Although oranges are the major fruit in the citrus fruits group, accounting for about 70% of citrus output, the group also includes small citrus fruits (such as tangerines, mandarines, cumquats), lemons and limes and grapefruits.
Citrus fruits and citrus juices have several beneficial health and nutritive properties. They are rich in Vitamin C or ascorbic acid and folic acid, as well as a good source of fiber. They are fat free, sodium free and cholesterol free. In addition they contain potassium, calcium, folate, thiamin, niacin, vitamin B6, phosphorus, magnesium and copper. They help to reduce the risk of heart diseases and some types of cancer. They are also helpful to reduce the risk of pregnant women having children with birth diseases.
Climate
Citrus are considered subtropical but will grow in most areas of New South Wales, the coastal areas north of Sydney are the most favourable for growth and early maturity because of their high summer and winter temperatures. Provided adequate irrigation is available, trees are tolerant of hot conditions, although exposed fruits may be sunburnt.
The cold-hardiness of fruiting citrus types varies significantly. This often influences where certain varieties or types can be grown successfully, that’s why its good to ask your local supplier what grows best in your area
Citrus trees grow best in deep (50 cm), well-drained, sandy loam soils. They will not tolerate very acidic or alkaline conditions, preferring a soil pH in the range 6–7. The maximum amount of sunlight is desirable for the growth, setting and maturity of fruit. Avoid positions that are low and frost-prone, or exposed to strong winds, particularly sea winds.
If trees are planted in tubs they will require exposure to the sun for part of the day to maintain healthy growth. Patios and small courtyards are suitable for tub trees such as Meyer lemons, cumquats and calamondins.
Propagation
Citrus trees grown from seed are normally very variable and do not produce true-to-type trees or fruits. Common varieties, available from local nurseries, are normally propagated by budding the required scion or variety onto specially grown and suitable rootstocks (understocks).
The most commonly used rootstocks for citrus are P. trifoliata and rough lemon, but Troyer, Carrizo and Benton citrange, Cleopatra and Emperor mandarin, and sweet orange are also used to propagate trees in some localities. Different rootstocks will adapt citrus to various soil conditions, such as light (sandy), heavy (clay), alkaline, saline, or soil containing citrus nematodes.
P. trifoliata and the citrange rootstocks have resistance to phytophthora root rot and are used in heavier soil types where drainage problems are expected or where trees are planted to replace others that have been removed. They may be used for all varieties except Eureka lemon. Benton citrange, however, can be used for this variety.
Rough lemon is very susceptible to phytophthora root rot but is suitable for all varieties except Ellendale and Imperial mandarins.
Planting
Citrus nursery trees are normally sold in containers, although some may be available as bare-rooted trees. Prepare the site by digging over several months before planting. Add organic matter and remove troublesome weeds.
The best time for planting a citrus tree is when the risk of severe frost is past, usually in August or September. Advanced trees that have been grown in containers may be planted during spring, summer or early autumn provided there is little disturbance to the main root system and they are watered frequently. Where summers are hot and dry, and winters are mild, trees may also be planted in autumn.
When planting any citrus tree, prepare the site well beforehand and plant the tree with the bud union at least 15 cm above the ground level.
Do not put artificial fertilisers in the planting hole. If the tree is planted too deep or allowed to settle below ground level, the value of the rootstock will be lost. Make sure the soil is pressed firmly around the tree’s base, and form a basin around the tree to make watering easier. Pour about 10 L of water slowly into the basin to consolidate the tree and provide a reserve of moisture. Rewater several times a week until new shoots appear.
Tree care
In the first year of growth in the garden, water the tree at intervals of 4 - 7 days, depending on the weather. Regular watering is essential during hot, dry, windy periods.
When the tree is well established, watering may be limited to a good soaking with a hose and sprinkler every week. Tub-planted trees will need to be watered more frequently, depending on the amount of soil and the growing position.
The root system of citrus trees is usually concentrated in the top 30 cm of soil and within the ‘drip ring’. If a mulch has formed or is maintained under the canopy, many of the important fibre roots will be found within 10 cm of the surface. Take care when digging around the tree not to damage larger roots, as an injury may result in invasion by wood-rotting phytophthora fungi. Heaping grass clippings or other material around the base or trunk of the tree can result in similar problems.
Remove weeds by light chipping or mowing until the canopy develops and reduces light under the tree, thus inhibiting weed growth. Problem weeds such as couch, kikuyu or summer grass may also be controlled by the careful use of registered herbicides. Some that are safe to apply near citrus may be injurious to other nearby fruit trees, shrubs or plants in the garden.
