This subject is dealt with exhaustively in Peter MacDonald’s definitive book The Manual of Plant Grafting (Timber Press 2014)

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These notes were developed as a response to explaining why a particular propagation technique could aid a student in gaining both useful knowledge and confidence in their ability in knifework and at the same time allow the instructor to impart the widest range of relevant and related information.

 

Woody Plant Propagation - Bench Grafting of Aesculus

 

During the 1970s the teaching of the practical aspects of Nursery Practices and the related plant propagation practice was still in its infancy and there was much knowledge to be collected, assembled and refined and then incorporated into the teaching programme. This was especially true when it came to the determination of which practical skills were to be taught, how the practice was to be demonstrated and what additional information could be disseminated in the process.

 

The ‘Bench Grafting of Aesculus’ as a teaching aid arose out of this exercise as it could be used to fulfil so many of the criteria expected.

 

One of the chief stumbling blocks faced by a student in propagating plants is the use of a knife – they are often chary of the implications and lack confidence in their ability not to damage themselves or to make the carpentry cuts correctly. However if they have been taught, initially, the ergonomics of how to hold and use a knife - then the crucial first step is to conduct operations on easily cut material (avoiding hard or otherwise difficult subjects).

 

The initial premise in seeking suitable subjects and techniques to develop was centred around refining the ease of the operation and that the materials could be readily used and manipulated in order to simplify the whole process. Thus among several techniques which were refined for this purpose, the bench grafting of Aesculus was one example but it can be used to demonstrate the simplicity of the approach and the wide spectrum of knowledge which can be associated during the process.

 

Prior to this stage, initial instruction would have included the demonstration of the types of knives (and the characteristics of their blades), the types of cutting actions involved in using secateurs and the acquiring of the skills involved in the sharpening of knives and secateurs as well as the inculcation of the importance of maintaining such kit in clean and usable condition. It is also at this stage that the basic ergonomic movements in cutting plant material to achieve flat matching surfaces would have been demonstrated.

 

The refinement of the process included:-

a) the choice of carpentry joint to be used

A based whip graft was chosen because it required fairly simple cuts and that the joint provided a stable and rigid system for the tying in process – something that the novice finds somewhat difficult to achieve when using the conventional whip graft.

 

b) the subject to be employed

Aesculus was chosen because the texture of the wood in both one year old scions and the hypocotyl of the seedling is relatively soft and hence easily cut – it is not dissimilar to cutting into a bar of hard soap. Thus the student could quickly achieve proficiency and gain confidence in their own abilities with a knife.

 

c) rootstock selection

Aesculus could also be used to initiate a discussion on the merits and demerits of rootstock suitability and scion compatibility - as at that time A. hippocastanum was used almost exclusively as the rootstock.

 

d) production of seedling rootstocks

A paper is appended (of later date) which deals with this subject in detail.

 

e) production and selection of scions

The establishment and treatment of stock plants for the development of suitable scion material in the quantities required.

 

f) the significance of hygiene in preventing fungal infection or red spider infestation during the process of callusing and union formation.

 

As the vegetative propagation of clonal selections of Aesculus species by stem cuttings is difficult, the use of bench grafting or field budding and grafting has been the norm.

 

The Process

 

When bench grafting Aesculus the stocks used are dormant, one year old, bare rooted seedlings. The operation is carried out in February–March in order to allow a union to form and so allow a full growing season for the new plant to establish.

 

The grafting is carried out on the hypocotyl of the seedling as this ensures that the rootstock does not develop suckers.

 

Prior to the operation the rootstocks are washed and it is ensured that the hypocotyls are clean. Whether the taproot is pruned and to what length will be a function of how the next stage in the production process is designated.

 

The scions are selected from young trees that have been pruned to develop the desired type of growth. The shoots are selected from the lateral shoots, ie not those which would be regarded as leaders. This is determined by the fact that these shoots will be of a similar caliper to the rootstocks. Most subjects will thus produce suitable shoots between 18 and 25cm long when cut just at the junction with the previous season’s growth, the terminal bud is left intact as it is unlikely to be a flower bud.

 

The caliper of the stock and the scion to be grafted should be selected to match as exactly as possible, so that when the joint is matched that the cambiums of both are immediately adjacent.

 

These notes accompany the line drawing below.

 

The scion is made, leaving the terminal bud intact so that a straight through leader is immediately present for the young plant. The main basal cut is made, obliquely, 2-3cm long, through the lowest pair of buds so that one bud remains on the side of the scion. Thus the scion will be about 15cm long.

 

The stock is then cut horizontally through the top of the hypocotyl, using sharp secateurs, so that the stem system is removed. A matching oblique cut is made into the stock but finishing after some 2.5cm and a short sloping cut is then made to meet the base of the first cut from the opposite side – thus developing the ‘base’ of the whip and so preparing the stock to receive the scion.

 

The scion can then be finished by making the basal cut from the opposite direction to match the slope of the short basal cut on the stock.

 

This then allows the two pieces to be put together snugly and with a sufficient rigidity and firmness that tying in can be achieved easily and without movement. That the correct length cuts have been made can be seen if there is a church window cut showing above the stock – this ensures that the cambiums are matching from as high as possible on both stock and scion.

 

The graft is secured using a thin elasticated rubber tie in a tight spiral under tension so that the union is fixed. The complete tied area is then sealed with a layer of paraffin wax in order prevent water loss.

 

The graft is then placed in a suitable environment for the union to develop – this usually implies that the temperature in the region of the union is at 20̊C.

 

Callusing of the cut surfaces will be apparent through the tied region and the ties can be cut when this stage is reached.