‘Some notes on an under-exploited method of Propagation’

 

Among the several intensive systems of vegetative plant propagation, the production of plants from root cuttings offers one of the simplest and least complex techniques available, as it makes effective use of limited quantities of suitable propagating material and does not require any of the sophisticated systems of environmental control favoured for the regeneration of leafy cuttings.

 

The object of the process is to initiate adventitious bud development in a separated piece of root and then to establish the resultant new plant as a liner.

 

The propagation of plants from root material is basically a simple, reasonably rapid and economic process but despite these not inconsiderable virtues it is a technique which has not generally been widely accepted or extensively practiced within the nursery industry - except for a few particular subjects such as Seakale, Horseradish, some Poppies, the various Sumachs, border Phlox and some Alpines.

 

The regeneration of plants from root material is by no means a new or innovative concept – for as long ago as 1662, John Evelyn was writing in his famous Sylva of the possibility of cutting a root from a tree and leaving it in situ to develop. By 1731, Philip Miller, in The Gardener’s Dictionary, was describing the propagation of certain trees from root cuttings as an established practice. Since that date various reviews of the literature related to this technique (eg Stoutmeyer 1968) reveal that hundreds of tree and shrub species have been demonstrated to respond to this technique. However it has nevertheless never become an established practice except as above.

 

Such surveys indicate the undoubted potential of the technique but it is relevant to enquire why the technique has not taken its place in propagating schedules when other more complex systems (eg mist propagation) have been widely accepted. The answer almost certainly lies in the perceived unreliability of the process – for despite propagating with apparently ideal material and providing the required environmental conditions, regeneration often fails.

 

The problem has been that in the past, quite simply, not all the components of the technique have not been fully understood – the process has never been intensively analysed - so that particular limitations have not been recognized. Because of this uncertainty of the outcome the whole concept was often ignored or discarded and relegated to the position of being ‘difficult’.

 

The technique however can be a successful enterprise and large quantities of plants can be produced from relatively small quantities of suitable propagating material. It is therefore incumbent on the propagator to analyse the process, highlight the relevant features and apply suitable process in order to eliminate uncertainty and achieve success.

 

The primary consideration in determining success in regenerating plants from pieces of root is that the material has an inherent capability to actually regenerate and that this is sufficiently dominant to allow an economic production – a feature that is accountable in any propagation system. An exemplar list will be found in the following article.

 

The chief problem associated with this technique has been the unreliability of success when otherwise apparently very suitable propagation material has been used. However it is now sixty years since this problem was elucidated. Hudson (1954, 1955) in a simple and elegant piece of work demonstrated for Raspberries that there was a seasonality of response and that this plant only responded in the winter period. It is therefore a mystery as to why this technique of propagation has been so shunned.

 

The factors which influence the successful propagation of plants from root cuttings do not differ significantly from other techniques of regeneration from other plant pieces. Thus as long as the process is properly analysed and the factors are sequenced then a successful outcome should be achieved.

 

A few plants (eg Rubus, Rhus) are readily regenerated from root cuttings - largely because they develop pre-formed adventitious buds as part of the normal growth process and can usually be recognized as they develop suckers readily.

 

Plants produced from root cuttings will by definition have developed their stem system from adventitious buds and this will, initially at least morphologically and physiologically, be in the ‘juvenile’ condition – however there is no indication that this effect is more than relatively transient and the plant soon reverts to the mature condition.

 

The adventitious buds are initiated in the pericycle close to the vascular tissue – but sites may vary. Normally parenchyma cells de-differentiate and then become thin walled and develop large noticeable nuclei and dense cytoplasm.

 

As with any system of vegetative plant propagation, the production cycle can be defined in five phases:-

 

1. Preliminary phase

 

The various factors which govern the selection of the propagation stock are the same as for stem cuttings in terms of identity, clonal variation, age of variety, health etc. (Reference should be made to Article 1)

                                                

2. Preparatory phase

 

There will, probably, be little opportunity to remove suitable material, for root cutting propagation, from a container grown crop or indeed a field grown crop - as would be possible for stem cuttings. It will therefore be necessary to establish stock beds for the purpose.

