Ericaceae

 

This very large Family, of four thousand or so species, is segregated into about 126 genera and these are then categorised in nine sub-families – with further sub-divisions in the larger groupings.

 

In habit it is extremely diverse – from trees and shrubs to dwarf shrubs, prostrate creepers and herbaceous plants. The species tend to occupy and tolerate acidic, depauperate, infertile soils and/or hostile conditions – often in extremes of arid, cold or damp conditions. They make mycorrhizal associations to compensate for this – some having complex arrangements. In general the species are evergreen.

 

The Family is virtually world wide in distribution – from the tropics outwards in both Hemispheres. The genera which appear below – itemised in their sub-families - contain species which have a distribution in temperate, alpine and sub-arctic climates and are suitable for cultivation in the UK – although often only in particular niches.

 

The Family is characterised as producing ‘dwarf’ seeds – ie small seeds with an immature (undifferentiated) embryo. The dormancy characteristics are not entirely predictable – in general the seeds, although requiring light for germination (even at only a short (30 minutes) duration per diem) - germinate at relatively high temperatures (20-25˚C) and do not exhibit any other constraints. However seeds of some species with cold temperate distributions – or other environments which experience cold winter temperatures (eg semi-desert in warm temperate areas) will respond to a period of chilling although it is not essential in order to achieve some germination.

 

A few genera have species which produce seeds with a protective hard seed coat or fruit covering (endocarp). This provides protection during passage through the gut after ingestion by a bird or animal and it also provides protection during wild fires – although it is not, always, necessarily a barrier to imbibition of the seed. Those species (from fire ecologies) are often triggered for germination not by the passage of heat but by the products of the fire ie smoke constituents and/or the leachate from burnt material (charate).

 

As with many subjects which produce small seeds, the seed samples often contain a fair proportion of non-viable seeds. These non-viable seeds tend to be small and light and can be distinguished from the plump viable seeds (although making this distinction may need the use of a lens).

 

Collection and Extraction

The fruiting bodies vary from dry locular capsules (eg Erica, Kalmia and Rhododendron) containing several to many seeds to fleshy, usually globular ‘berries’ which may contain one or more seeds (eg Gaultheria, Vaccinium, Arbutus and Arctostaphylos).

 

There is however still considerable diversity within these parameters in the characters of the fruit produced but effectively these can still be categorised into two broad groups.

 

a) sub-families with fruits consisting of dry capsules - these are usually made up of five locules which contain the seeds. These are produced in the late summer or autumn and gradually dry as the season progresses. When mature each capsule splits longitudinally liberating the seeds. The capsules are collected prior to this stage and then dried in a paper bag until they open - when the seeds can be shaken out; otherwise they can be broken up with gentle crushing. The seeds can be separated from the detritus by sorting the larger pieces and then the seeds can be separated from the dust and non-viable seeds by gently vibrating down an inclined sheet of rough surface paper (or cloth attached over a board) - so that the plump viable seeds travel all the way to the bottom leaving the lighter material snagged on the surface. This detail is relatively important if it is intended to sow known numbers of seeds in order to achieve a particular population of seedlings in a container.

 

b) sub-families producing fleshy fruits – generally described as berries - this fleshiness may be ‘juicy’ (eg Vaccinium and Gaultheria) in consistency or can be ‘mealy’ (eg Arctostaphylos). The seeds are extracted from juicy berries by the gentle maceration of the ripe fruits until the seeds disengage from the other constituents. The material is then swirled in water in a suitable bowl so that the viable seeds fall to the bottom and the dross can be decanted off – it will probably be necessary to repeat the process in order to obtain a clean sample. The seeds can then be surface dried to allow the seeds to be sown evenly. Extraction from mealy berries can be achieved in the same way or the fruits can be dried and then crushed to separate the seeds from the capsules and then treated as for seeds originating from capsules.

 

Germination - generalities

In general there are some universal applications, throughout the Family, which can lead to more assured germination and establishment. The process for sowing and germination although basically simple can vary considerably in detail according to the subject, the operator and the sophistication of the conditions and materials available.

 

The process should be conducted with fresh seed (ie from the current season’s crop) as storage (under practical conditions) does not usually extend economic life for twelve months.