Trees planted in tubs may require repotting into larger containers after several years, or root pruning if the tree is to be kept small. Artificial fertilisers may damage the roots and burn foliage, so it is safer to use blood and bone or slow-release, mixed fertilisers (pellets or tablets) for these trees. (See the section Nutrition for more information.)
Pruning
A light pruning of foliage may be necessary at planting if the root system has been badly damaged or reduced to fit in a tub. If the tree was grown in a shallow container it may be necessary to cut the tap root to remove spiralled growth before planting.
Apart from removing suckers and shoots from the rootstock and trunk, there is no need to prune the tree until the dead wood has to be removed. The citrus tree is self-shaping but in time the shaded inside twigs and limbs should be removed to open the centre and reduce the risk of fungal diseases like melanose.
Pruning hints
• Cut out all dead or diseased wood cleanly; don’t leave stubs.
• Remove crowded or crossed-over branches.
• Paint cuts that are larger than 2 cm across with a bituminous-type wound dressing.
• Prune in the spring when the danger of frosts is over.
The lower growth (up to 30 cm above ground level) can also be removed if necessary to assist in the tree’s soil management. This pruning is called ‘skirting’.
If individual branches impede access or are crowding other trees, fences or paths, they can be removed completely or cut back to any convenient position. Citrus trees readily produce new growth from near major pruning cuts.
Skeleton pruning
Orange and lemon trees that have lost vigour but have a sound root system and trunk may be rejuvenated by a skeleton pruning, that is, a severe cutting back in the spring. This can rejuvenate some citrus trees into completely new growth. Remove all the leaves and twigs cleanly, and cut all the main branches that form the framework at 2 - 3cm diameter so that only the tree’s skeleton remains. Seal the main cuts as described earlier and whitewash the limbs and trunk to protect against sunburn.
Nutrition
Most garden soils require some annual fertilisers for good tree development, fruit size and regular cropping. Citrus trees, being evergreen, have two main growth flushes (spring and autumn) and therefore require more than most deciduous trees or vines.
The main nutrition requirement is nitrogen to promote growth and fruit size. Phosphorus is also usually lacking, and, if the soil is a light sand, regular additions of potassium are needed to maintain tree health and fruit quality. Magnesium and calcium may be needed in some soils, together with small amounts of the trace elements zinc, manganese and copper.
Type and amount of fertiliser
The main fertiliser needs of citrus trees can be applied as organic/animal manures or artificial/chemical fertilisers.
For non-bearing trees, organic manures can be convenient and supply adequate plant elements and organic matter to the soil. Provided these manures are not too fresh, they may be applied safely to young trees in the autumn or winter. As a guide, apply 4–8 kg for 1-year-old trees, increasing annually up to 16–32 kg by the eighth year.
As bearing trees would require large amounts of organic manures (20–40 kg per tree), they are instead usually given a complete ready-mixed citrus fertiliser such as 10:4:6 or 10:3:6* at the rate of 500 g per year of tree age until the tree is 10 years old, then at the rate of 5 kg per year applied in the later winter. Give high-producing trees a supplementary application of nitrogen (700 g ammonium sulfate) in November.
*These ratios represent the proportions in which the constituent elements are present in artificial fertilisers:
nitrogen : phosphorus : potassium (N:P:K)
Mixed fertilisers can also be used for non-bearing trees, at the above rate. When used for non-bearing trees, three-fifths of the annual amount can be applied in the late winter and two-fifths in the summer. For very light soils a ‘little and often’ program in the growing period can be followed, particularly for non-bearing trees.
Application of fertiliser
Most citrus trees blossom once per year in the spring, and the applied nitrogen should be available at that time.
Whatever type of fertiliser is used, spread the required amount very evenly around the tree, avoiding the trunk, and water after each application. This spreading should be 0.5 to 1 m wide for young trees, and to the drip ring or foliage edge for bearing trees.
Correction of low soil pH to 6 - 7.5 with dolomite, magnesite or agricultural lime will improve the tree health generally and ensure that other fertilisers applied become available to the tree.
Deficiencies of nitrogen, magnesium, zinc and manganese are fairly common in citrus trees and can normally be corrected easily, but the correct identification of any nutrient deficiency is important.
Harvesting
For all varieties except limes, fruit should be left on the tree until they have developed full colour and flavour. Unlike many other fruits, mature citrus can be held on the tree for some time, but quality will eventually deteriorate and pest and disease problems may increase losses.
Citrus fruit should be handled carefully and picked by a twisting–pulling action which breaks the stalk but does not damage the button or the fruit. Do not pick cold or wet fruit, as shelf life can be severely reduced.
