Cashews: All You Ever Wanted to Know
Since Cashews are found only in the tropics, many people in the US, Europe, and Middle East have never seen a cashew tree , a cashew nut still in its shell, or a cashew apple. Please use the following links to learn about the taxonomy of the cashew tree as well as see pictures of this interesting tropical tree.
Taxonomy of the Cashew Tree
The Cashew tree, known by the latin name Anacardium occidentale, is a member of the Anacardiaceaefloral family, which includes the mango tree, pistachio tree, poison ivy and poison oak. The Anacardiaceae family contains 73 genera and 600 species. Anacardium contains eight species which are all native to South America.
The cashew tree is a small to medium tree, generally single-trunked and spreading in habit, up to 40′ in height but generally 10-20′ in cultivation. In older trees, spread may be greater than height, with lower limbs bending to touch the ground. Leaves are thick, prominently veined, oval to spatulate in shape, with blunt tips and entire margins. New foliage contains reddish pigment.
Flowering for the cashew tree is similar to the close relative mango tree: both male and perfect flowers are born in the same inflorescence (polygamous). Individual flowers are 1/4″ across, with crimson petals, often striped longitudinally and reflexed. They are borne terminally on panicles, generally at the beginning of the dry season. Flowering may occur over several weeks, and it is not uncommon to have ripening fruit and flowers on the tree at the same time.
Trees are at least partially self-fruitful, as lone trees can bear many fruit. One study of pollination biology showed no difference in pollen tube growth between self and cross pollinated flowers, yet final yield was higher when cross pollinated. In practice, cashews are often grown from seed, and cross pollination in orchards must occur to a high degree. Another study showed no indication of self-incompatibility, but a low percentage of fruit set (1-18%). Fruit set is highest in flowers that open first, and suggests some type of apical dominance with respect to fruit set, as found in the close relative the pistachio. Fruit set is similar for female and perfect flowers. Various insects, even flies and ants provide pollination.
The true botanical fruit is a nut, about 1″ long, shaped like a small boxing glove, hanging below a fleshy, swollen peduncle called the cashew apple or pseudofruit. The cashew apple resembles a pear in shape and size, is juicy, fibrous, and astringent tasting. It has thin skin of either yellow or light red color, and yellow flesh. Fruit are borne singly or in small clusters, and mature in 60-90 days. The nut develops first, followed by the rapid swelling of the cashew apple in the last few weeks.
Uses of the Cashew Tree (Traditional and Modern Medicines & Construction)
Use of Cashew Wood Resin
In terms of wood quality, the cashew tree is known for producing wood with high levels of resin. The resin from the cashew wood is made into a varnish which is known to prevent deteriation ants and other home-invading insects. The leaves, bark, and fruit also contain caustic oil which can cause skin irritations in some people.
In its raw form, the cashew nut is not edible. However through processing, the caustic oil is removed from the cashew nut shell. This caustic oil is referred to as Cashew Nut Shell Liquid (CNSL) and is located in the tissue between the two walls of the nut shell. CNSL has many industrial uses which include automobile and airplane brake fluid, adhesives, paints and varnishes, insecticides, electrical insulation, and anti-microbials. In addition to CNSL, resins and gums from fruit stems or bark is used as a varnish for books, wood, and flooring to protect from ants and other home-invading insects.
Traditional and Modern Medicines made from the Cashew Tree
There are many medicinal uses of cashew leaves , bark, and juice from the cashew apple. In Brazil, cashew bark teas were used to stop diarrhea while the caustic shell oil was used to treat skin infections, warts, intestinal worms, and parasitic larvae beneath the skin. Teas and fruit juices from the cashew apple and leaves are known to have antimicrobial, anti-inflammatory, astringent, diuretic, hypoglycemic, and other medicinal properties. The active ingredients in the teas and juices are thought to be tannins, anacardic acid, and cardol. Modern uses of shell oil and fruit juice include facial peels and scalp conditioners and shampoos. The cashew apple has also been a long time nutrional supplement as it contains up to 5 times more vitamin C than citrus and strawberries.
How to Grow Cashew Trees
The cashew is a strong plant that is renowned for growing in soils, especially sandy soils, that are generally unsuitable for other fruit trees. For the best production deep, well-drained sandy or sandy-loam soil is recommended. Cashew trees will not grow in poorly-drained soils.
Fresh seeds that sink in water are planted in an upright position in a planting bag containing a loose, sterilised soil mixture. Three to four seeds can be planted directly in the planting hole. The weakest ones are thinned out later and the strongest left to develop further. The seedlings are very susceptible to Phytophthora root rot. The plant bags should be 350 to 400 mm deep, as the tap-root grows very fast and bends around as soon as it touches the bottom.
Cashew seedlings are grown under shade (45 %) and hardened off before planting in the orchard. It is very important not to disturb the root system during planting. Young trees should be supported for the first 2 to 3 years so that wind will not blow the plants over.
Self-pollination and cross-pollination play an important role in the formation of cashew seed. Seedlings therefore show great variation and no “true to type” trees can be grown from seed.
Selected trees should preferably be multiplied by grafting or air layering because vegetative propagation will ensure the best production and quality. Trees that are precocious bearers and grow vigorously are selected.
Cashew Orchard Establishment and Maintenance
Planting distances of 8 x 5 m is recommended. The trees grow vigorously in the first 3 years and as soon as the crowns touch each other alternate trees should be removed until the permanent planting distance of 10 to 12 m is reached. Branches hanging on the ground should be removed because they interfere with harvesting. In other parts of the world cashew trees bear well, in spite of the little attention devoted to the orchards.
Growth and production of cashew trees can be enhanced by establishing clonal orchards, and improving fertilising and irrigation practices.
Two grafting techniques, namely side grafting and wedge grafting are practised with success. Grafting should commence as soon as possible (seedlings of 3-4 months old) and planted out in the orchard to prevent the tap-root from bending.
Flowering to harvest time
Flowering is affected by weather conditions and also varies from tree to tree, but continues for a period of 3 months. High temperatures lead to earlier flowering. Both male and bisexual flowers are borne on one cluster. The flowers are very susceptible to mildew and control thereof on the leaves and flowers is a prerequisite for good production.
Pollination is mostly by insects. After pollination it takes 6 to 8 weeks for the fruit to develop. The nut develops first while the apple develops and enlarges only 2 weeks before fruit fall. Nuts should be harvested as soon as possible, especially under wet conditions and should be dried before storage.
Irrigation is important during establishment of young trees because it doubles the growth tempo of young trees in a dry season. Due to the deep root system the trees can survive several months without irrigation. Mature trees should receive 1 800 l of water per tree every 2 weeks.
Grass strips in the inter-rows between the tree lines are ideal to prevent erosion and should be cut regularly.
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