 

The primary condition affecting successful regeneration is the age of the parent plant and this is a function of two aspects - the physiological age of the parent material and the actual age of the individual donor plant being used. Although an often ignored concept this is always a relevant consideration in determining the economic level of regenerative capacity required. In order to maintain vigorous growth of the stock plants it is prudent to have a regular replacement programme.

 

a) Stock Plant Manipulation

The preparation or manipulation of the stock plant to produce propagating material is effectively no different, in principle, to the processes required for the production of stem cuttings with improved capacity. As with stem cutting propagation – the emphasis is on engendering the highest regenerative capacity feasible in the roots which are to be used for propagation and in this respect the principles involved are very similar. Thus the stock plant is manipulated to develop, annually, a vigorous, rapidly grown root system in a ‘vegetative’ condition - a process which is analogous to that adopted for the production of stem material with a high capacity. However the process is inevitably made more complicated by the fact that the propagation material is developing underground and is not readily monitored.

 

The chief practical feature to be addressed is to determine the proportion of the root system which can be removed from the stock plant so that a sufficient amount remains for the successful re-establishment of the parent, but at the same time producing propagating material of sufficiently high capacity (produced as a result of rapid, vigorous growth) and inducing, by sufficiently rigorous pruning, the production of an adequate harvest during the following season. This factor is virtually ignored in the texts and in practice has to be determined by trial and error. For any commercial system it is important that a sustainable, regular and economic yield is achieved so that the quantity of propagation material meets a scheduled demand on an annual basis.

 

The production of an adequate supply of root cutting material can readily be achieved in most herbaceous plants if the roots of the stock plant are severely pruned – often virtually back to the crown – they will withstand this severity. However the response in woody plants is less tolerant and will normally require a less traumatic treatment. They will also require considerable attention to the development of suitable soil conditions for an adequate response.

 

b) Practicalities

The area where the stock plants are to be established and grown must be based on a properly and effectively ameliorated soil - which allows an unimpeded development of the root system. This conditioning will require the development of soil conditions which also allow:-

a) ease of lifting so that the entire root system can be extracted without causing damage to the individual roots and

b) a texture which is easily washed from the roots to provide a clean sample.

 

When the root system is lifted it is important that any separated roots are collected in such a way that the correct polarity can be maintained.

 

Attention will also need to be paid to the provision of adequate nutrition and irrigation in order to ensure that there is no limitation to root growth.

 

The logistics of the lay out of the stock beds also requires some attention in order to:-

a) avoid any overlap of root development from other adjacent stock plants, to prevent misidentification of relevant material,

b) determine whether there will be a replant issue (growth inhibition) if the stock plants are returned back on the same site, this is a potential hazard for several members of the Rosaceae and

c) ensure the land is clean and not contaminated with any remaining pieces of root from a previous crop which, potentially, may develop and cause confusion.

 

3. Regeneration Phase

 

This phase incorporates those factors which will influence the performance of the propagating material once it has been separated from the parent stock.

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a) Seasonal influence

Although it can be shown that a particular subject has the capacity to regenerate from roots there is, as with many systems of propagation, often a distinct variation in response according to the season. It is the seasonal response of woody plants that has caused the issues with the technique in the past as it was not widely recognized that the ‘on-season’ was limited to the dormant period - as the conventional propagation of many herbaceous plants from root cuttings do not exhibit any variation with season.

 

Virtually all temperate woody plants that will regenerate from roots exhibit a distinct seasonal effect insofar as the ‘on season’ is associated with the dormant condition. As the plant becomes dormant there is a rapid increase in the capacity to regenerate which develops on the expected sigmoid curve pattern – when the maximum has been reached the level remains constant until just prior to bud break when the capacity declines rapidly in a reverse sigmoid curve pattern.