Surplus fruit can be stored for several weeks in the crisper of the refrigerator. Lemons can also be cured and stored for long periods:
• clip dry fruit at first colour change from dark green to light green (silver);
• handle carefully;
• store in paper-lined or dry sand-filled boxes in a cool, dark place away from draughts but with good ventilation.
Problems
Summer fruit drop
Citrus normally shed large numbers of fruitlets shortly after blossoming in the spring and at early fruit set (pea size). However, it is also common for a sudden drop of small fruit (20 mm diameter) to take place in the summer, when warmer weather places stress on the tree. The problem is particularly severe in young navel orange trees and may be related to lack of water at, or soon after, fruit set. Root diseases and lack of nitrogen or trace elements may also be responsible.
Rind splitting
Rind splitting of fruit, particularly in navels, also occurs before or near maturity as a result of climatic factors, specifically a drop in average temperatures and an increase in relative humidity at a time when the rate of fruit growth is decreasing. There is no control for the disorder.
Preharvest drop
Preharvest drop in the autumn before fruit is fully mature is a common problem with navels and grapefruit. Some of this drop is natural, but in coastal areas fruit stung by Queensland fruit fly during the late autumn colour-change period is also very prone to drop. Some mandarin and lemon varieties will also shed fruit when damaged by the spined citrus bug. Brown spot infection in mandarins will result in fruit drop.
Alternate cropping (biennial bearing)
Alternate cropping (biennial bearing) is a common problem with many citrus varieties, such as Valencia orange, Wheeny grapefruit, and mandarins. After a heavy crop the tree often responds by carrying a light (or nil) crop. Once this cropping pattern is established it is difficult to return to regular annual cropping. Pruning or thinning of the heavy crop and early harvesting will assist in reducing the problem.
Second crop fruit
Sometimes orange trees will produce blossoms in the autumn after a stress period, and set a second crop. This fruit is often of poor quality (thick skins and low juice content) and is susceptible to fruit fly attack. With lemon trees, however, these intermediate crops are desirable.
Citrus care:
Limes grow well in the warmer areas of Australia. Worth a try in cooler zones, but protect from frost when young. A position in full sun is best. Keep trees well watered when the fruit is forming in spring and early summer. Trees growing in the ground should be fertilised in August and February. Complete Citrus Food alternating with Dynamic Lifter would be suitable. Feed citrus growing in pots every six to eight weeks. Once again alternate Complete Citrus Food with Dynamic Lifter, or use 3-4 month Osmocote applied in spring and early summer. Feed Australian finger limes with an organic fertiliser in late winter and spring. Water well before and after fertilising. Keep the area beneath your trees free of grass and weeds. Mulch with compost or other organic material, but make sure that the mulch does not touch the trunk of the tree.
Varieties
Oranges (Citrus sinensis)
Washington navel
An early, seedless variety that matures in May and June. Its fruit holds on the tree for several months under favourable conditions. The juice should be used quickly as it becomes bitter, even in short-term storage. In coastal areas excessive summer heat casues fruit drop or shedding of developing fruit, resulting in very poor crop.
Thomson navel
Matures slightly earlier than Washington. It is also seedless with a fine rind, but is not as juicy as the Washington.
Lane Late navel
Matures later than the other navels — it is not coloured until August. It can be very slow to reach satisfactory cropping levels in coastal climates.
Joppa, Parramatta, Homosassa, Siletta, Hamlin and Mediterranean Sweet
All these varieties are mid-season common seeded oranges that mature in July and August and have good juice content. Trees are easy to grow and cropping is regular, but fruit will only hold on the tree for a short time. All are suitable for home juicing.
Valencia
A late, seeded variety that matures in September and October. Fruit will remain on the tree for 6 months under favourable conditions, but only if it is not damaged by fruit fly or black spot as may occur in coastal districts. A seedless selection is also available. Valencia is the main growing and juicing variety in the State.
Blood oranges
Three varieties are currently available: Ruby, Maltese and Harvard. Although popular in some Mediterranean countries owing to their distinctive flavour and appearance, they have only a limited appeal in Australia. The amount of rind and flesh pigmentation, particularly under mild coastal growing conditions, is usually disappointing. The hot, dry inland areas are more dependable for pigmentation development.
Lemons (Citrus limon)
Eureka
This is a suitable variety for the coastal home garden. It bears several crops of fruit — in winter, spring and summer. The tree has few thorns and grows vigorously. Eureka is not compatible with P. trifoliata or citrange rootstocks and is normally propagated on rough lemon or the newer rootstock Benton citrange.
