Reprint From CMS Yearbook 1973

The conventional methods of grafting have all been examined in New South Wales, and it was found that the most suitable periods of the year for shade-house or field grafting of Macadamia were late Autumn (April-May) and early spring (August-September). The cold mid-winter and hot summer periods are to he avoided.

Bark grafts have given better results than side grafts. Health, vigor and stage of growth of both the stock and scion are important in obtaining successful grafts. The most suitable stocks should be vigorous, in an early flush stage of growth, and from 13mm to 17mm in diameter. Mature, vigorous scion wood cinctures [girdled] on the parent tree four to six weeks before use, gives best results. Scions with diameters about half as large as the stocks to be grafted were used.

In the case of Macadamia tetraphylla, scion wood that has defoliated naturally is preferred. This factor does not seem as important with Macadamia integrifolia, as it grafted satisfactorily when leaves were trimmed to a short petiole at the time of cutting the scion wood.

Scions having two bud whorls are chosen and to ensure that large areas of cambium are in contact, scion wood with long internodes is selected. This enabled the sloping cut made in preparing it for insertion into the stock to be at least 58 mm long where bark grafts were used.

Tying grafts with plastic tape and the use of suitable mastic for holding the graft in firm contact and completing the seal are essential. Macadamia forms callous tissue very slowly under natural conditions in New South Wales. As a result the scion may dry before union between stock and scion occurs. It is found that covering the graft and scion with a sleeve of plastic film reduces drying out and increased the chance of success.

The root system of the Macadamia consists of hunches of lateral rootlets forming first in mats just below the soil surface. Consequently, plants are sensitive to soil moisture fluctuations, particularly in the seedling stage. An adequate water supply is essential. Heavy losses may occur, in the nursery or shadehouse with potted or open grown stocks if the soil becomes too dry.

Macadamia wood is quite tough when it is more than a year old and it is difficult to make a cut on an even plane when preparing stock and scion for grafting. This is believed to have been a reason for the poor results in the early work with whip grafting.

Subsequently, field experience has shown that Macadamia can be whip-grafted in commercial quantities. Both stock and scion have to be well matched, and the cut surfaces prepared with a small wood plane type tool. This results in a very even surface and the necessary close contact between the cambium layers of stock and scion.

It was reported overseas that inserting a scion piece into the germinated seed after severing the developing seedling had successfully propagated Chinese chestnuts. This led to the trials with Macadamia in New South Wales,

The early work at Horticultural Research Station, Narara, was most encouraging and 1967 had developed a technique for use with Macadamia seed.

It was demonstrated that seed-grafts would develop successfully given suitable temperature and soil moisture regimes. Root development and vigorous growth continued when the young grafts were potted.

Examination of many successful grafts led to the conclusion that the roots develop only from, or through, the callus formed at the cut, surface of the cotyledons.

Careful separation of the roots from young trees several months after grafting gave no indication that any of these roots had developed from the scion.

When planted into their field positions the plants produced a vigorous root system. This strengthened, and commenced to penetrate more deeply into the soil and to form a stronger anchorage, in the second year after planting out.

Preparation of seed stocks and scion pieces for this method of propagation requires some care. Proper facilities are needed, and should be available where stocks and scions can be prepared and kept in good condition. A potting shed would be convenient, and could be used under most weather conditions.

Seed nuts should be selected from the mature current season’s crop, and kept in a dry cool place pending sowing.

In general, seed less than 2.5 cm in diameter is too small. The opening in the shell through, which the seedling emerges after germination must be large enough to accept the tapered scion piece without damage to its bark and cambium or causing the seed to split into two. Seed is soaked in water for 24 hours, and then immersed or dusted in a fungicide such as one of the mercuric compounds used for seed dressing. It is best planted in shallow trays or boxes, using a mixture of equal parts of sharp sand and peat moss.

Germination should commence within 4 weeks of planting under glasshouse conditions but will be much slower in the open if air temperatures fall below about 24⁰C.

Where glasshouse conditions are not available for germinating seeds in the colder months, the sunny sides of buildings and fences might be used, combined with clear plastic or glass covering of the germinating media, to provide added heat. Winter germination of seeds is needed as some of the best grafting results have been obtained in winter and early spring.

Seedbeds should not be allowed to dry out, otherwise germination will be affected. On the other hand, too much moisture tends to promote fungal activity, and losses may occur through infections either penetrating the seed as it becomes exposed during germination, or attacking the developing shoot.

An advantage accruing from seed grafting is that use can be made of scion-wood of smaller diameters. This method, when used in conjunction with conventional grafting, results in very little waste of scion material.

