Preliminary Considerations - The Seed

 

The seed itself is the basic raw material of the seedling production system and as such its qualities define the potential status of the end product. The implementation of the commercial practicalities of the production system will necessarily have an influence on the individual quality of the end product in terms of appearance, size and vigour but essentially, however, it is the genetic constitution and integrity of the seed which determines the commercial outcome. This is derived on the basis of a reliably determined identity and quality (ie fitness for purpose). The integrity and quality of the seed, as the raw material, is therefore paramount in the success of the operation.

 

Determining the production inventory

 

There are effectively two approaches to the commercial production of seedling ornamental woody plants in the context of these notes. The first is to operate a controlled inventory in which particular subjects are identified for an annual production schedule and for which there is a proven and substantial market place or, secondly, to be opportunistic – always being on the lookout for the unusual and often new subjects - and grow whatever comes to hand - as long as there is a potential (and preferably a premium) market available. In practice an amalgam of both is quite possible or indeed a business may well concentrate on the seedling production a particular genus or group of plants (eg Acers, conifers etc)

 

All of these approaches have their advantages. The ever increasing number and diversity of exotica which are maturing (and thus producing seeds) - as a result of material collected, introduced and planted, in the UK, in recent decades - makes any of these courses a reasonable speculation and has much to recommend it. In the last forty to fifty years there has been an unprecedented explosion in the variety of ornamental woody subjects which have been introduced to cultivation in the UK. Many of these have now come to maturity and - with many as yet still immature – their discovery, locally, will provide an ever increasing potential inventory.

 

Whatever the decision, the production levels for each particular subject will inevitably be relatively small unless a specific market, where a national distribution network - usually within garden centre outlet system - can be accessed.

 

Sources of Seed

 

a) Seedhouses and seed collectors

There are, throughout the world, seedhouses supplying the seeds of woody plants aimed at either commercial or non-commercial customers. They supply various categories of material. In Europe, especially, there are several large seedhouses (often acting as wholesalers) which source from both home and world wide contacts and offer a comprehensive selection. Elsewhere there are many seedhouses offering material chiefly of native species – in Australia, New Zealand, Chile, India, Japan, South Africa and the USA – which, in general, offer a comprehensive range and an excellent service.

 

Nowadays there is no necessity to enumerate a list of these suppliers (as would have been the practice in the past), as access to the internet provides most of the information necessary and tends to be current in its application and status.

 

In general the more specialist is the inventory of the commercial supplier the more reliable will be the material and will allow the possibility of negotiation as to aspects of the collection procedure and season of dispatch. Inevitably this relationship will depend for its success on the level and value of the business transaction.

 

Although these suppliers are very useful in obtaining stocks there is no substitute for the collection, by the practitioner, of any subject which can, satisfactorily, be sourced locally. Such a situation can allow the determination of the identity and quality of the material and can control the timing of collection. This creates the potential for greater certainty and thus offers a much more reliable outcome. The local provenance also provides an indication of the performance of that particular species in the local environment and climate and that it is functionally suited to the area. These comments do not imply that the process will cost less money but simply that control of the sequence of events will start at an earlier stage.

 

There are intermediate possibilities between commercial sources and self collection - as it is possible to make useful contacts with professional collectors or other reliable personal contacts, both at home and abroad, who will identify, collect and dispatch particular species to order.

 

b) Self collection

Identification of the parent and genetic status of the seed

Whatever the starting point - the most important consideration is to determine and confirm, with as much certainty as is possible, the identity of the subject to be propagated. Not only in the context of one’s reliability and responsibility as a supplier but also for determining the likely protocols for production.

 

Although there are many reliable and very competent commercial seedhouses dealing in plants from temperate climates, which supply seeds in excellent condition; it is still the most satisfactory starting point if the seeds can be collected by the propagator at a particular time that has been predetermined and that selection in relation to quality can be assessed.