Lisbon
A vigorous, thorny variety that is more cold-tolerant than Eureka and more suited to cooler growing situations. The main crop matures in autumn/winter, but light crops ripen in spring in coastal areas. Selected strains are compatible with P. trifoliata and citrange rootstock.
Meyer
A hybrid that is more cold-tolerant than other lemon varieties. The fruit, which is produced throughout the year and is almost orange in colour, has a high juice content and a mild, low-acid flavour.
Lemonade
An Australian seedling originally located in Queensland. A vigorous, heavy-cropping tree with fruit similar in appearance to a lemon but with lower acid and a distinctive ‘lemonade’ flavour.
Mandarins (Citrus reticulata)
A large number of mandarin and related hybrid varieties (tangor and tangelos) are available. Some develop rind colour early, but each variety is mature and retains its fruit quality for a relatively short period (5–6 weeks). Compared with most other citrus types, some varieties are hard to pick and require clipping to reduce rind damage at harvesting. The following popular varieties are listed in their approximate order of maturation, from very early (April) to very late (September).
Satsuma
The Silverhill selection is a very early variety and the first to mature (April and May). The tree is very cold-hardy and small. Fruit is seedless, medium-sized, juicy and has bright orange flesh. Also known as C. unshiu.
Imperial
An early variety that matures in May. The fruit has an excellent flavour, but tends to be small when heavy crops are produced. Thinning fruitlets will improve fruit size.
Clementine
The Algerian selection is widely grown in some Mediterranean countries. It is early-maturing, commencing in mid-May. Fruit is well coloured (red-orange), medium to small in size, sweet and juicy, but seedy.
Emperor
A mid-season variety that matures between June and August; a late-maturing selection is also available. It has good flavour and is easily peeled. The fruit is subject to brown spot in coastal areas, and the trees tend to overcrop, resulting in smaller fruit size and tree decline.
Thorny
Also a mid-season variety. Fruit is small, sweet and juicy with good flavour, but seedy.
Minneola
A mid-season hybrid, maturing in early July. The fruit has a distinct neck and thin bright reddish-orange rind, which is difficult to peel. Fruit is also subject to brown spot when grown in coastal areas.
Ellendale
A late variety, maturing in August in the warmer districts but sometimes harvested as late as October in cooler areas. It is not a true mandarin but a natural tangor of good eating quality.
Seminole
An attractive late hybrid mandarin with reddish-orange rind. It is very juicy with a distinctive flavour.
Kara
This is a good quality, very late maturing variety (end of September).
Grapefruit (Citrus paradisi)
Grapefruit require hot, dry growing conditions to ensure good flavour and quality.
Marsh
A nearly seedless variety with straw-coloured flesh, it can be harvested from July but is sweeter if left until October in the Sydney area. A natural physiological preharvest fruit drop is a common problem and can occur just as the fruit is colouring or before it is fully mature.
Wheeny
A natural hybrid with light, straw-coloured flesh, a distinct lemon flavour and many seeds. It grows vigorously and produces large fruit but tends to have the problem of alternate cropping
Pigmented varieties
Of the pink-fleshed varieties, Thompson is similar to Marsh but has a faint red-pink flesh. Ruby (shown at right) and Red Blush have an orange-pink flesh and some colour on the skin under favourable hot and dry inland climatic conditions.
Poorman
A hybrid with large, pale orange rind and flesh. This variety is more suitable for growing in colder districts than other true grapefruit varieties.
Chironja
A newer imported variety that is apparently a natural hybrid of grapefruit and orange. Fruit size and appearance is similar to grapefruit but with an orange flesh that is juicy and sweet, lacking the acid and bitterness of normal grapefruit.
Limes (Citrus aurantifolia)
Limes are suitable for growing only in frost-free sites. They do well in the more tropical areas.
Tahitian lime (Citrus aurantifolia)
This variety produces very juicy fruit all year round. The plants grow to around 3x3m (10x10') tall, and they do well in the garden or in pots. The seedless fruit is small and green when ripe, although it can be left on the tree until it turns yellow. Tahitian limes are easy to grow. They are reasonably frost tolerant, but in very cold areas you should wait until spring before you plant.
Kaffir lime (Citrus hystrix)
Kaffir limes will reach 1.5 metres (5') tall, but because the leaves are constantly being picked for cooking, the plants usually remain small in size. They also make good pot specimens. The leaves of the Kaffir lime are an essential ingredient of many Thai recipes, including green curries, fish dishes and soups. The flesh of the fruit is usually thrown away, but the rind and zest is sometimes used. Each leaf is 'waisted', and looks like two leaves joined together.
