Propagation of the cashew tree by vegetative methods - Layering & Grafting

- Age of mother tree and choice of layers: Most often, layers from young mother trees will take root faster than layers from old mother trees. A study on the layering method carried out at the Ulhal Cashew Research Station, Margalore, India (Rao V.N. Madhava, Hassan M. Vazir, 1957) found that layers from 10-year-old mother trees took roots earlier than layers from 20-year-old mother trees or older. For 1-year-old trees, layers took root the earliest in just 22 days and can be detached from the mother plant within 35 days. When choosing layers, it should be given priority to branches that grew from the previous year but have not yet flowered, are 10 - 12 months old, healthy, straight and 30 - 40 cm long. The diameter at the layering site is 7.5 - 8.5cm, the branch has light brown bark and has 3 to 4 small trunks on each branch.

- Materials used as tools to stimulus root growth: 

At the layering point (below the sprout), strip a piece of bark 2.5-3.5 cm long, use a sharp knife to scrape off the cambium carefully in a way that does not damage the wood layer underneath. Next, use a 0.1mm thick plastic wrap (25 by 15 centimeters in size) that contains sphagnum mosses (grow in canals and rivers, absolutely no use that grow along the coast), damp sawdust or coco peat to wrap around the peeled part and then tie two heads of wrap tightly to keep moisture from escaping out. Use indole butyric acid (IBA) as a rooting hormone that promotes more root growth to moisten the outside of plastic wrap. After 45 - 60 days, young shoots growing at the layering site can be seen through the plastic wrap. After 80 - 90 days of layering, the layer can be separated from the mother plant.

- Separate the layer from the mother tree and transplant to the field. There are three following steps to separate the layer from the mother tree, as shown below:

Step 1: Make a V-shaped cut right underneath the layering point. The depth of the cut is one-third of the layer's diameter

Fig.1 - Layering

(a) Strip the bark

(b) Cover the stripped part with a plastic wrap containing sawdust, coco peat, water hyacinth roots, etc.

(c) Tie both heads of plastic wrap

(d) The layer begin growing young shoots

Step 2: after 7 - 10 days of step 1, make the first cut deeper (two-third of the diameter)

Step 3: After 7 days of step 2, separate the layer from the main plant, and prune some leaves on the layer to limit drainage. Carefully remove the plastic wrap without damaging the young shoots, then dip the shoots in 10 liters of solution including cow dung and 10 grams of nitrogen fertilizer for 2 hours. Next, plant the layer in a potting soil bag made of polyethylene plastic (45 by 35 centimeters in size) which has potting soil with enough nutrients as known, then place the pots in a cool place to nurture the layered plants. Remember that the young shoots are very susceptible to breakage, so cover the shoots with a layer of fine soil and use a watering can with a shower head to water the pot during the growing period. After 3 weeks of nourishment, the layer is transplanted to the field. The survival rate of trees can reach over 85%.

Grafting 

Grafting is an asexual propagation method in which the stem from one tree is inserted into a cut in the stump of another tree. If the graft is successful, these two parts will grow together and form a new tree. There are various grafting types for cashew trees as follows:

- Wedge grafting (includes epicotyl grafting and soft wood grafting)

- Veneer grafting

- Side grafting

- Approach grafting

Season:

In Indian experience, the best time of year to graft cashew trees coincides with their maximum growth phase. In any case, after grafting, if  the temperature is favorable in the next two weeks, grafting cashew trees will grow in the best way.

Root stock:

In all types of grafting, seedlings from seeds of a suitable age are often used as rootstocks. So far, there have not still been any negative effects caused by the rootstock on grafted trees nor has there any incompatibility phenomenon between scion and rootstock (Angles G.K. (1969). Anacardium occidentale. Ecology and botany in relation to propagation F.A.O. Conference on propagation on tropical and subtropical fruits . September 1969, London).

Selection and handle of scion:

Scions are selected from mother trees that have been selected to collect seeds. It would be better if seeds are selected from a garden of mother plant seeds. Selected branches are usually 6 - 8 months old (depending on requirements of each grafting style), have round fat bud tips, light brown bark, about 8 - 10 cm long. Do not choose scions that have flowered in the previous season or have excess buds. It is necessary to remove leaf blades from selected grafting branches (usually 8 - 10 leaf blades corresponding to a length of 8 - 10cm), only keep the leaf petiole 7 - 10 days before cutting the grafted branch. It is best to use all the severed scions for 1 day. The leftover grafts, especially the bud tips should be placed in a clean plastic bag (absolutely no use a plastic wrap containing fertilizer) containing peat moss or damp sawdust to prevent them from drying out. Maximum storage time 1 - 2 days.