 

Seed imbibition

The technique required for the imbibition of these dwarf seeds will depend on the actual seed size of the individual species. It is probably best achieved by ensuring that the sowing medium is sufficiently damp either before or immediately after sowing and thus allowing the seeds to imbibe in situ. ‘Large’ seeds can be mixed with damp sowing medium for a few days prior to sowing – and the whole sown evenly. It is relevant that during imbibition – however the process is implemented - the temperature is maintained at a reasonably warm level in order to ensure that the process is effective and optimal.

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Sowing and the germinating medium

As the seedlings are, initially, usually small it is prudent to provide a fine, even and flat surface for sowing. The fresh, dry seeds are broadcast evenly onto the surface of the compost and left uncovered. The container is then placed under intermittent mist (or similar) until the medium is sufficiently moistened for the seeds to imbibe. It is then necessary to maintain a humid atmosphere in order to prevent any drying of the medium.

 

Containers

The seedlings tend to produce a shallow and delicate fibrous root system initially – thus the container which holds the germination medium can be quite shallow.

 

Mycorrhizal association and the production of the medium

It would appear that, in the majority of genera, the roots need to make a mycorrhizal association quite soon after germination in order to achieve successful establishment and growth. Thus it would be prudent to inoculate the medium to ensure that the action of the process is maximised. Most of the relevant fungi require an organic matter base - with an acidic reaction - in which to proliferate.

 

The sowing medium, which readily fulfils these parameters, is, conventionally, sifted peat moss – this should have a uniform structure throughout its profile – whether it is extended with an inorganic material (to improve the air filled porosity) is a matter of individual circumstances and management system. This sowing medium is most easily inoculated before sowing – this can be achieved if the coarse peat and the inoculum are thoroughly mixed prior to sifting so that the medium is inoculated uniformly. The most suitable inoculum is obtained by collecting the immediate surface soil scraping and the lower strata of decomposing leaf litter from a planted stand of an ericaceous species (as closely related as is feasible) - this material will usually contain the necessary ‘ericoid’ fungal associates.

 

Summary

The germination medium should therefore be:-

a) acidic,

b) uniform in particle size, compatible with seed size and the subsequent separation for moving the seedlings on,

c) capable of water retention,

d) capable of maintaining adequate aeration,

e) of a suitable organic constitution to support fungal activity and

f) contain sufficient and suitable fungal associates.

 

Light

The germinating seeds require light to trigger germination and are thus best sown on the surface of the medium and then maintained in an environment with natural light and daylength - although observation has found that very limited photoperiods of exposure (as little as half an hour per diem) may be sufficient to provide the required stimulus for many species. Germination should be programmed to occur as daylength increases in order to provide as long a growing season as is feasible.

 

Temperature

Germination is temperature driven - with greater success as temperatures rise from low levels; generally the seeds of all species tend to emerge most successfully in the range of 20 to 25˚C - with some evidence that temperatures above 25˚C starts to see a decline in numbers emerging.

 

Seed Chilling

As has already been alluded to above, there are many species which are native to cold temperate environments, which will exhibit a vastly improved germination response, both in terms of the speed and the synchronisation of seedling emergence, following a period of chilling. However the issue with seeds which are so small - is how to achieve this effectively. It will prove tedious to try and treat the seeds in a conventional way - as mixing the seeds with a suitable extending medium (with an adequate aeration and water holding capacity) and then trying to sow this evenly after chilling becomes difficult. On a relatively small scale it is simplest to chill the seeds once they have been sown and the medium is moist – ie to place the container with the sown and imbibed seeds into refrigeration for the determined period. During this period it is important that the medium does not dry (as the seeds are surface sown). This can be achieved if the container is enclosed in a polythene bag (or other method which achieves the same end). As this system depends on a satisfactory imbibition of the seeds, for effective chilling, it would be prudent to leave the sown container in a warm environment for a few days prior to treatment, both to ensure imbibition and the commencement of embryo maturation should this be needed.

 

It is apparent that during the process of chilling that the presence of light is not required.

 

Moisture

After sowing it is of paramount importance that the medium is kept adequately moist by maintaining a humid atmosphere but still maintaining a high degree of air filled porosity in the medium.

