Propagating – Grafting Under Glass

The practice of grafting under glass

The process of grafting involves the bringing together of the cambium layers of selected stocks and scions, there being several methods of achieving this, only a limited number of which have application under glass. Grafting is a more likely operation under glass commercially than in amateur spheres, although there is nothing to stop the gardener with specialized interests carrying it out successfully. If there is one vital clue to successful grafting in most of its forms, including budding, it is to appreciate the importance of a union of cambium layer, without which failure is inevitable.

Grafting under glass allows work to be carried on over a longer period, and as humid conditions can invariably be created, the normal protection and sealing of the graft area is not so essential.

 

Saddle grafting of rhododendrons

saddle grafting of rhododendrons

credit: apps.rhs.org

Perhaps the most usual operation is the saddle grafting of rhododendrons, when selected variety scions are grafted on to R. ponticum stocks. Stock and scion should be approximately of the same diameter, the stock being previously potted up 23-30cm (9-12in high) in a 13-15cm (5-6in) pot and brought into the greenhouse from outside in January, February or March, and after a few days cut back to 3.8 – 5cm (1-½ – 2in). By making a cut with a sharp knife an inverted V is formed, compensating cuts being made in a 8-10cm (3-4in) four-leaf terminal bud-bearing section of the selected scion. After fitting the scion carefully in the stock, ensuring that at least one and preferably both sides of the respective cambiums correspond, the union is bound up tightly with raffia.

They are then placed in a warm propagating case until a union is effected, when fresh vegetative development appears on the scion. Magnolias and fruit trees can be raised by this method also. Many rhododendrons previously saddle grafted are now rooted from cuttings.

 

Splice grafting is also practised under glass, especially with Cytisus battandieri, which is a weak grower on its own roots and is frequently grafted on to a stock of common laburnum, potted up in the same way as rhododendrons and brought under glass. A splice graft is the same as a whip and tongue graft without the tongue, and simply involves two slanting cuts, it once again being imperative that one side of both stock and scion corresponds, before binding up with raffia. Roses and clematis (on C. vitalba stock) are also propagated in this way, although clematis are now frequently grown on their own roots. The method is to take soft shoots from a greenhouse-grown plant of the desired variety and make two scions from one but cutting the stem down the centre. Roots of C. vitalba raised from seed sown in the previous March are cut back to 5cm (2in) above ground-level and a compensating 2.5cm (1in) cut is made in the stock, with a lip, allowing the half stem of the variety to be fitted in and bound with raffia before placing in a closed propagating frame.

Double forms of Gypsophila paniculata are grafted on to seed-raised stocks of the single form, but in this case wedge-shaped stocks of the double form are inserted in split root stocks before being bound up with raffia. This is usually carried out during February or March. Here again this practice has tended to fall into disuse as double forms are now raised successfully on their own roots or with tissue culture. Cacti also are grafted in this way.

Inarching or approach grafting is sometimes carried out under glass, when the stock and scions of roughly similar size and growth, and in pots, are bared of a 5cm (2in) slip of bark and brought together by tying with raffia.

Conventional budding can be carried out under glass with roses and other species, although there is no great advantage in this other than perhaps speed for the specialist gardener or plant breeder.

 

Sources of propagating material

These are many and varied. With increased specialization taking place in most spheres of greenhouse culture, there are many central sources of cuttings produced by specialist raisers under strict supervision (virus diseases and fungal disorders are readily distributed by vegetative propagation). Due to poor winter light in Britain, provision is also made for cutting material from the Mediterranean zone, particularly with chrysanthemums and carnations.

The constant stream of new varieties to replace worn out or deteriorating stocks of plants, more especially chrysanthemums and dahlias, necessitates constant reappraisal of the situation and the purchase of reliable new stock at fairly regular intervals, a lot depending on the intensity of culture. With specialized pot plants such as pelargoniums the same pattern prevails, and specialist producers of young plants retain stock plants under strict supervision; the same is true of specialist fruit tree or soft fruit producers.

Many species of plants which were formerly grown from cuttings are now largely grown from seed. This is because of intensive breeding work (eg. pelargoniums). The same is true of tissue culture.

 

19. April 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Propagating | Tags: , , , | Comments Off on Propagating – Grafting Under Glass

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