Citrus Trees
Lemon trees (and other citrus) are one of the most popular trees grown in home gardens. Nurserymen, landscapers and garden experts of all types, from London to Sydney and everywhere between, will tell you they get more questions about citrus than anything.
Citrus can be grown in cool temperate climates through to the tropics, but some types are not as tolerant of cold as others. Fortunately they are well suited to growing in containers though; so in a cold climate, they can be simply containerised and brought inside over winter.
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 If you want the best from your citrus, you do need to understand basic horticulture including soils, nutrition, pest and disease management, water management, etc.
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Citrus Profile:
Genus: Citrus Common Name : Citrus Family: Rutaceae
Appearance: Mainly small to medium evergreen, sometimes spiny trees. Thick, leathery, glossy leaves. The leaves and skin of the fruit usually contain numerous oil glands.
Flowers: White or purple, often fragrant flowers, in axillary clusters or solitary, or in pairs.
Features: Aromatic, leathery skinned fruits with fleshy, often very edible pulp.
Requirements: They have shallow roots and will do best in light soils with good drainage. In alkaline soils, deficiencies of Zinc, Iron and Manganese may require treatment with foliar sprays. Most Citrus are frost sensitive, particularly during growth flushes, and may need protection from wind.
Culture: In mature trees, fertilizer applications will often require addition of supplementary Nitrogen, the exact amounts depending largely on the variety. In young trees Calcium is an important nutrient. Little pruning of Citrus is required other than to maintain shape and vigour. If necessary, they can be lopped quite hard, but limbs exposed by this operation (or any sort of pruning) should be coated with a white paint, as the bark is quite sensitive to sunburn. Spacing in the past has been about 5m x 5m. Current trends however, are towards closer plantings and even to hedgerows. Propagation is by budding or grafting of selected varieties onto suitable seedling stocks mostly of Sweet Orange (C. sinesis), Sour Orange (C. aurantium), Rough Lemon (C. limon), Grapefruit (C. paradisi), and the closely related Trifoliate Orange (Poncirus trifoliata). The selection of stock will depend on their disease resistance, soil type preferences, vigour or dwarfing effects and the variety being propagated. Most respond well to watering in dry periods. Mulch well, but keep away from trunk.
Pest & Disease: Their are numerous diseases and insect pests which may affect Citrus including a number of scales and other sucking insects, fruit fly, a variety of fungal infections of the leaves, fruit or roots, and several viruses. The latter may be potentially the most serious. Exactly what pest and disease problems occur will depend on locality, variety, rootstock type and cultural practices.
Species & Varieties:
About a dozen or so species of tropical and subtropical origin.
LEMON (Citrus limon)
Lemons are relatively hardy provided you use a grafted plant growing on a rootstock suitable for your area. They are fairly tolerant of high humidity and occasional strong winds but do not tolerate waterlogged soils at all. Lemons need more potassium (ie: potash) than other citrus. Trees can take 10 years or more to reach full size but can last a lifetime. Lemons are picked after they turn yellow
Most lemons grow 4‑7m tall and while adapting to cold climates are frost sensitive. Meyer lemons are smaller and more frost tolerant than other varieties. Other popular varieties include Lisbon and Eureka. Lisbon is sometimes preferred because it has less thorns than Eureka.
GRAPEFRUIT (Citrus paradisi)
Most varieties need more warm weather than lemons. "Marsh Seedling" is the most common variety, however Wheeny is a more cold tolerant variety. Pink fleshed varieties such as "Ruby" are best suited to warm climates (eg. Southern Queensland). Fertilize generously particularly with nitrogen and phosphorus, but avoid heavy feeding close to harvest. Too much nitrogen near harvest can cause a coarser skin. Observe frequently near harvest as grapefruit often drop from the tree before fully ripe.
LIME (C. aurantifolia) (NOTE. This is not the same species as the sour orange ‑ C. aurantium).
This fruit is very frost sensitive and is generally not suitable for cultivation south of Southern Queensland. The Tahitian Lime is more cold tolerant and not as acidic tasting as the West Indian Lime. West Indian Limes do not grow well on the coast. Fruit ripens over a period of time and will drop off the tree when ripe.
MANDARIN (C. reticulata)
Mandarins are more frost tolerant than oranges and lemons however like grapefruit, they do best in warmer areas and may be frost damaged or develop dry less tasty fruit in cool areas. The satsuma mandarin (Citrus unshiu) is a similar but more cold tolerant fruit. Soil, fertilizer, water and other requirements the same as with oranges. Mandarins may tend to set very heavy crops and develop a biennial bearing habit. This can be prevented and controlled by careful thinning of fruit.
SWEET ORANGE (Citrus sinesis)
Sweet Oranges require temperatures between 13 to 40 degrees celsius to grow, and average temperatures in summer which are above 16 degrees celsius. They can tolerate low temperatures if they are not growing, but if there is any fresh growth at all, they are very susceptible to cold or frost. Fruit can mature any time from mid winter to late summer depending on the locality and the variety.
Fertilize two to three times each year with nitrogen phosphorus, and to a lesser extent, potash. Keep the roots moist at all times but never waterlogged.
SOUR ORANGE (Citrus aurantium)
Sour Oranges (C. aurantium) are only occasionally grown and then only for various process products such as marmalade. Valencia is the most popular variety, sometimes grown because of it's disease resistance and ability to hold ripe fruit on the tree for up to six months over spring and summer.
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