The propagation of woody plants by hardwood stem cuttings is, by definition, limited to deciduous subjects which are designated as being in the dormant condition and therefore leafless. The technique involves the regeneration of pieces of the current season’s stem growth which have fully matured and are then also at a reduced level of physiological activity during this period.

 

Structurally the stems at the hardwood stage have completed their growth and differentiation for the year and are fully mature. All the cells have completed their differentiation and the lignification of the relevant tissues has been fully developed. At this time the cells are farthest removed from an ability to de-differentiate and to re-orient for root initiation - thus the process will be at its most complex and time consuming - and the capacity for root production in such material will inevitably be less than when in the soft or semi-ripe condition.

 

Consequently the range of subjects which will respond to this method even with support and enhancement techniques is necessarily relatively few.

 

The seasonal effect on the expression of the capacity for root initiation

 

One of the most underestimated aspects of the process is the significance, for those subjects with a marginal response, of the influence of season on the capacity for regeneration ie the development of root initials. The season available for making hardwood cuttings will begin at leaf fall and end with bud break. The assumption could well be made that the capacity for regeneration remains constant during this period - however it does not necessarily respond in this way. For subjects which already have preformed root initials present in the stem, the seasonal influence will be negligible. However for those subjects which have an inherent capacity but do not have the preformed root initials, the process of root initiation has to begin from scratch and it is for these more critical (and therefore more difficult) subjects that a varying response can be detected.

 

If comparable cuttings are made at intervals throughout the dormant season it will be found that the rooting response varies. It is highest at leaf fall, declines quite quickly to a base level, in the sigmoid curve fashion, then remains more or less constant until just before bud break when it rises again, in the reverse sigmoid curve fashion as the buds break – although that base level is not necessarily particularly low.

 

The above concept looks fairly foolproof - however it lacks one defined parameter – the definition of ‘leaf fall’ and in those subjects which are marginal in their response it could be economically significant. As far as the plant is concerned the plant has become dormant when it abscisses its leaves and it is this activity that appears to be crucial – leaf fall is not when the leaves actually detach but is when the abscission layer is complete.

 

The actual timing of the completion of the leaf abscission layer is not a constant feature to all deciduous woody plants and in any one species may vary according to the level of maturity of the individual stock plant and its manipulation, the particular growing season and the latitude. In this particular context, where the stocks have been stooled to produce vigorous stems in a vegetative condition, leaf retention is more often than not delayed beyond the normal mature condition. Stooled apple rootstock stems, for example, do not absciss their leaves until December in the south of the UK whereas stems in a ‘normal’ growing condition will absciss some six to eight weeks earlier.

 

Maximising the capacity of the material to regenerate by the manipulating the stock plant to develop maximum vegetativeness

 

The capacity for the initiation of roots in hardwood cuttings has been shown (Article 2) to be a response to the increased growth rate of the stock plant stems – described as ‘vegetativeness’. This is characterized by the upright growth of the stems and a lack of flowering potential. This has been achieved by the establishment of stock plants and then stooling or hedging them during the dormant season to induce a considerable reduction in the existing root-shoot ratio and hence cause the development of rapidly grown and vigorous shoots during the growing season to return the growth pattern to balance.

 

Methods employed to encourage successful propagation

Unless it is otherwise indicated, the separation of the cutting from the stock plant is conducted at leaf abscission. What this concept indicates is that there is a fairly attenuated season (over several weeks) for propagation by hardwood cuttings when dealing with the diversity of subjects available.

 

For conventional purposes hardwood cuttings are made according to the following template – to achieve ease of handling, the productive use of the available resource and a cutting which is of a sufficient size to survive, regenerate and establish.

 

If the subject has a reasonable capacity to root it would be assumed that the whole length of the year’s growth, which is mature and fulfills the parameters, can be used. If the subject has lower capacity it will probably be prudent to limit to the cutting which incorporates the basal swelling

 

Cuttings +/-15cm long

Bud at top diagonal cut above

Horizontal cut at the base (or below a node if hollow stemmed or soft pith present)

Thick caliper for food storage

Use sharp, scissor style secateurs (not anvil type which will crush the tissue).

