The successful production of a crop of liners, derived from seed, continues with moving the seedling from the germination medium into the liner unit (pot, plug etc) with the minimum amount of damage to the seedling and especially its root system. A check to growth at this stage may set the seedling back quite dramatically.

 

The objective of the liner production phase is to achieve as uniform a crop of young plants as is reasonable and one which has produced the maximum amount of growth compatible with the maintenance of the typical characteristics of the particular species. To achieve a uniform crop, in a commercial situation, it may be necessary to be prepared to cull extremes unless they have particular value.

 

In a wholly controlled seedling production process the constitution and components of the germination medium have a relevance in the development of a rapid and efficient system of liner production. If an effective protocol has been derived for the pre-germination stage and the seed is sown with germination following on quickly - then it is probable that the seedlings can also be moved on into a liner unit in a relatively short time. The objective is to achieve a minimal disturbance of the root system of the seedling in the transfer and so create an efficient process and rapid establishment of the liner. Thus the sooner the seedling is moved on – ie before the root system has had a chance to develop much more than radicle elongation – the more is the likelihood of an uninterrupted development. The constitution of the germinating medium will reflect this and has been discussed previously.

 

In whatever liner production system is adopted, there are a number of factors which will not only determine the amount of growth produced by the plant, in this phase, but will also affect the quality and the ability to grow it on successfully in the production phase:-

a) the liner container - individual size and shape and hence the handling system;

b) the growing medium – physical, chemical and biological attributes and

c) the growing environment and its management – whether open or protected; and the manipulation of temperature, water and light.

 

The detail of the container and the compost used in liner production has received much attention in recent years and this has resulted in the various proprietary materials which are now on the market. There is little relevance in discussing these in particular as they will undoubtedly be superseded and developed still further in the future.

 

The Container – the liner unit

When operating in a modern outfit with integrated systems of production and the need to use growing spaces and resources economically it would be prudent to adopt a specific modular handling system – which standardises the shape, size and number of the various liner units and the carrying module – so that a multiple module layout (ie in terms of the area and its shape - so that each carrying unit occupies x1, x2 etc of space in a particular shape) can be adopted and so maximising the use of the relevant space.

 

The size (volume) of the individual liner unit should be chosen from a limited range of containers (see above) and should be commensurate with, and allow for, the potential size of the particular plant at the end of the (liner production) season – given the specific growing environment – so that there is no check to growth and the development of a typical specimen will be anticipated.

 

The individual shape (ie the volumetric configuration) and the construction detail should be chosen to provide the most suitable conditions for root development and to suit the type of root system developed - as well as taking account of the typical shape and spread of the stem system. The major issues are to avoid the potential for root circling and to allow for the development associated with those subjects producing a tap root.

 

The Growing Medium

The actual constituents of a growing medium, suitable for containerised liner production, are not, in themselves, significant as long as the individual material or mixture provides the texture and physical attributes required for the desired level of plant growth - which are:-

a) the ability to maintain the relevant and required structure without deterioration over the anticipated ‘life’ of the medium;

b) the ability to absorb, retain and release water without a significant pressure gradient;

c) the ability to maintain a desirable air filled porosity ratio;

d) the provision of suitable, textural surface conditions for the retention and availability of the required chemical materials needed as plant nutrients or to maintain a relevant pH;

e) the provision of conditions for the development of any relevant biological agencies;

f) that they do not contain or develop any phytotoxic or other growth inhibiting materials and

g) have a textural compatibility with the growing medium to be used in the production phase or with open ground conditions - when the liner is moved on.

Thus the actual constituent, component materials of the medium are then determined by availability and cost; as well as, potentially, any ‘green’ considerations.

 

Although the nutrition of the seedling in the liner phase can be taken care of by the provision of slow release fertilisers it is probably more efficient and economic to provide this only at a base level at the start of this production phase and to provide ongoing nutrition as and when required by a process of liquid feeding which is tailored to the needs of the crop so providing a more constant, suitable and relevant pC status.

 

Most of the plants that will be grown in a general inventory of woody subjects will flourish within a reasonable range of pH values. However for those subjects which are adapted to conditions at more extreme levels it will be necessary to modify the pH value of the growing medium to suit the particular species. This is achieved either by using chemicals or more satisfactorily by choosing the physical components of the medium which have a natural pH attribute suited to the particular individual species.

 

Biological considerations - Symbiotic associations

It is now generally recognised that a very large number of woody plants will form a symbiotic relationship with a micro-organism(s) with some degree of dependence, intensity or reliance.  These relationships usually enhance the supply of a particular nutrient and/or water from the micro-organism to the host - which in turn receives, in exchange, elaborated materials – normally in the form of carbohydrates. This is part of an evolved (at least by the host) strategy to enhance the ability of an individual to compete and survive in an otherwise difficult environment – usually waterlogged, depauperate or deficient soils. In such a competitive situation the ability to obtain water and/or minerals efficiently and effectively or from an alternative source will give a particular species the edge in creating itself a successful niche.

