Coniferales

 

Pinaceae

 

This family of conifers; includes the genera Pinus, Picea, Abies, Keteleeria, Cedrus, Larix, Pseudolarix, Pseudotsuga and Tsuga. These have much in common insofar as they all produce their seeds in dry ‘woody’ or ‘leathery’ cones. They all develop winged seeds and the food reserve in the endosperm is stored as fats or oils to a greater degree. It is also the normal pattern for the seeds of most species as a whole to exhibit, in some degree, an endogenous dormancy condition which requires the seeds to be exposed to a period of chilling to speed, unify and synchronise emergence at germination. Cathaya and Nothotsuga also belong here but, as they are ‘not in cultivation’, they are discounted for lack of information.

 

The interest in conifers as decorative shrubs and trees has been a feature of ornamental horticulture in the UK for well over two hundred years and this group contains many stately and shapely trees with many decorative characteristics. The benign nature and subtle variations of the climate in the UK also allows the cultivation of an extraordinary range of these species, from around the world, and most of these can be found reasonably locally and growing in a mature condition (and therefore, in general, cone bearing). This provides the opportunity to collect seed locally and manage the entire production process in such a way that high quality seedling production is achieved.

 

The seeds of virtually all of the temperate species within these genera of conifers are unusual in that they exhibit an endogenous embryo dormancy (caused by the presence of inhibitors in the endosperm – as excised embryos germinate without hindrance). Usually this effect is at a fairly low level and is often one that is not constant within the sample or even from year to year. Chilling of the imbibed seed, for c42 days at c3ºC, prior to sowing will normally be sufficient to unify germination and synchronise emergence. This level of treatment (although the need may be considerably less) appears to be enough to overcome this variable constraint in all species and lots and satisfactorily prime all the viable seeds within a sample. Although it is possible, in many instances, to obtain satisfactory results by chilling the naked seed it is more reliable to stratify the seed, within a suitable medium, in the conventional way.

 

To achieve the chilling requirement by stratification it will be prudent to use materials that are of a relevant size and texture so that the seeds can be separated if required. Many species of Silver Firs and Pines, for example, have relatively large seeds and for high value, ornamental production can be handled individually and station sown.

 

Pinoideae

 

This monogeneric sub-family contains only the Pines (Pinus). It is a large genus of 90 to 115 species – the number indeterminate as the species seem to be regularly ‘lumped’ or ‘split’. They are natives of the Northern Hemisphere with a circumpolar distribution from the Arctic tree line to the tropics – although at tropical latitudes they usually occur at altitudes with sub tropical or warm temperate climates. They are the largest and most diverse genus of conifers varying from tall, erect trees (P. lambertiana and P. coulteri) to short multi-stemmed shrubs (P.mugo and P. pumila). They also contain some of the oldest individual trees alive today (P. aristata and P. longaeva). Many are important timber producing trees. Many of the species are highly ornamental trees and are used as specimen trees in relevant situations. The five needle pines, as a group, for example, contain a significant number of decorative species with long needles, a bright green foliage colour and a graceful habit – however several of these are not particularly hardy and are only suitable for mild areas of the UK. Some species have a very extensive geographic range (eg P. sylvestris) while others have a very local and endemic occurrence (eg the Pinyon pines of the Mexican canyons, P. torreyana and P. canariensis). About forty species are native to the United States.

 

The Pines are typified by their thin, generally long, needle-like leaves which occur in sheathed bunches (fascicles) containing one to five leaves (one of the diagnostic features for a species); the resinous wood  and the resinous, more or less round, woody cones.

