts speeding up, bearing in mind that Citrus trees store excess food in their leaves, so removing too much leaf may result in a poor fruit crop.
In general, it is best to report just before or during the growing months, rather than in the Autumn or Winter. Either loam or peat-based composts can be used, mixed with extra sand or grit to improve drainage, and preferably crocks in the bottom of the pot. The ideal pH is 5-5.5, so a slightly acid (ericaceous) compost is beneficial. Loam-based composts will give welcome increased stability for a tree outside, but it will also make large trees extremely heavy to move in and out of doors with the change of seasons. Where possible, pot up one size each time, with enough depth to cover the top feeder roots and give a small reservoir for water.

In normal conservatory conditions, where the size of tree is to be restricted, a final pot size of 45cm will be adequate. Once it has reached this size pot, instead of repotting into a larger size, take the root ball out of the pot at the end of February: if it is overcrowded with roots, cut off about 2.5-5cm all around the edges, and put back into the same pot with fresh compost around the edges; or, if this has been done before with the same tree, remove 3 vertical triangles of compost from the root ball, and refill gaps. Try and roughly match the original compost, i.e. use a loam-based compost, if that has been used before. Keep the tree fairly cool and shaded for the following month, in order not to make great demand on the root system until it has started re-growing.

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