 

The relevant symbiotic associations developed by woody plants are usually of two types – a) the fixation of atmospheric nitrogen by a micro-organism through a process which turns atmospheric nitrogen into a form in which it is available and useable by the host plant; this agent is housed in specialised root nodules developed by the host plant for the purpose and b) the development of an association with a fungus (or a range of fungi) which, in effect and in some degree, acts as a substitute for the root system of the host in accessing water and minerals.

 

 Nitrogen Fixation

Nitrogen fixation is carried out by bacteria (described as diazotrophs), generally in anaerobic conditions, in the root nodule. Nitrogen from the atmosphere is combined with hydrogen (from water) by the enzyme nitrogenase using energy liberated from ATP as it is reduced to ADP. This results in the formation (usually) of an ammonium radical which is assimilated into glutamate and then transferred to the host plant.

 

 The Rhizobium/Fabaceae symbiotic association

Nitrogen fixation by the (diazotrophic) bacterium Rhizobium occurs in specialised root nodules in the family Fabaceae. These bacteria, despite being involved in this very specialised symbiotic arrangement are also generally found as free living organisms in the soil. Hence in open ground seedbed conditions inoculation of the roots of a potential host is not a problem; however in situations where the subject is to be grown in an artificially constituted and ‘sterile’ compost it may be necessary to provide an inoculum.

 

 Actinorhizal symbiotic arrangements

A similar arrangement is found in a number of other woody genera – (inter alia - Alnus, Ceanothus, Cercocarpus, Comptonia, Eleagnus, Hippophae, Myrica and Shepherdia) in which the nitrogen fixing (diazotrophic) organism is the actinobacterium Frankia. This actinorhizal association also occurs in root nodules on the host.

 

 Mycorrhizal associations

Many plants will enhance their ability to compete for or to utilise resources more effectively as a result of creating a symbiotic relationship with particular fungi. These fungi will develop in conjunction with the root system of the host and will, in some degree, replace it in terms of the processes by which water and nutrients are accessed from the soil by the plant. These symbiotic relationships between fungi and the roots of higher plants are described as mycorrhiza (literally fungus roots).

 

The fungal mycelium, at the ultimate, may virtually completely replace the roots as the organs of water and mineral uptake from the soil – the fungus fulfilling this particular function and transferring these materials into the plant roots - the particular arrangements for transfer being dependent on the structural interrelationship. Broadly there are two types of arrangement – those in which the mycelium penetrates the root tissue and those in which the mycelium develops as a sheath around the surface of a specially modified root tip. In return for providing the plant with these raw materials the fungus extracts elaborated carbohydrates for supplementing its own energy resources.

 

Mycorrhizal associations are normally at their most effective, for the host plant, in demonstrating their benefits in nutrient deficient situations and especially those where phosphate deficiency is prevalent. Conversely where nutrients are readily available mycorrhizal development is often impaired and no additional growth increments will accrue from, or can be attributed to, the association.

 

Mycorrhizal associations not only provide the plant with an opportunity to improve the ability of the roots to obtain water and minerals more consistently but also provide the plant with a more penetrative and efficient system for the exploitation of the entire soil profile than can be provided by the roots alone. It provides the plant with the opportunity to forage more efficiently through the entire volume of soil. It also creates the scenario in which these fungal ‘roots’ are not only more efficient foragers but it also implies the ability to adapt by the rapid development and redeployment of the mycelium into any particular area as a result of the changing patterns of water and mineral availability in the soil.

 

Those fungi which are capable of developing a mycorrhizal association are otherwise normally saprophytic and are part of the normal soil flora and ecosystem.

 

The necessity for any particular species of woody plant to develop a mycorrhizal association is variable – some species at the extreme, as has already been indicated, are obligate in their arrangements and have adapted the development and structure of their roots to the most efficient arrangement (eg Pinus) and if the seedling is not ‘infected’ with the potential symbiont it cannot grow and develop successfully. Other species can develop a facultative dependence and will develop the arrangements depending on the environmental circumstances and the availability of suitable symbionts; at the other extreme many plants have not evolved to need this arrangement and manage quite successfully without.

 

For those species which have a degree of dependence the capability for growth and development will be enhanced by an opportunity for the development of the association soon after the deployment of the radicle at germination. In the process of germinating seeds it is therefore prudent to expose the radicle to the necessary symbionts at an early stage in order to encourage the association as soon as the process is feasible and so enhance the potential for the enhancement of growth at the earliest opportunity.

 

Fungi are, in general, particularly adapted to soils with an acid reaction and tend to form associations with subjects growing in these environmental conditions. In these habitats however the converse may be true in that these plants are able to survive in these environments because they have the inherent facility to make a suitable mycorrhizal association and so compete successfully.