 

The male and female flowers are produced, on the same tree, in May/June and pollination occurs at that time. The female flowers are produced in the crown of the tree and the male flowers lower down. The ovules in the female flowers are not fertilised until the following spring and they then develop into the familiar cones and produce seeds - the process of maturation is then completed by the autumn; however in a few species (eg P. leiophylla and P. pinea) this does not occur until the following autumn. The cones are, in general rounded in shape but may be elongated. They consist of well developed, spirally arranged woody scales which fit together tightly and these normally open readily on drying (unless they are serotinous). The cone scales open to shed the seeds during the late autumn and early winter. The spectrum of size of cones is considerable from the huge cones of P. coulteri to the tiny cones of P. mugo. A number of species are described as ‘closed cone pines’ (ie serotinous) and these respond to a particular scenario to liberate the seeds. In the case of species from Mediterranean climates especially (eg P. nigra and P.radiata) the cones remain on the trees for many years – often decades and require the flash heat of a wildfire to liberate the seeds. Other species from hostile environments (eg P. cembra) shed the complete closed cone which either eventually rots on the ground or is torn apart by animals seeking the seeds.

 

Coning in most species of Pines does not occur until the tree is well into maturity and the cones tend then to be produced high up on the tree. Collection is consequently difficult (David Douglas collected cones in the wild by shooting them off the tree!). The variation in quality of the trees in pine species can be quite substantial so that it is advisable to select ‘good’ specimens as parents in order to achieve a reasonable proportion of acceptable seedlings.

 

For those species in which the cones open conventionally it will be necessary to collect them just before the cone scales open in the late autumn to early winter and extract the seeds by drying them gently in warmth and shaking out the seeds – commercially this is carried out in a ‘kiln’. It is as well to keep the drying to the minimum required for liberation as excessive drying may enhance or develop a hard seed coat condition. Closed cone pines can be dealt with by smashing them or inducing the relevant species to open with a short exposure to extreme heat – eg plunging them in boiling water for a couple of minutes.

 

Those species which produce serotinous cones are natives of ecological niches dependent on fire for regeneration - the cones are opened by the application of flash heat provided by a wild fire. Cones of the following species fall into this category in some degree – P. attenuata, P. contorta latifolia, P. muricata, P. pungens, P. radiata and P.serotina. In all species the seeds are liberated readily and promptly from an opening cone.

 

The seeds are winged - albeit in some of the species with large seeds the wing is rudimentary. The seeds vary in size from the large P. (cembroides) monophylla @ c2K per kg, through P. cembra @ c4K per kg to the small - P. mugo @ c150K per kg. The seeds of Pines tend to have a harder seed coat than that found in the preceding families. De-winging is only useful if large quantities of seed are to be processed. The seeds of Pines store their food reserves more generally as fats than as oils and dry storage thus tends to be marginally less damaging. However the harder seed coat also helps to conserve moisture and so the storage of seeds by conserving the moisture content at dispersal and maintaining them at c3ºC is still the best option.

 

Some species develop a hard seed coat which will prevent imbibition if not treated. This is especially true of the seeds of large seeded species such as P. cembra and P.pinea – prolonged soaking in warm water will usually be effective but short term, warm stratification with an added compost activator is also a workable treatment.

 

In a few species it is possible to detect the presence of an immature embryo condition - this is almost invariably marginal and can readily be mitigated by a period of warm stratification. This condition may well be physiological rather than morphological but nevertheless it needs to be treated in order to unify the germination response. Pinus cembra, P.koraiensis, P. parviflora and P. sibirica should all be treated as falling into this category.

 

Although this genus of conifers will, in general, germinate without further treatment they undoubtedly benefit from a period of chilling. A period of 35 days at c3ºC will unify and synchronise germination. Those species which produce large seeds – eg P.cembra, the Pinyons (the P.cembroides group), P. pinea, P. bungeana and P. gerardiana can be station sown. The largest seeds are probably those of P. maximartinezii. Species with a warm temperate distribution, however, do not appear to develop any type of chilling requirement.

 

Several of these species which produce a nut-type seed appear to have a tough and impermeable seed coat – in the great majority of such species the problem is mitigated by an extended warm water soak to ensure water penetration and imbibition. If the seeds have had any extended dry storage the problem becomes enhanced and the degradation of the seed coat will have to be achieved by a period of warm stratification (preferably this is enhanced with the addition of a biological agent such as Garotta). An alternative technique is to soak the seeds in a 1% solution of citric acid for four days. The following pines fall into this category – P. albicaulis, P. armandii, P. bungeana, P. cembra, the P. cembroides group of species, P. coulteri, P. flexilis, P. gerardiana, P. koraiensis, P. parviflora, P. peuce, P. pinea, P. pumila, P. sabiniana, P. sibirica, P. strobiformis and P. torreyana.