Australian finger lime (Citrus australasica 'Rainforest Pearl')
Formerly known as Microcitrus australasica, the Australian finger lime grows from 3-10m (10-33') tall. It is a hardy, spiny, shrub or small tree which is closely related to the domestic citrus. The fruit is long and narrow, and is a brownish red colour. It has a sour taste and is probably best used for Thai cooking, jams, garnishes, sauces and drinks. If possible, before you buy an Australian lime, taste the fruit to see if you like the flavour.
Mexican (or West Indian)
Fruit from this variety is very acidic with a strong lime aroma and flavour, but is smaller than that of Tahiti. The trees are also bushier and not as vigorous.
Sweet limes or sweet lemons (C. limettioides)
These are widely grown in India, where they are considered to have medicinal value. They have an insipid taste because the fruit lacks acidity, and they are not grown commercially. The fruit is medium-sized with a smooth, thin rind which is greenish to orange-yellow. It has good juice content and few seeds.
Rangpur
An attractive hybrid mandarin–lime variety with orange-coloured fruit which is very acidic and suitable only for drinks.
Tahiti (or Persian)
A variety similar in size and shape to a small lemon, but the flesh is seedless and pale green in colour. It should be harvested while the rind is still green, or it may develop stylar end rot and drop from the tree. The juice is not as acidic or aromatic as that of the Mexican lime, which is preferred for fresh juice drinks. The trees are upright, of medium size and vigour.
Calamondins (Citrus madurensis)
This fruit has frequently been confused with the round cumquat, and many home gardeners do not know it by its correct name — both cumquats and calamondins are attractive ornamental trees and produce distinctive small fruit which have a wide range of uses.
Although resembling a cumquat, the calamondin is similar to a very small mandarin and has a loose, bright orange skin when ripe. The tree blossoms in the spring at the same time as mandarin trees, whereas cumquats flower in early summer. Variegated calamondin leaves and fruit are shown at right.
The calamondin has been used mainly as an ornamental tree in the garden or in a tub, but the fruit, although it lacks sweet flesh and skin taste, can be made into a tasty marmalade.
Cumquats (Fortunella species)
Cumquats belong to a citrus subspecies. The oval cumquat (Nagami, shown at right) is available from nurseries and can be easily identified by the oval shape of the fruit. It makes an attractive ornamental tree in the garden or in a tub. The fruit has a sweet, oily skin and can be eaten whole, crystallised, or used to make jams, conserves and liqueurs. The main blossoming does not occur until after November, but the crop matures unevenly and fruit is on the tree for long periods.
Nurseries have often confused round cumquats with the calamondin. The round cumquat resembles Nagami but is more rounded and has more segments and a sweeter flesh and rind flavour. The round cumquat available in the Sydney area is thought to be the Meiwa selection. As with the oval cumquat, blossoming is delayed until after November because of the tree’s dormant habit, and this makes it particularly well suited to colder climates with late frosts.
A variegated (light green and pale yellow) foliaged cumquat selection is also available.
Other citrus varieties
Chinotto (Citrus myrtifolia)
Fruit from this variety is bitter and not pleasant to eat fresh but is sometimes used for candying or crystallising. The tree has unusually shiny small leaves on thornless branches, and with its large, orange fruit (shown at right) it makes an attractive tub specimen.
Seville (Citrus aurantium)
This variety has a very bitter, orange-coloured fruit known as ‘sour orange’. It is used commercially for marmalade. Both smooth-skinned and rough-skinned selections are available.
Pummelo (Citrus maxima)
Indigenous to the Malayan peninsula, this variety was carried to other countries by traders. It is also known as a ‘shaddock’. It has very large, yellow-green fruit with coarse pink or white flesh and a very thick white rind, which makes it suitable for marmalade. It can be used as a substitute for grapefruit. The local selections are highly variable and have little use in the home garden, except for the novelty value of the very large fruit.
Citron (Citrus medica)
This is one of the oldest citrus species known and grown. It has special religious significance. The fruit is large, oblong, thick-skinned and very aromatic, but it is bitter and virtually non-edible; however, it can be used for marmalade or the peel can be candied. The tree is small, weak-growing and short-lived because it is very sensitive to wind, frost, heat and the common tristeza (‘quick decline’) virus disease.
Kaffir lime (Citrus hystrix)
This variety is an exotic type with an unusually large-winged petiole leaf that is dried and used widely as a seasoning in cooking, particularly in South-East Asia. The fruit is very bumpy, full of seed and with very little juice.
