Multiplication of cashew plants by sexual method through directly sowing seeds is one of the methods of creating freshly cashew trees.There are several advantages and disadvantages of this method. The procedure of sexual propagation includes following steps:
Selection of mother trees
There are several specific criterias to select main plants:
- A dwarf tree with a dense canopy, many branches, and a high rate of producing flowers on side branches.
- The trees in which the immature stage is short, the flowering phase is shorter and the difference between stamens and pistils is smallest.
- The mother cashew tree must be 15 - 25 years old, with a harvest yield of 15 kg of nuts per year.
- An average bunch has 4 - 5 fruits, cashew nuts have average size (125 - 150 seeds/kg), thin shell, beautiful shape and high specific gravity.
(Source: "Package of Practices for Cashew" published by "Central plantation crops research Institute" Kerala India in December 1982)
Mother cashew trees can also be selected according to the following criteria:
- Tree age: choose a 10-year-old or older cashew tree, with an average yield of 10 kg of nuts/year.
- Foliage: choose a cashew tree with a dense canopy, with at least 5 branches on 1 main branch.
- Inflorescences: must have at least 60% of flowering branches, flowering time must be within 30 - 60 days. The ratio of bisexual flowers per inflorescence is at least 10%.
- The number of cashew nuts on the bunch must be at least 5/bunch.
- Seed size: cashew nuts are average in size (120 - 150 nuts/kg).
- The cashew kernel after peeling must account for 25 to 30% or more.
(Source: "Cashew production and processing techniques" of the Cashew Research and Development Project UNDP/FAO/VIE/85/005 (1989))
Seed treatment
During harvesting cashew seeds
Only collect cashew seeds from selected mother trees of the currently harvested crop (latest crop) and at peak times (February to April). After picking, take the cashew nuts with a balanced shape and average size (112 - 125 nuts/kg) and soak them in salt solution 10% (or sugar solution 15%), then pick up the sinking seeds (with specific gravity from 1.025 - 1.05). Next, rinse selected nuts with cold water, then dry in the sun for at least 16 hours to a few days, then store in a cool place for use. Storage time has a great impact on the quality of cashew nuts. According to a survey on the germination capacity of cashew nuts over storage time (Rao V.N. Madhava, Rao I.K Sambashiva, Hassan M.Vazir 1957 in the work "Studies on seed viability in cashew"), cashew nuts stored in a tin container with a tight lid will obtain the best rate of germination (80 - 100%) within 8 months from harvest, then it will gradually decrease and completely lose germination capacity (0%) after 14 months. On the other hand, cashew seeds stored in sedge bags (or rice straw) will completely lose their germination ability after only 11 months. Using cashew seeds with high specific gravity helps germinate faster, grow seedlings stronger, increase harvest yield, and increase the kernel ratio of a nut (Turner D.J. 1956; Rao Madhava V.N. el al. 1957; Auckland A.K. 1961; Northwood 1967). Seed germination time is usually about 15 to 25 days according to temperature, humidity and the depth of seed burial in the soil. At a temperature of 35 degrees Celsius, the germination speed is fast (8 days) and at 25 - 30 degrees Celsius, this spread is moderately fast (11 - 12 days) (Rocchetti G. and Panerai, 1968).
To sum up:
- Select seeds from mother cashew trees harvested in the latest crop.
- Harvest at the peak time (February - April).
- Only choose medium sized seeds (112 - 125 seeds/kg) with specific gravity greater than 1.0 (1.025 - 1.05).
- Dry the seeds in the sun for a few days before storing them in a cool, airy place (best to use a tin container with a tight lid to store in a cool place).
- Only use cashew nuts stored for a maximum of 8 months from the time of harvest.
During sowing cashew seeds
In addition to the quality of the seed, when sowing cashew seeds directly into the nursery (or into soil pots), factors including humidity, temperature and location of the seed in the soil has also been playing an important role on the germination rate, germination time and health of seedlings. According to a survey on 5 positions and at 5 depths (between 2 cm and 10 cm) of seeds in the soil, if the seeds are in the position in which the seed stalk is facing straight up and tilted at a 45 degree angle (the curved waist of the seed is slightly face down) and they are at a depth of 2 to 6 cm in the soil, the best result will be obtained (high germination rate, short germination time and beautiful shaped seedlings). In this position, it is also beneficial that when the seedling emerges from the ground only 5% of the cotyledons are exposed, which protects the cotyledons (the first leaves of the seedling) from being damaged animals (birds, rodents, monkeys) (Bhuhanga Rao C. et al., 1962; Ascenso J. et al., 1971).