 

Piccoideae

 

This sub-family contains only the genus Picea, the Spruces. This genus chiefly consists of tall, upright, conical to cylindrical, generally quick growing, decorative trees. They produce their branches in whorls and these have dense green, grey-green or bluish foliage. There are about forty species – several of which are significant forest, timber producing species (P. abies, P. glauca, P. sitchensis and P. pungens) with substantial value. In forest conditions the lower branches absciss but when grown as well spaced specimens they will remain clothed to the ground until of considerable age. They are natives of the Northern Hemisphere with a circumpolar distribution and southwards into warm temperate climates. Several of the Chinese species are native in continental climate locations and so tend to flush early and in the UK are susceptible to spring frost damage and therefore need a favoured location to grow normally.

 

Once they reach maturity, trees produce cones annually although they tend to produce heavy crops only in every third year or so. In the intervening years seed production is light and often tends to be infested with Seed Flies (Megastigmus spp.) The trees produce both male and female flowers during the spring, on different branches of the same tree, the female cones tending to be around the crown of the tree. Pollination occurs in the spring and the cones develop during the summer – maturing in the autumn when they become pendulous. The cones are leathery to woody and tend to be sausage shaped. The seeds are liberated during the early winter as the scales dry and open, this occurs over a fairly long season as the seeds are shaken free and the cones remain on the tree until well after the seeds have dispersed.

 

The cones should therefore be collected without delay at maturity. The cones can be induced to open by gentle drying and shaking out the seeds.

 

The seeds are small, thin coated and ‘oily’ to the touch - but are not fleshy as in Abies or Cedrus. They have a relatively large wing but de-winging is not advised as the small seeds are easily damaged. In common with other genera in this group, storage is best achieved in moisture conserving conditions at c3ºC.

 

Most of the species of the Spruces exhibit an endogenous dormancy condition mitigated by a period of chilling. This feature however is not constant throughout a sample or from year to year. A seed lot will, in all probability, germinate without chilling but the germination will be slow and erratic with some of the sample failing to respond. A short period of chilling will unify and synchronise germination; stratification for 35 days at 3ºC will usually be sufficient. Those species which have a definite chilling requirement include P.rubens, P. glehnii, P. jezoensis, P. omorika and P. orientalis.

 

Observation has shown that P. omorika will germinate more successfully in the dark – therefore the seeds should be covered at sowing. The critical feature in the germination of seeds of this genus is the temperature at which the process will initiate. Many species will not respond below 15ºC and optimally at 20ºC. This relatively high range of temperature required for successful germination is part of the strategy for avoiding a winter or early spring germination (in the absence of a strong chilling factor – thus delaying germination until late spring or early summer when the soil temperature has risen. Despite this the following species will germinate at low temperature – P. engelmannii, P. pungens, P. koyamai and P. polita. The seedlings of Spruce species are particularly susceptible to frost damage and thus the level of temperature at which many species will germinate can also be protectively critical in this respect. A high threshold temperature for germination is thus not unusual - ensuring that germination (despite a satisfactory chilling) does not occur until well into the spring when the danger of frost has passed. It would therefore be prudent to germinate all the species at 20ºC.

 

Abietoideae

 

This sub-family includes six genera the Hemlocks (Tsuga), the Silver Firs (Abies) the Cedars (Cedrus), the Golden Larch (Pseudolarix) and Keteleeria as well as the little known Nothotsuga. The relevant five closely related genera of conifers are characterised by the production of relatively thin coated seeds which contain an oily (sometimes virtually liquid) food reserve. This implies that successful storage may be a challenge and that mechanical seed treatment (ie de-winging) can cause considerable damage.