 

The Effects of Fire

There are many species in this Family which are native of various fire ecologies (eg Erica species in the Fynbos of South Africa and Arctostaphylos in the Chaparral and Scrub communities in California) in which germination is promoted by the passage of wild fire – rarely is the effect one of the effect of heat, almost invariably the effect in promoting germination is attributed to the penetration of smoke or the leachate from the charred remains of vegetation on the surface. For such species the use of commercially available ‘liquid smoke’, to treat the seeds initially, is the most practical option.

 

Nomenclature

Below are listed the genera which are potentially relevant to these notes. They are dealt with in relation to their current taxonomic positions in the sub-families and using what are currently designated as the recognised generic names (2012).

  

Hence the following do not appear - Bruckenthalia (Erica), Ledum (Rhododendron), Leiophyllum, Loiseleuria (Kalmia), Menziesia (Rhododendron) although Pernettya still appears until it is amalgamated officially into Gaultheria.

 

Enkianthoideae

Enkianthus

This genus consists of about seventeen species from Eastern Asia in temperate to sub-tropical niches. They are very ornamental, deciduous, small trees and shrubs which are noted for their prolific flowering and intense autumn colour. The veined flowers – white, yellow or pink, are produced in May and develop dry, beige to brown capsules in the autumn; each capsule containing three to five, very small, dust-like seeds. E. campanulatus is fairly widely grown but generally the genus is not fully exploited in the UK.

 

There are no dormancy constraints to germination which succeeds best at 20-25˚C.

 

Arbutoideae

Arbutus, Arctostaphylos, Comarostaphylis, Ornithostaphylis & Xylococus

This sub-family is characterised as having many species which are native in wild fire environments (which implies that germination may be enhanced as a result of the passage of fire).

 

All the species produce medium sized, fleshy, globular fruits and as described above the seeds can be extracted either by macerating the fresh fruits, swirling in water and decanting the fruit pieces to allow the seeds to separate and fall to the bottom or allowing the fruits to dry and crushing the fruits to a powder, when sound seeds can be separated by sieving.

 

Arbutus

This small genus has a limited distribution. It has an evolutionary history which has produced two disjunct populations. One in the Old World – the Mediterranean Basin, some Atlantic Islands and Western Europe (the Strawberry trees) - which includes A. unedo (also in SW Ireland where it is a remnant of the Lusitanian flora), A. andrachne, and A. canariensis; the other in the New World - Western North America - chiefly in warm temperate areas, the Madrones (Spanish for Strawberry tree) which includes A. menziesii, A. arizonica and A. xalapensis.

 

The genus consists of up to 14 species (depending on the taxonomic view) of trees or shrubs, which are evergreen, have exfoliating bark in summer which is orange to mahogany red in colour. The flowers are small, white – sometimes pinkish and bell shaped (similar to Arctostaphylos) in pendant clusters, the flowers are followed by the edible, fleshy, red fruits.

 

The fruits are collected in the late autumn when they are ripe and soft – prior to this if they are being predated by birds. The seed coat is brownish black to black. Best results are produced from the current season’s crop of seeds – ie fresh, unstored seed.

 

Germination will usually occur at some level without pre-treatment at 20-25˚C. However the seeds from more northerly provenances (eg A. menziesii) and temperate semi-desert conditions (eg A. xalapensis) respond to chilling at 3˚C for 49-63 days while A. unedo responds to 63 days. With this pre-treatment germination will be more rapid, better synchronised and will occur at the low temperature range of 15-18˚C.

 

Arctostaphylos

This genus of small trees and shrubs (the Manzanitas) is chiefly from the West and South West of North America (90% in the Californian floristic province). There are also three species (the Bearberries or Kinnikinnick)) which are circumpolar in distribution and have a cold temperate (high montane) to sub arctic occurrence in the Northern Hemisphere. This evergreen genus contains about 55 to 60 species (depending on taxonomic niceties). Most of the more southerly species are natives of wild fire ecologies.

 

The drupe-like (usually globular) fruit contains a single nut-like structure with a hard bony endocarp which contains two to ten seeds which are individually nut-like and may be separate or sometimes fused together – they also have a hard coat.