1. Softwood grafting

Fig.2 - Softwood grafting (Milheiro Vaz, 1969)

1. Buds are used as scions
2. Cut across the rootstock
3. Place the scion on the rootstock
4. Wrap tightly scions by PVC tape 

Fig.3 a 4-month-old grafted tree grafted at 10 weeks, ready for transplanting to the field (Ascenso Milheiro, 1973)

Grafting is done on the soft wood part of the rootstock. In this grafting technique, use 30-40 days old seedlings as rootstock. When grafting, cut on the top of the root stock, retaining only 2 pairs of bottom leaves. Then, make a cleft of 3.5-4.5 cm deep in the middle of the cut end of the stem. 

The cut end of the scion is mended into a wedge shape as long as the split line by chopping off the bark and little portion of wood from two opposite sides, then place the graft tightly into the split line on the rootstock and use a 0.05mm thick polyethylene plastic tape to tie it in place. After that, cover the grafted buds with a 15 x 10 cm polythene cap to retain moisture. Be careful not to let the cap touch the top of the bud as this will cause the bud to rot. After grafting, the grafts should be provided with partial shade within 12 - 15 days later, the graft will sprout new buds. After 2-3 weeks, the polythene caps are removed gently and the grafts are shifted to open condition in the nursery.

Care for the freshly grafted seedlings:

- The shoots arising from leaf axils on the root stock (below the graft joint) should be removed frequently.

- Cashew grafts should be watered daily twice a day, morning and afternoon.

- Bordeaux mixture spray (1 %) may be given as a prophylactic control measure to reduce fungal infection of young grafted plants.

- After 45 days of grafting, cut off the remaining 2 pairs of leaves on the root stock. If the grafted plant has good growth prospects, rotate the grafted tree's pot every 45 days interval until the tree is transplanted to the field.

- After 80 - 90 days of grafting, remove the polyethylene plastic tape and transplant to the field.

2. Epicotyl grafting

In this grafting technique, use 10 days old seedlings from the time the seeds germinate as rootstock. Firstly, cut on the top of the root stock, retaining only 2 pairs of bottom leaves. Then, make a cleft of 3.5-4.5 cm deep in the middle of the cut end of the stem. The cut end of the scion is mended into a wedge shape as long as the split line by chopping off the bark and little portion of wood from two opposite sides, then place the graft tightly into the split line on the rootstock and use a 0.05mm thick polyethylene plastic tape to tie it in place. After that, cover the grafted buds with a 15 x 10 cm polythene cap to retain moisture.  This technique is quite similar to the softwood grafting technique. It should be noted that because the rootstock is still young and weak while the graft is heavy, therefore, as grafting is done, the grafted tree should be tied to a bamboo stick inserted next to the root to initially support the tree.

In both grafting methods on softwood and on seedlings, it is best to ensure that both the rootstock and scion should be of equal size in diameter. In cases of inequality (usually due to the diameter of the scion being larger than the diameter of the rootstock at the grafting site), when grafting, the vascular cambium of one side of the scion or bud must be aligned with the vascular cambium of its one other side. In Indian experience, the success rate of wedge grafting is highest when it is done in July and August.

3. Veneer grafting

The rootstock used in the veneer grafting method is a 6 months old seedling with a diameter of about 10 - 15 cm. Then place the graft tightly into the split line on the rootstock and use a 0.05mm thick polyethylene plastic tape to tie it in place (when tying, it must be tied from bottom to top and then tied up from top to bottom). After that, like the above grafting methods, cover the grafted buds with a 15 x 10 cm polythene cap. After 3 - 4 weeks, when the graft union is completely healed, use scissors to cut the rootstock at the top of the graft union (making oblique cuts to avoid water stagnation and rotting of the rootstock). When cutting, remember to divide into 3 short intervals to help the grafted plant achieve a higher success rate.