 

The common characteristics of the seeds of these five genera are that they store their food reserves principally as oils and have a thin seed coat. This indicates that there are potentially some issues for the plant propagator to address:-

a) The thin seed coat potentially allows the food reserve to dry relatively quickly unless moisture is conserved in storage and that, when drying does occur, the subsequent reconstitution of the food reserves with a water component for mobilisation at germination is severely constrained. This causes a consequent depression of vigour at germination – the level depending on the degree of drying which has occurred.

b) The storage of the seeds at other than cool temperatures (c3ºC) can cause the oils eventually to become rancid with the consequent deprivation to the embryo.

c) Any mechanical treatment to extract or dewing the seed is likely to produce an unacceptable level of damage both in terms of enhanced drying and the physical loss of the food reserves and perhaps more importantly the development of fungal infection.

 

In effect the most satisfactory outcome for the production of a high quality sample of seed is to collect, extract and clean the seed by hand, leaving the wings intact. If storage is necessary the seed should not be allowed to dry in storage (the water content at dispersal being maintained) and that a relatively low temperature should be provided. It would be prudent to move as quickly as is feasible into the treatment and germination phase. As most of these species of conifer are only likely to be grown in relatively small quantities and will be treated and germinated under controlled and protected conditions - this option will generally be economic. The major implication is that it will be necessary to collect seed annually.

 

If the seeds are stored for extended periods at other than low temperatures under moisture conserving conditions the seeds are prone to fungal rot and especially so if there has been physical damage to the seeds during extraction.

 

A common physiological feature of the seeds of the majority of the species in these five genera is that they exhibit (albeit at a low level) a form of endogenous dormancy which requires the seed to be imbibed and chilled (even if for only a short period) if a uniform and synchronous germination is to be achieved. Some germination will usually be achieved without chilling but it will often be slow, erratic and incomplete.

 

All of these genera only grow effectively with the conventional mycorrhizal association and it would be prudent to consider the addition of a suitable inoculant to the germination substrate.  

 

The Silver Firs (Abies) are an important genus, of some 40 to 50 species, of tall, upright, shapely, cylindrical to conical trees. Most of the species are significant either as ornamental subjects or as timber producers.

 

The trees have a strong central axis and tend to remain clothed to the ground until well into maturity. The foliage is usually dense and in bright shades of green – or occasionally ‘blue’. Some species are currently fashionable as Christmas trees. They are chiefly natives of the temperate regions of the Northern Hemisphere and occupy in the northern ranges low elevations but the more southerly species tend to occur at higher elevations – in northern Africa and Central America. The species from continental climate provenances usually do less well in the UK as they tend to be early flushing and the young growth is often damaged by spring frosts.

 

At maturity the cones of both sexes are produced on the same tree on the previous season’s shoots - to the outside of the tree. The cones are held erect. The female flowers of Abies spp are produced in the early spring and these develop along the top side of the branches. Pollination will occur in the late spring and the cones mature in the autumn of the same year. The mature cones are upright and disintegrate on the tree over the winter as the seeds and the scales are dispersed - leaving the central axis of the cone still attached to the tree. The greater proportion of viable seed appears to develop on the basal scales of the cone. The cones themselves can be ornamental and are often brightly coloured – green, blue and purple.

 

The cones should, therefore, be collected just prior to this disintegration phase and then gently dried so that the seeds can be separated when the cones break up. If the seeds are much lighter than the scales this can be achieved by winnowing - otherwise it may be necessary to sieve the material with a relevant gauge sieve. The seeds are angular  and three dimensional, oblong to triangular in shape thin coated, soft and with an oily, virtually liquid, food reserve; such a condition means that they are easily damaged by physical activity during extraction. If they are not properly stored (in the short term) they can desiccate quite easily and lose viability, or the food reserve deteriorates and becomes rancid or the seed becomes subject to fungal infection. There is often much unsound seed (ie with an aborted embryo) although the seed appears normal with a normal oily food reserve. This characteristic is often more prevalent on isolated specimens – suggesting that there could be a degree of self-incompatibility.

