ACS Distance Education UK
SWEET ORANGE (Citrus sinensis)
Sweet Oranges require temperatures between 13 to 40° Celsius to grow, and average temperatures in summer which are above 16°C. They can tolerate low temperatures if they are dormant, but if there is any fresh growth at all they are very susceptible to cold or frost damage. Fruit can mature any time from mid-winter to late summer depending on the locality and the variety.
There are many options to choose from and typically with the exception of dwarf varieties they all grow quite tall at about 5 to 6 metres. Some of the more popular ones are 'Navel' and 'Valencia' oranges. Valencia does well in many climates from warm temperate to tropical. They are fast-growing and relatively hardy trees with good disease resistance and reach about 4m in height. The fruits can be left on the trees for several months over spring and summer without any noticeable deterioration in quality. Whilst the fruits contain seeds there is a 'Seedless Valencia' variety which tastes very similar but bears fruit a little earlier than the standard Valencias towards the end of winter.
Navel oranges grow to 3 to 4m tall. They are usually seedless and have easily peeled skins. The Washington Navel has particularly sweet fruits which are borne in early winter. The main drawbacks are that it is very prone to a wide range of diseases, it is not a heavy cropper and the fruit quality and fruit juice quickly deteriorates. The ‘Leng Navel’ can be harvested in autumn and is generally suited to growing in inland areas only. It has a thinner skin which tends to split. The ‘Thomson Navel’ is seedless and less juicy than Washington navel, and is also harvested in autumn.
Navelina is a smaller growing cultivar which only attains a height of about 3 to 5 metres. Its fruits mature earlier than other navels being ready to harvest towards the end of autumn. Fruits have thin rinds and are oval shaped with a very good taste.
Seedlings of oranges should be planted in full sun in well-drained and fertile soil. They take 2 to 3 years to bear fruit. Choose a variety suited to your local climatic conditions. Water plants at least once a week during hot weather and apply mulch. Fertilise 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. Most oranges bear more fruit if they are not pruned.
Harvest oranges when the fruits begin to turn orange. They can be removed by twisting and pulling or cutting the stem but care needs to be taken to avoid damaging the branch. Oranges may be stored for 2 to 3 weeks at room temperature or longer if refrigerated but they are best stored in plastic to avoid mould form developing on the rinds.
SOUR ORANGE (Citrus aurantium)
Sour Oranges are only occasionally grown and when they are it is only for various processed products such as marmalade because they taste too sour to eat fresh. The rinds and oil are also used for a variety of foodstuffs and other products like soaps, perfumes and potpourri. These fruits are also known by other common names including as Bitter Orange, Seville Orange and Marmalade Orange. They are not very hardy and grow best in frost free zones but will tolerate short bursts of frost. These trees bear hermaphroditic flowers in spring and summer and are self-fertile.
C. aurantium are largish thorny trees reaching up to 9 metres tall and 6 metres wide, although there are forms which only grow to 3 metres tall. Plant them in sheltered positions to avoid cold damage. They do best in well-drained but moist soil. Dig in some well-rotted manure to enrich the soil if it is not very fertile.
They can be grown in containers which is useful in cooler climates. The containers can be moved under cover over winter. If grown in containers care should be taken not to over water or under water the plants. They don't like to dry out and they won't withstand waterlogged conditions. They will need more frequent fertiliser applications in containers since nutrients are more readily leached out.
The thick skinned fruits are fragrant, rounded and orange with a knobbly surface. As they mature they become a dark reddish-orange colour and this is when they should be harvested.
There are many different sour orange varieties to choose from. C. aurantium var. bergamia which is known as the Bergamot Orange has small flowers and pear shaped fruits with very aromatic rind. C. aurantium var. myrtifolia or the Myrtle-leaved Orange is a thornless type with a compact habit and small fruits. C. aurantium var. khatta has purple tinged flowers like lemon flowers and sour orange fruits and is possibly a hybrid of lemon and orange.
There are also several groups of sour oranges known as the normal, aberrant and bittersweet groups and in each of these groups are many cultivars. 'Daidai' from the aberrant group is grown widely in China and Japan. It is a thornless, almost dwarf, species with large, very thick skinned, seedy fruits. The flowers are large and fragrant and used to flavour tea. 'Bouquet' is another from this group which is often grown as an ornamental. It has small fruits with few seeds. 'Paraguay' from the bittersweet group is quite a hardy and large tree with medium sized and fairly sweet fruits.