 

In general, for any particular species, there is one normal defined gene pool and virtually all of the seedlings from a representative sample of parents will deliver typical specimens. However there are some species which have a very extensive geographical range and which vary, in their characteristics, quite considerably, through it – often responding in various characteristics to the local environment so that the gene pool has become locally specialised. This range nevertheless is often clinal in variation so that these changes within the species are not easily delineated although, often, particular extreme variants are given specific names viz Acer saccharum (the Sugar Maple) changing gradually into Acer floridanum in the south and becoming the ‘Caddo Maple’ in the farthest west of its distribution (a drought tolerant form in Oklahoma). Similarly Acer grandidentatum (the Canyon Maple) varies from a shrubby tree in Utah to the single stemmed upright ‘Uvalde Maple’ in the east of Texas – on the Edwards Plateau towards San Antonio – the variation responding to increased rainfall. In the UK this variation can be demonstrated for native Oaks where the smaller stature and thick leaved characteristics of the type from the Cornish Atlantic Rainforest ecosystem differs widely from the type to be found in the drier East Midlands such as Sherwood Forest! As long as this sort of variation is accounted for in the assessment of the parent sample, and is recognised, it does not become an issue. Hence any description of the provenance of a sample is a useful guide to expectations.

 

Once the identification has been confirmed then the next consideration is to assess the available individuals which are producing seeds - in order to select the most suitable parent. More often than not, however, there will probably only be one seed bearing parent plant available - or with luck there may be several specimens which exhibit sufficiently similar characteristics to demonstrate a reasonable uniformity among the available donors. It will be useful to determine whether these potential parents are seedlings, or are of vegetative (clonal) origin, as this will govern the limits of the available gene pool and its potential for variation.

 

It is probable, therefore, that the most significant consideration up to this point has been to assess the likely genetic integrity of the parent plant(s) - insofar as the seed crop has been produced as a result of self pollination or cross pollination from another individual of the same species. This will help to ensure that the offspring will be, reliably, of the same basic genetic constitution and be reasonable representatives of the species. In the great majority of subjects it is likely that particular selection will not be necessary as all of the available plants will provide a relevant genetic constitution whether growing in an advantageous or less suitable position.

 

c) Stock beds and Seed orchards

If it is the intention to establish a reasonably fixed inventory, there is much to recommend the establishment of one’s own source of seed – where this is feasible - by planting suitable stock and allowing it to reach seed bearing maturity. In practice this will only be suitable for subjects (chiefly shrubs) which will reach sexual maturity in a relatively short time. The possibility of hybridity however should not be overlooked.

 

The chief issue to be addressed will be the availability (and cost) of the land – for the space required is inevitably an extensive exercise.

 

Hybridity

 

In a situation where other relevant, related species are growing in the same area, such as in Gardens and Arboreta, the potential for cross pollination and the production of hybrid offspring should be allowed for and thus seed collection should be avoided, from such potential parents - unless there is some particular value accruing.

 

Apomixis

 

There are a few species of woody plants (especially in the Rosaceae – eg some of the species in Cotoneaster, Sorbus Malus and Aria) in which the seed is produced by a process described as apomixis or agamospermy - an unusual process in which the ovules are produced by mitotic cell division (ie there is no halving of the chromosome number) and not by the normally expected meiosis (reduction division). Although the flower may well be pollinated, pollen tube growth is prevented from reaching the ovary and fertilisation does not occur - however seed development is still stimulated and it is the mitotically developed ovule with the 2n chromosome number which develops to become an embryo and eventually an otherwise normal seed. Thus seeds with the same genetic complement as the parent are produced and, to all intents and purposes, the new generation are identical to the parent. This phenomenon has much to recommend it insofar as the chance of hybridity or other genetic variation is virtually eliminated from parental selection.

 

Some Factors influencing Seed Quality

 

Selection of the parent(s)

If there is an opportunity to collect seed from more than one parent plant then it will be prudent to make some assessment of the specimens in terms of their trueness to type and that they represent the species. There is some value in selecting seed from a single stand alone specimen as there is every opportunity to make this assessment of all the features. This is not to imply that other specimens growing in a more competitive arena will not be acceptable but simply that the assessment is less rounded and complete.

 

In the forestry industry there is much selection of elite individual trees in order to ensure the very best qualities for timber production in particular environments. However, with ornamental plants, this is not significant unless selection is being made for a particular trait – such as flower colour (Daphne mezereum) or leaf characteristics (purple and/or dissect leaved Acer palmatum). There occurs, in some genera, the existence of undesirable traits - which more often then not relate to the quality of timber production but do detract ornamentally viz the development of straight stems, as opposed to spirally twisted stems in Quercus robur and Castanea sativa. Otherwise no selection of parent plants is needed.

