Citrus are subtropical but hardy plants and many will do well, even in areas which experience cold winters and frosts. They are susceptible to some pest and disease problems in all climates.
When planting citrus in a cold climate you will need to choose the area carefully. A warm, sheltered position is essential for the plant to do well. A north, or north-east facing position is ideal and the more tender species such as limes, should be sheltered by the eaves of the house (or similar) and positioned near a masonry wall so they receive the benefit of stored heat. Citrus plants make good subjects for espalier and this is often a good idea in cold areas as the plants receive shelter and warmth from the wall on which they are espaliered. In any climate citrus need good drainage so if you have heavy clay soil, or poorly drained soil, plant your citrus in raised beds. Plentiful water is essential in the warmer months, and regular fertilising, as citrus are shallow rooted and heavy feeders.
Choose the site for your plant and water the area well, then leave for a day. Dig a hole which is twice as wide as the root ball of the plant and add some well rotted compost, but don’t put any manure or fertiliser in the planting hole. The soil should come no higher or lower on the trunk than it is in the container in which you purchase it, and by doing this you will ensure that the graft union is above soil level. Fill in the planting hole and firm down. Sprinkle around the plant with blood and bone, and composted manure and water well. To overcome transplant shock, it is a good idea to add some seaweed extract to the water at this time. Mulch around the plant during the warmer months but make sure you keep the mulch back from the stem of the plant to avoid collar rot. Grass or ornamental plants should not be grown too close to citrus trees as they can also cause problems with collar rot if they are too close to the trunk of the tree at soil level. Remove all fruit for the first 2 years to allow the plant to develop properly and form a strong framework.
Some citrus may be slow to grow in the first two years or so and this may be due to the fact that they are grafted onto trifoliata root stock which can be quite slow growing. Remember that watering and feeding in the warmer months is of the utmost importance for citrus. Take care when fertilising as the soil must be evenly moist BEFORE any fertiliser is applied. The fertiliser must then be watered in well after application. Container grown plants will need more frequent fertilising, AND watering, than those grown in the ground, but it is best to use only organic or liquid fertilisers on container grown plants as some chemical fertilisers can be harmful when used for pot culture. If growing citrus in a container, ensure that the container is of sufficient size with good drainage holes and raise the pot off the ground using pot feet or bricks, so that the drainage holes do not become blocked. Container grown citrus should be turned regularly, particularly if positioned against a wall. Turning enables the sun to reach all sides of the plant, making growth more even and improving the appearance of the plant.
Spraying with Pest Oil every 4 weeks in the warmer months is a good preventative measure against scale and leaf miner which are two common citrus pests. Seasonal fertilising of plants in colder areas should be done in November, January and at the end of March using organic fertilisers. Inorganic, or chemical fertilisers, are best used ONLY to correct nutrient deficiency problems. Watering in hot weather is critical, especially at petal fall when the new crop is forming. Drip irrigation is best as this will assist in avoiding some fungal problems.
Citrus do not need too much pruning and generally this is only done for shaping, height reduction, or rejuvenation of old trees. However some citrus grow very thickly and the centre of the plant may become congested which can result in fungal problems. If this occurs then the centre of the tree should be thinned. In most instances pruning is best done lightly and often, rather than radical pruning. Pruning can also be done to remove dead or diseased wood and to ensure that fruit is not too low to the ground where it may be exposed to soil borne fungal diseases.
To rejuvenate an old citrus tree, usually Eureka or Lisbon lemons, cut out all the deadwood in late spring and then remove all small branches, leaving the main framework of the tree almost bare. Water and fertilise well immediately after pruning and then water deeply every week during the warm weather as lots of water is needed for production of new growth. Exposed areas of the trunk should be whitewashed after severe pruning as citrus bark is susceptible to sunburn.
In cold winters citrus leaves often become yellow, this is actually called ‘winter yellow’ and is caused by cold weather and cold soil. In frost free climates citrus trees can be fertilised in winter, but not in cold areas. In frosty areas the soil is usually too cold for fertiliser to have much effect, but fertilising should not be attempted after March in cold areas as it may promote soft growth which will be burnt by frosts. Ensure that citrus plants are well fed in the warmer months and receive sufficient fertiliser in March before the onset of cold weather, to see them through the winter.
Mottled leaves with veins quite green are usually a sign of nutritional deficiency. Possibly insufficient magnesium, zinc, or even reduced availability of iron due to alkaline soil. Mix up some seaweed extract to the recommended rate and then add a heaped teaspoon of Epsom salts and chelated iron to this mix. Spray onto the foliage once a month during the warmer weather.
Harvest with the stalk on, and pack into a dry cardboard or wooden box with lots of crumpled newspaper. Ensure that the paper surrounds each lemon so none are touching each other. Store in a dry position indoors. Only clean, unblemished fruit should be stored. Lemons can be stored in this way for about 2 months.