Before sowing, cashew seeds are soaked in plain water for 12 - 48 hours, then soaked in copper sulfate solution 1% for 10 - 15 minutes, then washed with lime water and finally rinsed with plain water. This treatment not only supports seed germination but also kills fungal spores that cause disease in future seedlings. Seeds should be sown in the field at the beginning of the rainy season to ensure that the seedlings will continue to grow well thanks to water stored in a root system. Sow 2 - 4 seeds in 1 pre-prepared hole (see section on cashew cultivation techniques), the distance between 2 seeds is 20 - 30cm. If sowing 3 seeds, each seed is placed at one vertex of a triangle with sides of 20 - 30cm. When sowing 4 seeds, each seed is placed in one corner of a square with sides of 20 - 30cm.
Many seeds are sown in one same hole in case one of these seeds does not germinate and after 1 year of planting, only select 1 best tree.
In summary, when sowing cashew seeds directly into the garden, here are a few things to bear in mind:
- Treat seeds before sowing: soak seeds in water for 12 - 48 hours, then put them in copper sulfate solution 1%, rinse with lime water and rinse again with plain water.
- Place the seed in the soil in the correct position with the stem facing up and tilted at a 45 degree angle, the curved waist of the seed is slightly facing down. The depth of the seeds in the soil is 2 - 3 cm.
- Sow 3 - 4 seeds in 1 hole. In the early stages of growth, it is necessary to keep the tree away from direct sunlight. After 1 year, prune weak trees, select only the best tree.
Sowing directly has the advantage of requiring less labor and the seedlings grow well, but it has the disadvantage of consuming cashew seeds and the quality of the seedlings is difficult to be consistent. To overcome these limitations, people sow seeds in pots to create seedlings of sufficient quality to transplant to the field.
Sowing seeds in pots include following steps:
- Step 1: Create a nursery (scale depends on the number of seedlings needed), closest to the planting site, choose a location with sunlight, and avoid places in the shade.
After being treated (as in sowing directly into the field), these seeds are put in a sedge bag, watered to keep them moist, and incubate for another 48 hours to help the seeds germinate quickly. Select seeds that germinate early and sow them in soil pots. Remove seeds that do not germinate or germinate too slowly. Each seed is planted in one soil pot.
- Step 2: The soil pot can be made of woven palm leaves, banana leaves, bamboo, plastic, etc. Currently, plastic is used commonly. The size of the soil bag is usually 15 cm in diameter, 25 - 30cm high (sometimes 35cm x 53cm in diameter), about 40 holes is perforated around the soil bag, they are evenly distributed from the top to the bottom of the bag for drainage (or cut corners at the bottom of the bag).
- Step 3: The soil put in the pot includes topsoil in the planting area, river sand soil (absolutely do not use salty sandy soil) and manure. For every kilogram of soil put into the pot, add 3g of phosphate fertilizer.
- Step 4: Sow the seeds into the soil pot in the way in which the seed stalks face up and are at a 45 degree angle (seed waist facing down). These seeds are at a moderate depth of 1 - 2 cm in the soil. After sowing seeds, water them twice a day, morning and afternoon, to keep the soil moist. Put the soil pot in the shade or keep it away from direct sunlight. In addition to watering, all weeds growing on the surface of the soil pot should be picked up.
- Step 5: When the seedlings are 1 month of age at a height of 15 - 25 cm, or at any height, transplant the young plants to the field before the young roots reach the bottom of the pot. When planting, make careful cuts in the potting bag so as not to damage the roots. Usually plant 2 trees in 1 hole. In the first month, if water is available, water regularly every 2 days to help the tree grow quickly. When the tree is about 0.7m tall, prune the bad tree to leave the best tree.
This method is very suitable for planting on large areas and being proactive about the planting season.
Plants created from sexual propagation (through seeds) have the genetic traits of both the parent plant. These traits are very complex and heterogeneous. Even though the seeds are harvested from the same tree, it is impossible to know which seeds were pollinated from the good father tree and which seeds were pollinated from the bad father tree. Therefore, there is only a small number of good seedlings, and a large number of trees will have different traits. This drawback can be overcome by selecting the plants that have best traits of the parent plant by checking continuous statistical data for a number of years for each plant and giving their next generation self-pollination to establish a pure line. These pure lines will be the main source of seeds for planting new cashew gardens. Thus, establishing a seed nursery (Clona seel garden) requires a lot of time and effort (Argles G.K, 1969; Paiz R. Guillen, 1975), the fastest way to immediately obtain a sufficient number of plants with the superior traits of the parent plant is vegetative propagation.
REFERENCES
Thanh, P. D. 2003. Cashew nuts - Production and Processing.
Argles, G.K. 1969; Paiz, R. Guillen. 1975.
Package of Practices for Cashew. December, 1982. Central plantation crops research Institute.