 

The seeds of Silver Firs are more consistently dormant than seeds of Spruces or Douglas Firs and several species with an extensive range exhibit a variation in the depth of the dormancy condition and this can be attributed to the provenance. As is typical for this group of genera the seeds of most species will germinate without any period of chilling but under these circumstances is then erratic, incomplete and attenuated. A period of chilling synchronises emergence, maximises production and decreases the time taken to emergence. A chilling treatment of 35 days at below 3ºC will usually be more than sufficient to ensure that the encumbrance is mitigated. Germination rates for Silver Firs are almost invariably poorer than expected as many seeds which appear to be sound are not and so fail to develop. Best results are achieved when germination is at its speediest and this occurs at a temperature of around 18 - 20ºC.

 

This genus Keteleeria is only satisfactorily grown in mild niches in the British Isles as the young growth is particularly susceptible to frost and the relatively early flushing habit makes it significantly prone to damage. It is unusual to find cone bearing trees in the UK (if at all) and production is usually achieved by using commercial seed imported from Mediterranean countries. The seeds are very similar to those of Abies and should be dealt with in the same way.

 

The Cedars (Cedrus) are all, potentially, large trees of substantial stature and require considerable space. They are therefore principally used in public parks, cemeteries, parklands and those properties with extensive garden areas. There are nominally four species in the genus but these are difficult to delineate clearly and are, by some authorities, treated as extreme geographical variants of one species. It is therefore important that the provenance of any seed lot is determined and recorded in order to provide a valid expectation. All four species grow quite satisfactorily in all but the northern parts of the UK. They will also set viable seeds once the individual tree has matured and the vegetative growth phase is no longer the plant’s priority. The presence of cones is not an indication of seed production and the production of sterile cones is more often than not a symptom of this emphasis on vegetative growth.

 

Male and female flowers are usually produced on the same tree although not always so. The flower cones are erect and produced on the short spur shoots in July/August and are fully developed by September/October. Pollination occurs in October and the cones begin to grow the following spring - by the following autumn they are about two thirds developed and complete development to full maturity does not occur until the following autumn (ie about two years after pollination). The cones are substantial and barrel-shaped – up to75mm tall and 40mm broad: when mature the cones break up on the tree during the winter - dispersing the seeds and scales and leaving the central axis on the tree in the same fashion as the Silver Firs. The cones are erect, relatively large and woody and initially tightly closed.

 

The cones should be collected in late November/December as they ripen and change colour from green to purple-brown and before they begin to disintegrate. Extraction is easier the closer the maturity of the cone is to disintegration. The cones seem to break up more readily under damp conditions – so that the most useful method for extraction of the seeds from a closed cone is to immerse the cones in warm/hot water until there is an indication that the scales are opening.

 

The seeds are relatively large, angular, thin skinned, soft and oily – containing a clear oil - with a large membranous wing; it is unwise to attempt to de-wing them as this is likely to cause damage. Because of the thin seed coat the seeds are prone to dry quickly so they should be kept cool under moisture conserving conditions until they are ready to be processed.

 

Much of the commercial seed offered (which is generally of high quality) is collected in Southern Europe; however there appear to be considerable variations in the hardiness of individuals and it would be prudent to collect seed from a local specimen in order to provide an indication of hardiness - as well as the ability to define the name and provenance.

 

The dormancy control, in this genus, requires a period of chilling to eliminate the effects. It is however very marginal and normally a seed lot can be germinated quite effectively without treatment but with an attenuated and erratic emergence; however experience will show that a short period of chilling will enhance germination by speeding and synchronising emergence. A stratification period of 28 days at 3ºC will achieve this. Optimal germination is achieved by providing a temperature of 18-20ºC when emergence will occur in 5-7 days.

 

The seeds of all the ‘species’ are remarkably similar – only varying in their 100 grain weight:- C. atlantica - 7.5gm; C. brevifolia – 8.5gm; C. deodara, c12.5gm and C. libani – 8.0gm.

 

The genus Pseudolarix is monotypic and thus has only the one species – the Golden Larch (P. amabilis) from Eastern China. It is a tall, erect, more or less conical, deciduous tree prized for its autumn colour. It has not been widely planted in the UK because of a perception that it is of doubtful hardiness. There are however a number of fine specimens in the South West of the UK. This reputation is more associated with its slow growth rate which is probably a reflection of the greater light levels and higher temperatures that it experiences during the summer in its native continental climate – its winter hardiness is not suspect but it will be susceptible to late spring frosts.