Bundle in convenient sized units

Treat with a liquid PGRS

 

When the cuttings are inserted they should be to a depth that about 3-4cm are left exposed

 

There are, in general terms, four methods (relating to the degree of influence which may be available for environmental control) for subsequently dealing with hardwood cuttings in order to achieve regeneration:-

Direct sticking

Callus bin treatment

Cold frames

Heated (Garner) bin treatment

 

Direct Sticking without pre-treatment

Unless the environmental conditions, during the winter season, are particularly benign this method is limited, in cold temperate climates, to those subjects which have pre-formed root initials already present in the stem and are therefore ‘ready to go’ and will demonstrate an acceptable level of productivity. The system involves making the cuttings (which may or may not have been cold stored) and inserting directly into field conditions without further treatment.

Examples of suitable subjects include relevant Willows (Salix), Poplars (Populus) and Flowering Currants (Ribes).

 

Callusing bin prior to direct sticking

As has been shown elsewhere (Article ) that although callus formation is usually a precursor to root initiation and the same conditions are needed for its development, it is a concurrent process and not directly related. The advent of root initiation in hardwood cuttings appears, nevertheless, to benefit from the same conditions that encourage the development of callus tissue and thus treatments that encourage callusing will usually enhance the rooting potential. For those subjects which will respond and initiate roots fairly readily, within this scenario, the callusing of the base of the cutting prior to sticking will enhance the process.

 

The process may be further enhanced by the application of rooting hormones to the base of the cutting, using a suitable concentration.

 

Callusing is usually achieved by ameliorating the environment around the cutting, especially at the base where the increased temperature, provided by the protection from frost etc, will enhance activity. A sufficient effect can be realized by using a straw bale bin to achieve the desired results – the details of construction are well documented in the literature.

 

The following genera contain species which will respond to this method - be aware that the response is not all encompassing but is species (or even varietal) related:-

Forsythia, Jasminum, Spiraea, Holodiscus, Physpocarpus, Neillia, Rhodotypos, Stephanandra, Deutzia, Philadelphus, Sambucus, Tamarix, Buddleia, Cornus (coloured bark dogwood types), Viburnum (V. farreri and V x bodnantense types) Euonymus, Symphoricarpus, Ligustrum, Laburnum, Platanus and Weigela.

 

The usual practice is to line out from the bin (as for direct sticking) in the spring as soil temperatures begin to improve.

 

Cold Frames

The benign environment of a cold frame will achieve the same end product as the previous callus/direct sticking method but can be operated in the early winter and the cuttings left in situ over the winter season and for the ensuing growing season (with protection removed in the early spring.

 

Heated (Garner) bins prior to direct sticking

Fundamentally this method is a sophistication of the callus bin technique but chiefly improving the rooting response by the use of bottom heat.

 

During the post war years Robert Garner at East Malling Research Station was continually attempting to improve the rooting response of fruit tree rootstocks from hardwood cuttings – chiefly apples but also plums, cherries and quince. He recognized the quality of ‘vegetativeness’ and then by gradual sophistication he modified the various environmental factors that improved rooting in a callus bin. This culminated, in the mid-1960s, in the development of the ‘Garner’ bin – this concept harnessed, the use of bottom heat and ‘rooting hormones’ together with the manipulation of the stock material to produce better regenerative response. This work was carried on by Brian Howard and reference should be made to his papers of that time. Subsequently advances in technology have improved the equipment available, the critical aspects of the use of rooting hormones and the materials for construction of the bin. The details of construction can be readily access.

 

This technique has also been applied to various ornamental subjects and has proved successful (inter alia) for Morus and Platanus - but has not ‘caught on’.

 

 

There are many variants on the theme in modifying hardwood cutting for particular end products:-

Longer cuttings where the end product is required with a leg – currants and gooseberries

disbudding – removing all but the top two buds to avoid subsequent suckering - Rosa multiflora for rootstock use

Longer cuttings for fruit tree rootstock use to be budded at +/-20+cm

Long cuttings to create ‘standard stems’ – Rosa rugosa, Salix for top working with Kilmarnock Willow etc.