Cutting of scions from parent trees on which branches have been cinctured at least 4 weeks before removal and preferably have not produced flowers is essential for good results. The appearance of freshly cut wood will usually reflect the general health of the tree. Any that is not light ivory-white in color should be discarded before time is wasted in further preparation such as trimming of side shoots and removal of leaves. Removal of leaves from the scion after cutting is optional, and it is then wrapped in moist paper and plastic film. This reduces loss of moisture, and the pieces remain fresh if stored in a cool place. For convenience the scions may be cut in pieces of grafting length on which two whorls of buds remain.

It will be found that scions in the diameter range of about 13mm to 6mm are most commonly suitable for use. Larger diameters require additional paring to permit insertion into the seed. However, smaller diameters may be used successfully if moisture and humidity can be kept at an adequate level.

The best period for grafting is of short duration as the germinated kernel soon begins to harden and become brittle. Hence it is necessary to perform the graft when the radicle, which emerges first, is about 50 mm to 75 mm long and the aperture in the shell is wide enough to accept the scion piece.

The developing seedling is severed at the point of emergence from the split shell, leaving its base and the cotyledons exposed.

A scion is prepared by shaping the budstick to a thin wedge, leaving two complete whorls of buds above the point of entry. The cut surfaces of the wedge are allowed to protrude slightly above the surfaces of the cotyledons to promote callusing in this region.

The scion is inserted by forcing into the cotyledons as far as possible without splitting the nut into two pieces. The insertion must be made at a 90 angle across the divided cotyledon faces, not into the division, and should also be perpendicular to the cut surfaces.

Investigations have indicated that better results may be expected when grafting is done in the period May to November in New south Wales even under glasshouse conditions. Temperatures over about 26 C within the striking media, such as occur during summer in the glasshouse, appear to be damaging and may also cause a problem of excessive drying out.

Sealing the cut surface at the top of the scion with suitable mastic as soon as grafting is finished will reduce loss of moisture.

The seed with inserted scion is then buried in a suitable media such as peat moss, sharp sand or a mixture of both, to a depth sufficient to cover the seed and lower portion of the scion. Overwatering of newly made grafts is to be avoided as this may lead to losses from development of moulds and rots.

Under glasshouse conditions the first sign of growth may be expected in 3 to 4 weeks from grafting, but the scions may continue to "break" for some months.

It is possible to commence moving the young plants to pots after two-leaf whorls have been produced and hardened. Pots should be large enough to avoid the need for repotting before planting out. Heavy losses may occur if plants are moved when in flush growth, or earlier than the two-leaf stage, unless moisture and humidity remain adequate.

Care is needed during transplanting to avoid breaking or injuring roots, which are soft at this stage, and to prevent drying out. Macadamia trees react very quickly to moisture stress. And the root system of seed-grafted plants is developed very close to the surface in the early stages; these are more than usually vulnerable.

Investigations at Alstonville indicated that chip budding is a useful method of propagating Macadamia in the field. It lends itself to use when reworking unwanted varieties or for working stocks in the field. Because it is possible to obtain early flowering on chip-budded stocks, the method would be useful in providing an accelerated test for variety identification.

Buds taken from cinctured scion wood are essential, and that as the starch content in the scion increases so does the degree of success in the operation.

A simple iodine test is adequate on the cross-section of a budstick. The intensity of purple of the color reaction indicating starches concentration.

Stocks and scion material are prepared as for conventional grafting, and buds may be taken from large wood, which would be too bulky for use in grafting. Plump buds are preferred and are best taken from recently matured growth.

Spring budding is preferred to autumn. Buds placed in stocks in August to October under field conditions in northern New South Wales come into growth in 3 to 4 weeks. Autumn budding often results in delayed growth from the bud for a period extending into the following spring.

Chip budding is done in the conventional manner, ensuring that the bud-chip matches the portion removed from the stock. If the contact between the cambium layers is not good at all points a week bud union will result in failures or losses.

Bud placement is preferred on the shaded side of the stock as this protects the bud from strong sunshine and scalds and prevents desiccation.

After budding, branches (or stocks) are cut hack to within 6 inches of the insertion or at a point, which leaves two whorls of leaves above it. New growth from the bud may he tied to this stub of branch for stability in the early stages of development .

Cincturing (girdling) of scion-wood is essential and should be done at least 4 weeks before use.

Stocks should be in the early flush stage of growth.

Small seed is unsuitable both for stock production and seed grafting.

Scion wood should be used when fresh and when high in starch (as determined by the potassium iodine test, which shows an intense color).

Seed treatment with a fungicide reduces fungal development.

The type of media used is important when germinating seed for stocks and seed grafts.

Overheating under glasshouse conditions adversely affects germination and seed-graft development.

Better results are obtained when seed grafting is done during winter and spring in the glasshouse and in the spring for chip budding. Conventional grafting may be done either in autumn (March/May) or spring (July/September) in Australia.

Three methods of propagation are available, which lend them to use in three different situations, namely, on seed, on conventional rootstocks and on older trees.