 

Male and female flowers are produced in the spring and pollination and fertilisation occurs in April. The erect cones – which are described as ‘being like small globe artichokes’ are characterised by the open, loose and pointed appearance of the scales, these develop and mature in the one season. The cones gather a purple flush as they ripen and finish a reddish brown; each scale carries two seeds. The cones are 15-18mm long and 10mm broad at their widest. The seeds are dispersed in November-December as the cone completely disintegrates and falls from the tree.

 

The seeds are white to pale beige, egg shaped and 6-8mm long with a long wing which is shiny and yellow brown in colour and is somewhat oily to the touch. The cones break up fairly quickly so collection should not be delayed. Extraction is achieved by gentle drying until the cones can be broken up to allow the seeds to be separated.

 

The production of seed in the UK is not common - if at all – but how much this has been a function of the maturity of the available specimens is not clear and indeed whether the summer climate is light and warm enough to achieve seed production. It is likely however that the trees are self sterile and hence isolated specimens are unlikely to produce viable seeds – even though they cone regularly. Crops of good quality seeds are however available, annually, from northern Italy. The longevity of the seed is short but satisfactory storage for a few months can be achieved under water conserving conditions at a low temperature (c3ºC). 

 

As with all of this group of conifers, the germination of the seed will be unified and synchronised by a short period of chilling. A period of chilling for 35 days at 3ºC will achieve this treatment satisfactorily. Germination at 18-20ºC provides the optimal range of temperature for germination.

 

The Hemlocks (Tsuga) are a small genus of ten to fourteen species (depending on the current view of the taxonomists) of straight stemmed, upright, elegant, stately and graceful evergreen trees of North America and Eastern Asia in the Northern Hemisphere. The Asiatic species are only successful in the UK in areas where late spring frosts are not common as they tend to flush early in the season. The crowns are generally tall and conical and the mature branches are horizontal to drooping with very thin, lightweight twigs.

 

The flowers are produced in the early spring – just before vegetative growth begins; male and female flowers are produced on separate branches of the same tree. The female flowers are produced terminally on the lateral shoots of the previous year and the male flowers in the axils of these shoots.

 

The cones mature in the same year that they are pollinated and are small, brown in colour and consist of leathery overlapping scales. They are pendulous and remain attached to the tree for most of the winter. The cones open in the autumn to early winter and the seeds are liberated slowly throughout the winter and into the early summer of the next year – when the cones are empty they eventually detach themselves. In general the Hemlocks fruit consistently each year and are not periodic in their response. The cones should not be collected until they are fully mature in the autumn just before they are ready to open - as cones in the green condition do not open readily and many of the seeds are immature (the seeds do not mature until just before they are shed).  Gentle drying will cause the cones to open readily and the seeds can be shaken free. The seeds are very small and are produced in abundance but viability is generally low.

 

The seeds are small and thin coated and are very susceptible to bruising so should be treated gently. The seeds can be stored ‘dry’ (ie at their dispersed moisture content) at 3ºC in the medium term.

 

The species and their geographic variants vary greatly in their germination requirements however in order to provide a generalised formula for a protocol - the seeds should be stratified at 3ºC for 42 days and then germinated at 20ºC, this regime requires that the seed should be covered to exclude light. In common with Birch species, from the same ecological niches, unchilled seed will achieve reasonable levels of germination if exposed to light.

 

Laricoideae

 

This sub-family contains the three genera Larix and Pseudotsuga plus the ‘non-cultivated’ genus Cathaya.

 

The Larches (Larix) are a genus of deciduous conifers consists of about sixteen species which is native to the Northern Hemisphere where it has a circumpolar distribution – it is essentially an alpine or polar forest genus but there are outlying species further south – as far as the Himalayas - but always at medium to high altitude depending on latitude. Many of the species have an extensive geographical distribution and have developed races in particular provenances. Several species are important forest timber trees in the colder northern climates. They are usually tall trees with a single tapering trunk which retains the branches if grown as individual specimens at reasonable separation. They are sufficiently decorative to be used in an ornamental context – especially for their autumn colour in difficult or cold situations.