 

A depauperate plant, even, is not to be ignored if it is simply growing in a very competitive situation – poor apparent status does not necessarily imply poor genetic constitution.

 

Parental influence on the Quantity of seed produced

When dealing with the collection of seeds from plants that have only recently matured (and hence have reached a phase for flower and thus seed production), it will be evident that the plant, at this early stage in the mature phase, is still producing vigorous vegetative growth and that the greater part of photosynthetic product is being used in this growth. Normally the level of seed production will be relatively sparse and the quality of any seeds produced – especially in terms of the amount of stored food reserve – may be low.

 

As the plant progresses further into the mature phase and reaches middle age, the amount of annual vegetative growth declines and seed production in both quantity and quality potentially improves. At this stage, vegetative extension growth has moderated considerably and the plant has reached the greater part of its ultimate height and spread. This increase and stabilisation of the growth pattern is accompanied by maximum leaf area (ie photosynthetic area to produce carbohydrate) and by an increased propensity to produce flowers. As a result of this combination of factors there will be the potential to produce high quality seeds until the senile phase approaches when there will probably be a flurry of heavy crops prior to death.

 

The established mature plant thus is diverting materials for the development of both the embryo and the food reserve to the seed and a good crop can be anticipated. However, especially in the UK, this theoretical level of seed production may be modified by various environmental conditions in any particular year eg frost occurrence at flowering which may concurrently also determine the availability of suitable pollinators; low summer temperatures and the associated poor light levels - as well as the incidence of leaf pests and diseases.

 

The advent of ‘middle age’, in this context, is not easy to forecast for any particular species without some experience - for example, in Cornwall, Cornus capitata will flower and fruit prolifically from the age of four years whereas Magnolia campbellii will probably need from twenty to thirty years even to flower and a further period before fruit and viable seeds are produced with any regularity.

 

Currently there are many species, resulting from relatively recent collections, which are moving into early middle age and are producing reasonable crops of seed and in the near future will be producing good crops of seed (eg Aesculus assamica).

 

Season of collection

Normally it is anticipated that seeds will be collected just prior to dispersal when all of the biological processes involved with the development of the maturity of the seed will have been completed. At this stage the seed is mature and will be suitable for proceeding to germination or where relevant - storage. There are however a significant number of species of woody plants in which this is not necessarily the best strategy to adopt as the seed will have developed intransigent dormancy conditions by this stage. It is therefore prudent to collect the seeds at an earlier stage – when the seed has matured and is complete – so that particularly difficult seed coat conditions can be mitigated or even avoided.

 

Periodicity of seed production

It is a well documented fact that there are many tree species, especially, which exhibit a periodicity in the production of seed crops. The production of prolific seed crops will only occur at more or less regular intervals. Some knowledge and/or observation of this phenomenon is therefore important to the propagator in order to be able to gather sufficient crop in the ‘on’ years to account for the deficiency in the ‘off’ years by storage.

 

This more or less genetically controlled condition should not be confused with a failure to crop due to a particular prevailing environmental condition such as frost incidence - which may affect the initiation or development of the seed at flowering, pollination or fertilisation; poor summer light conditions etc.

 

The amount of seed produced annually by a particular plant is a function chiefly of the amount of food, excess to vegetative requirements, which the plant diverts to storage for seed production – this may dictate the level of seed/fruit production in any one year and will reflect the nature of the season – in terms of photosynthetic light reaching the leaves and creating a particular level of carbon assimilation. In those plants which do not accumulate sufficiently to produce crops annually, the response, in mature plants, will determine the frequency of seed production year on year. This is demonstrated by Beech which produces good mast years on infrequent occasions (five to seven years) or some subjects which will respond by biennial bearing. In the intervening years seed production may be absent or of poor quality or the production of unsustainable seeds without an embryo or sufficient food reserves.

 

Scope

It is quite possible, with the right enquiry and observation, to discover an extensive array of seed bearing species within a few miles of the production facility. It is always surprising how many subjects are growing locally and as collection and cultivating of new introduced material continues to expand this type of sourcing will undoubtedly intensify.