 

Nowadays with the routine use of sterile growing media in the germination of seedlings and in the liner production phase, and the provision of protected environments, the provision of an inoculum is needed to ensure the required status is achieved. As the relevant fungi are saprophytic under natural conditions it is necessary that the growing medium has an adequate organic matter content to ensure the rapid proliferation of the mycelium to a sufficient level throughout the profile as the seedling root system is developing. In ‘open’ growing situations the growing medium will be subject to the normal levels of ‘spore rain’ and inoculation will occur naturally.

 

This account of mycorrhizal association is not intended as a description of mycorrhiza themselves – as their study is in itself a complex and difficult discipline to understand; however the emphasis here is on the fact that, for many species, this arrangement is an integral part of successful seedling production. In these cases they are an essential and beneficial component of the process, and therefore a necessary provision within the overall pattern of husbandry.

 

The particular relationships between fungi and host are complex but it is evident that there is, in general, little specificity between the two components. There is a very large range of fungi which are capable of developing such an association and the host may embark on an association with several different species of fungi at the same time - as long ago as 1896 Hartig recognised 400 species of fungi forming symbiotic arrangements with the European Beech (Fagus sylvatica). In woody plants, in general, there are few specific host/fungal relationships; it is therefore only necessary to provide an adequate range of fungi to ensure success.

 

The Growing Environment

The growing environment provided for the development of the seedling during its establishment and growth to the stage of being a suitable liner plant is a function of:-

a) Light - many plants are adapted for seedling growth to occur within the forest floor or in other habitats where full light is limited as a result of the shading created by mature plants. In order to make the most use of the season of available light, the seedlings of many deciduous plants do not absciss their leaves until well after the adults of most canopy species have shed their foliage. It is also quite possible that because of this adaptation that exposure to full light – especially ‘the noon day’ sun - may produce tissue damage (scorching) during the main season of growth. Thus the early and mid-season growing will be best undertaken in a shaded environment.

b) Temperature – within reasonable limits, the growing temperature should approximate to the ‘natural’ growing conditions for the particular plant and so avoid atypical growth induced by too high a temperature. Few plants produce seedlings which are able to resist freezing conditions – it is therefore relevant to provide frost protection for the seedlings - as even at marginal levels it will be a check to optimal growth rates.

c) Water and humidity

Apart from the competition for light the seedling is also subject to competition for water and this can be a factor in limiting growth rates under natural conditions. As the various plant species are adapted to varying degrees of water stress the optimal conditions for growth are necessarily difficult to describe in general. In practice it is easier to vary the texture of the growing medium and so alter the water holding capacity than to alter an irrigation programme. Optimal growth conditions will be achieved the greater is the length of time that the soil is as close to field capacity as is reasonable and water stress is minimised. Hence the watering regime should be tailored to this status. Similarly the humidity of the atmosphere will vary according to the needs of the individual plant and the growing environment should be tailored with that in mind. The maintained humidity may also influence the incidence of various pests and diseases such as Red Spider Mite and downy mildews.

 

Pests and Diseases

One of the inevitabilities of growing a high density of large numbers of a crop, under some level of artificial conditions, will be the occurrence of both general and specific pests and diseases. Particular attention should be made to this element of growing and the necessary measures for control put in place. The maintenance of high levels of hygiene in the growing environment to reduce potential occurrence is a non-sequitur. The prime consideration will, however, be the provision of those relevant conditions which preclude occurrence and thus maintain an environment which will not encourage infection or infestation. Where general, non-specific problems are likely to become an issue it would be prudent to maintain biological control materials or even the maintenance of companion plants which create an uncongenial environment.

 

However if such organisms do appear it may be necessary to resort to chemical controls but this should always be used in such fashion that it takes account of any side effects - such as the depression of mycorrhizal activity.

 

Weeds

If a ‘clean’, ie weed free, site is maintained the ingress of spores or seeds is much depleted and the odd occurrence can be removed before it becomes troublesome.

 

Mosses and liverworts - sources of infection within the site should be eliminated so that spores cannot access the growing areas and the irrigation regime should be tailored to prevent the surface of the compost from becoming too wet and so provide the conditions for rapid development

 

Ubiquitous ephemeral weeds – eg annual meadow grass and hoary bitter cress - although it is difficult to deal with these sorts of weeds in open situations the maintenance of clean conditions within structures should eliminate any primary source of local seed production.

 

Weaning

The final part of the development of any crop of liners will be the weaning of the crop from any protected environment to ‘outside conditions’ which then allows an unfettered move into the production phase.

 

Facilities

The facilities required for the production of a crop of seedling liners will reflect the type of crops to be grown - especially in relation to their optimal light and temperature requirements and this will necessarily require the provision of structures such as shade tunnels, poly tunnels, combination (hybrid) tunnels, glasshouses and possibly frames - as well as the necessity for outside growing space.

 

Water

Although the provision of a source of water for irrigation will be a necessity, attention should be paid to the water strategy for the whole nursery. This includes the collection or impounding of (rain) water, the impounding of drainage and run off water, recycling and the necessary treatment procedures to maintain and ensure a clean supply. Water is becoming an increasingly expensive commodity and effective and economic use is an essential component of the nursery practice.