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b) Treatment

 1. Polarity

As with stem cuttings the best rooting response is achieved if the cutting is propagated in a vertical position and ‘the right way up’. It is often the practice that those subjects with a fibrous root system and which develop thinnish roots are chopped into suitable lengths and set horizontally. This practice will normally produce regeneration but often with many pieces not responding (a suitable method if the quantity of material is not limiting).

 

However if the roots are selected for reasonable caliper and are inserted vertically the response is virtually 100% and the time taken to regenerate is significantly quicker.

 

The conventional practice for making root cuttings, and to ensure that the ‘right way up’ can be recognised, is to make the cuttings with a diagonal cut at the base and a horizontal cut at the top.

 

2. Plant Growth Regulating Substances

The conventional chemical aids to rooting in stem cuttings – NAA, IAA, IBA, Giberellins etc - do not aid adventitious bud formation in roots and in fact are generally regarded as counterproductive. Although there is evidence to suggest that cytokinins (kinetin is often quoted) will enhance bud initiation in roots there is no readily available resource from which to obtain these materials and there is little or no information to determine which chemicals are most effective and certainly not in assessing the concentration to use.

 

3. Fungicide

As the cuttings are to be effectively buried in soil or compost, and as such are not readily monitored, it would probably be prudent to dress the cuttings with a fungicide for protection.

 

c) Environment

Depending on the ‘hardiness’ of the subject the regenerative environment can be as stark as the open ground or the limited protection of a cold frame or, for less successful material, the benign protection of a glasshouse. The potential for success will generally increase with increased protection and higher temperatures. This is usually represented by a more speedy regenerative response.

 

Until such time as regeneration begins there is virtually no need to provide moisture as the root cutting virtually has no capacity to take up water – it is merely necessary to maintain an adequate surrounding humidity to prevent desiccation of the cutting.

 

d) Practicalities

The portion of the root exhibiting the highest capacity (ie the fastest growth rate) will be the proximal end of the root where it emanates from the existing root system and grew fastest – for ‘difficult’ subjects this may be the only cutting that will respond.

 

1. Size of cutting

Factors governing the size of the cutting will be of significance as this will determine the quantity of cuttings which can be obtained from the resource available.

 

The size of the cutting will initially be related to the caliper of roots that the particular subject develops. However in theoretical terms the root will need to contain

a) a sufficient food reserve to initiate and develop an adventitious bud and

b) enough food reserve to sustain the root through this process until it produces a leaf area sufficient to become autotrophic.

 

The length of the cutting and hence the actual amount of food reserve required will be determined largely by the environment (chiefly the temperature) to which it is subjected as this will in turn determine the length of time the cutting has to survive before it becomes self-supporting.

 

As there is a wide variation in the characteristics of the root systems and the available environments to be used it will require an individual assessment of each subject to determine the most economic size of cutting.

 

2. Insertion

The cutting is inserted vertically into the compost or soil so that the top of the cutting is at the surface. The medium is then top dressed with perlite/ vermiculite/ grit to ensure good drainage and maintain maximum aeration at the site of regeneration

 

The root system of the new plant may develop from the root cutting so that the cutting becomes an integral part of the new plant or it may develop from the base of the developing adventitious shoot

                                                

4. Establishment Phase

 

Once regeneration has occurred (ie the cutting has produced an adventitious bud) the cutting can be moved on to a relevant liner container although it would probably be prudent to wait until the new root system has begun to  develop – the operation will largely depend on the current workload and programme schedule.

 

Development will then depend on the provision of a conventional growing on programme with attention to the balance of the environment, nutrition, water etc.

 

 

References

Hudson, J.P. (1954) Propagation of plants by root cuttings. I. Regeneration of Raspberry root cuttings. Journal of Horticultural Science, 29:27-43.

Hudson, J.P. (1955) Propagation of plants from root cuttings. II. Seasonal fluctuations of capacity to regenerate from roots. Journal of Horticultural Science, 30:242-251.

Stoutmeyer, V.T. (1968) Root Cuttings. Plant Propagator, 14(4):4-6.