All citrus need warm conditions in order to thrive, although they can cope with surprisingly cool temperatures so long as they are not over-watered and have plenty of ventilation. Some daring growers with advantageous microclimates have even been able to grow the hardiest citrus trees outdoors all year round. If you want to try this, you will need to have a south- or west-facing brick wall that will radiate back any stored heat from the day to the plants at night. You will also need to be living on the west coast where the influences of the warming Gulf Stream are at their peak.
Otherwise, check that you have a suitable home for your citrus for both the summer and winter months. If you don’t have a frost-free greenhouse or conservatory, it is possible to keep your plants in a bright position indoors. If you want to do this, you will need to make sure that the humidity is kept high (see box, ‘Overwintering’). You’ll find that the sweet-scented flowers, glossy leaves and colourful fruits have considerable ornamental appeal when grown under cover throughout the cold weather and outside on the plot in summer.
The other major consideration when cultivating citrus is the compost. The plants need a nutrient-rich growing medium that is also free draining, as they can’t abide waterlogged or excessively damp roots. Buy some purpose-made citrus compost or create your own. For a homemade citrus compost combine four parts loam-based compost, such as John Innes No 3, with one part drainage material, such as bark chippings, perlite, horticultural grit or sharp sand. This mix will achieve the ideal balance of moisture retention with good drainage
All citrus crops are best grown from bought-in plants, and a number of specialist nurseries have sprung up to supply a dizzying selection from which to choose. These are available at any height from 15cm tall to a mature tree. Most varieties will have been grafted onto a rootstock, which will limit their ultimate size and make them suitable for container culture. Smaller plants will take longer to reach the fruit-producing stage but will be more adaptable and likely to settle in quickly. Larger types will be ready to crop immediately, but will cost considerably more to buy.
Plants can be bought and planted up at any time of the year. Pick a container that is only slightly larger than the one your citrus arrives in; about 5-10cm wider and deeper is perfect. It is better to pot on in stages than to ‘swamp’ a plant in too large a tub. Ensure there are plenty of drainage holes and prop it up onto pot feet so that any excess water can escape unhindered from the base. Place a few crocks or stones into the bottom of the container before adding some of your compost. Position the citrus rootball on top of this and infill around the edges, pressing down on the compost as you do to remove any air pockets. Thoroughly water-in the newly potted plants to further settle the soil and top up with more compost if necessary.
Position citrus plants in a bright and warm position. During the summer, they will love being outdoors where the access to fresh air and higher light levels will give them a real boost. Any pests that have built up under cover, such as scale insects, will clear up once the plants are moved outside. As mentioned, a sunny, sheltered patio is ideal – the extra warmth will encourage fruit formation. Plants can be moved outside from May, as soon as the possibility of frost has passed.
Citrus need very little pruning, but some formative work on young specimens will help them achieve a desirable shape. Remove lower branches to the main stem so that you encourage a single, clear trunk. Any badly placed, excessively long branches or those that are damaged or diseased may also be cut right back. Do this in spring, just as the trees are starting into growth, and use a sharp, sterile pair of secateurs for the job. Additional pruning can be carried out as needed during the growing season.
Once your tree is mature, almost no pruning will be necessary but unproductive branches can be removed, as should long, straight stems emerging near the base of the plant, known as ‘water shoots’. Very long and flexible branches can be tied down to the main stem to encourage fruit production in a technique called ‘festooning’.
Watering with consideration is essential and different times of the year will dictate different approaches. The trick is to keep plants moist in the summer, but never wet. In the winter, when temperatures are low and plants are growing very slowly, watering sessions should be dropped back to just once or twice a fortnight so that the compost almost dries out entirely in-between.
Container plants will need regular feeding if they are to flower and set fruit successfully. Special citrus feed is available, but during the growing season it is also possible to use a liquid tomato feed or liquid seaweed solution. Apply this every week or two, according to the instructions. Established plants will need to be potted on into the next size of tub as soon as the roots reach the base of their existing pot. Do this in winter and use fresh compost around the sides. Once the container size reaches 60cm in width and depth, it will be tricky to pot your citrus on much further. To give such plants an additional boost, scrape back the surface layer of compost from the pots each year in late winter and top-up with fresh compost mixed with a little general purpose organic fertiliser that is rich in trace elements.
Pests can become a nuisance under cover where the air is still. To counteract this, provide optimum ventilation – even during the winter (so long as it isn’t too cold outside). Aphids, scale insects and whitefly may be suppressed under cover using ‘biological controls’ – these are predatory insects that can be introduced to feed on the pests. Alternatively, you could spray the plants with a homemade soapy solution. These problems usually disappear when the plants are moved outside for the summer. If you want to keep them under cover all year, regularly damp down the floor and provide some shading during the sunniest, hottest months.
Citrus fruits take up to 12 months to mature, the exception being grapefruits which take even longer! For this reason, it is quite normal to have both flowers and fruits at different stages of growth on one plant. Young plants should only be allowed to set four fruits, but older ones can be left to produce as many as they can – any excess will naturally drop off. The heaviest crops are achieved when average night-time temperatures are above 16°C for the six months following flowering in spring. If you want maximum fruit production then you may need to supply additional heat in early autumn when plants are brought back in under cover.
Consider supporting heavily-laden branches and pick your crops as soon as they are the right size and colour. Ripe fruit may be left on the plant for up to two months without deteriorating, and this is the best option to maintain their juiciness. Cut them away using a sharp knife or a pair of scissors, taking care not to pierce the branches.