 

The male and female flowers are produced on the same tree in advance of leaf bud break. Pollination and fertilisation occur at this time and the cones develop and mature in the same year. The cones are erect, woody, relatively small, ovoid in shape - about 20-30mm long and 20-35 mm wide. The scales, in most species gradually open and shed the seeds over a long period from the autumn to spring – the spent cones are retained on the tree for several years. The cones should be collected in the late autumn before the scales begin to open (as some species liberate their seeds quickly eg L. occidentalis). The cones should be gently dried until the scales open and the seeds can be shaken free. Storage without further drying at 3ºC is effective over the medium term. The cones generally develop good quantities of seed.

However seed production can be periodic and in years of poor seed production most are infested by weevils.

 

The seeds are small and red brown in colour and have a relatively large wing which allows a good dissemination by the wind. De-winging although feasible is not easy without damage to the seeds. The seeds are somewhat slippery to the touch. However even in good years the quality of the seed is poor and there is a high proportion of non-viable seed. Viable seeds can be readily recognised by the well developed red brown embryo set in a light coloured endosperm.

 

The germination characteristics of this genus are variable – the seeds will germinate if spring sown (when soil temperatures are high) but germination is likely to be erratic, slow and incomplete. The most reliable practice to achieve a reasonably complete response of all the viable seeds is to stratify the seed for 42 days at 3ºC – and then sow and germinate the seeds at 20ºC. This should take care of any extended variation in chilling requirement which relates to a particular provenance. It would appear that a longer chilling period (up to 63 days) marginally speeds, unifies and synchronises germination – although L. leptolepis often exhibits a deeper dormancy than the other species and would normally be stratified for 56 days.

 

The Douglas-Firs (Pseudotsuga) are a small genus, of about six species, of very tall, upright evergreen coniferous trees producing their branches in whorls. In the past they have been included in many of the above genera as they have apparent individual affinities with them all. They are easily recognised by their cones in which the bracts have a three pointed external extension – this distinguishes them from all the other genera in this group.

 

The most widely known and recognised species is the Douglas-Fir (P. menziesii) which has, in its widest interpretation as a species, a distribution which covers large tracts of the western side of North America. It is a significant forest timber tree and can grow to immense height (90+m) and size. In its early life it can exhibit considerable growth rates. Because of this wide geographical distribution there are many variants and it is relevant to know the source of the parent tree.

 

All the species can be ornamental as individually sited specimens - however the species from warm temperate or continental climates (of which there are three) do not generally succeed or thrive in the UK as they are susceptible to late spring frosts and are therefore only found in locations with a mild spring climate.

 

The male and female flowers are produced in the spring with pollination and fertilisation occurring then. The female flowers are produced in the upper crown of the tree and soon become pendant. The cones and seeds mature during the one season as early as August – September and they begin to release their seeds in September-October, the seeds are dispersed fairly quickly. The cones are medium sized (8-10cm x 4-5cm in P. menziesii), blocky, light brown, pendulous and leathery. The cones should be kept under observation and collected as soon as the first cone scales begin to separate. The cones are gently dried until the seeds can be extracted by shaking them out. Storage under moisture conserving conditions at c3ºC maintains quality in the medium term.

 

The seeds look very similar to those of Larch are dark brown and shiny and have a large rounded wing. As the seeds develop, the side of the seed which lies on the scale is flat and the upper side is rounded – it has been shown that the seeds germinate faster when sown with the flat side of the seed is oriented upwards – ie away from the medium. The food reserves are high in fats and oils and thus require storage at cool temperatures under water conserving conditions under such a regime they will retain viability for several years.

 

Seeds which are sown without any pre-treatment germinate poorly – slowly and erratically. If there is no dormancy control the seeds will germinate in 12-15 days if a temperature of between 20 and 30ºC is provided although in this genus there is no light sensitivity. The response of the seeds of P. menziesii depends on their particular provenance – as does their size.

 

In general the best germination results are achieved by a stratification treatment of 35 days at 3ºC followed by germination at 20ºC.