Veneer grafting can also be done on seedlings in the field, in which case the age of the seedlings can be up to 15 - 20 months old.

In Shantigodu cashew orchard, S.K. Karnataka India, success rate of veneer grafting in the three years between 1985 and 1988 was 32.9% in April and 32.7% in July. According to the report of Nagabhushanam and Venkata Rao (1977), the success rate of 15 - 20 month old seedlings grafted in the field at the Bapatla Cashew Research Station in India was 49% in September and 36% in August.

Fig.4 - Veneer grafting

a: Bud before leaf removal

b: Bud after leaves removed

c: Selected and handled rootstock and grafts 

d: Position of the graft on the rootstock

e: Tie the graft union with a thin polyethylene wrap

f: Grafting was done

4. Side grafting

The rootstocks used in this method are 1 or 3-year-old seedlings with poor growth.  In the stock at a height of around 15 cm, a rectangular downward cut (4 cm by 1.25 cm in size) is done forming a thin “tongue”. About 8 cm from the bottom of the selected scion , a second cut is also done forming a thin “tongue” as on the rootstock. The scion is inserted into a downward cut in the stock. Then, use a banding rubber to tie it tightly. When the side grafting is completed, cut off a part of the upper foliage to stimulate the growth of the scion. Most often, after 3 months, when the scion has begun to sprout and produce young leaves, completely cut off the rootstock right above the graft union and remove the banding rubber. The best season for this type of grafting is from June to August with a success rate of up to 70%.

5. Approach grafting

This grafting method can be done on cashew trees, mango trees as well as other tropical fruit trees. The seedling used as rootstock is 11 months old, about 60 cm tall. The circumference of the graft union is 4 - 5 cm (diameter 1.2 - 1.5cm) similar to the scion’s. On the same side of both the rootstock and the scion, two cuts that are shaped as thin tongues and have the same size are done. Then the graft union is tied tightly with a banding rubber and the rootstock’s soil bag is also fixed with a support stake. After about 90 days, when the graft union is healed, cut off the top of the rootstock and separate the scion from the mother tree underneath the graft union (in the same way as to separate the layering branch from the mother tree).

In India, the success rate of this method reaches 75 - 100%.

In addition to the grafting methods mentioned above, there are two advanced other methods as follows:

- Y-cutting grafting

This grafting method experimented in India is a combination of approach grafting and layering. (Rao Madhava V.N., Rao Sam bashiva I.K., Hassan Vazir (1957 - 1958) Studies on the vegetative propagation of cashew (Anacardium occidentale L.))

The "Y cutting" grafting method is used to produce at the same time 1 grafted tree and 1 layered tree on 1 bud. The best time to apply this method is between February and May. Done in February, 10 grafted trees and 10 layered trees can be obtained from 10 shoots.

Fig.5 - "Y-cutting" grafting

Source: Rao Madhava and Rao Sambashiva (1958)

- Cut the top of the cashew plant

This is a technical measure that affects old cashew trees from 8 to 15 years old with poor yield and quality of nut products to transform them into young cashew gardens with high productivity and quality of nut products and to improve economic efficiency of existing cashew orchards.

After harvest time and about 2 - 3 months before the rainy season, cut the top of the old cashew plants at a height of 0.5 - 0.75 m from the trunk. After cutting off the tops, many shoots will grow in the leftover root, selecting 8 to 10 of the best elite shoots that evenly growed around the root to use as rootstock when grafting on soft wood. After about 60 to 70 days, select only about 5 best trees. Bordeaux mixture spray and tar may be given on the cross-section to reduce fungal infection and avoid rotting the trunk. After cutting off young shoots, apply 50% BHC solution immediately to prevent stem borers. The success rate can reach from 50 to 80% depending on the season.

Table - Comparison of tree survival rate when using 3 grafting methods within 3 years (1985 - 1988) at cashew seed garden Shantigodu, S.K. Karnataka India. (Average survival rate of grafted plants%)

Total number of grafted plants within 3 years: 132,923.

Total number of successful grafted plants: 78,121.

Table - Effect of grafting season on success rate of side grafting 1977 - 1978 and 1978 - 1979 cashew cultivation Shantigodu, S.K. Kartanaka, India (Room Singh)

REFERENCE:

Thanh P. D. Cashew nuts - Production and Processing. (2003)