 

The seeds are extracted, conventionally, by maceration etc of the fruit and liberating the seeds by twisting and breaking the endocarp.

 

Most species (the Manzanitas) are native to fire ecologies and there is virtually no natural regeneration until the passage of wild fire when subsequently germination is extraordinarily prolific. However seed germination is not triggered by heat – it would appear that the leachate of the charred remains of relevant woody vegetation (charate) is the trigger. In artificial conditions the use of commercial ‘liquid smoke’ is effective. Domestically the burning of suitable vegetation and leaching the charred remains onto the imbibed seeds is also possible. The hard seed coat itself is normally impermeable to water but the presence of a small ‘hole’ (periole) allows the uptake of water. References to the need for acid scarification is thus likely to do more harm than good. Germination is then normally prolific at 20˚C or above.

 

The Bearberries (A. uva-ursi etc) do not respond to this treatment but do benefit by chilling the imbibed seeds at 3˚C for 35 days.

 

Cassiopeoideae

Cassiope

This genus of nine to twelve species of small, low growing, sub shrubs has a circumpolar distribution in the Arctic and from there southwards in high montane areas (eg C. mertensiana as far down as California).

 

They are characterised by their bright green leaves which are tightly adpressed to the stems and the brilliant white flowers. The fruit is a dry capsule which contains many seeds – the viability of which is influenced by the length of the particular summer – ie the time between snow melt and the first snow fall and hence the amount of time available for carbon assimilation. The various species are cultivated in specialist conditions such as peat banks and alpine gardens.

 

The capsules can be collected before they split to release the seeds and then dried in a paper bag and shaken to liberate them. The fresh seed will then germinate without pre-treatment although a more uniform and speedy emergence is achieved after a chill for 21 days at 3˚C. Germination will occur prolifically at 20˚C.

 

Cassiopes demonstrate a particularly integrated mycorrhizal arrangement (the mycelium penetrates and ramifies through most of the plant) and these fungi are an essential component of successful growth patterns and establishment. Thus the inoculation of the medium, preferably with leaf litter from existing plants, is a significant part of the propagation process.

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Ericoideae

Empetrum

This genus is attributed, by some authorities to its own Family – Empetraceae. It contains up to four species which have a circumpolar distribution in the very north of the Northern Hemisphere. All the species are low-growing sub-shrubs that act as ground cover in coniferous forest.

 

The flowers are hermaphrodite and produce the fruit in one season. The fruit is a black, soft, thinly fleshy drupe which contains up to 9 pyrenes (‘stones’). The seeds are extracted by gently macerating in water and squeezing to separate the stones; they are then washed thoroughly to remove any remaining flesh and then surface dried for short term storage.

 

The seeds show low viability – usually about 50 to 60%. The viable seeds are plumper than the unfilled and can be identified visually and are easily separated.

 

The seeds have a stony endocarp which has traditionally been regarded as impermeable to water; which has engendered recommendations for acid scarification and/or warm stratification. However recent critical observations have determined that this coat is not impermeable. However the warm stratification is essential for the complete ‘maturation’ of the imbibed embryo – ie before treatment by chilling can begin effectively.

 

Seeds will imbibe rapidly in a warm water soak completing water uptake in about three hours.

 

A protocol to achieve germination consisting of:-

a) 12 weeks stratification at 20˚C,

b) 20 weeks chilling at 3˚C (which can be conducted in the dark) and

c) germination, in light, at 20 to 25˚C

will deliver c90% germination.

 

Calluna, Daboecia & Erica

These three genera tend to occur in temperate climates and all have species native to the UK - where they principally occur in moorland and heathland. The largest genus – Erica – has a wide geographic distribution with a centre of diversity in South Africa chiefly in Mediterranean climate conditions (Fynbos).

 

The fruit is a dry, five-valved, locular capsule which can be readily collected and extracted in the late summer to autumn. Extraction of the seed can be achieved by crushing and shaking but usually by drying in a paper bag and then shaking to liberate the seed and so produce a sufficiently clean sample.

 

The South African species, which are native to areas subject to wild fire, will germinate much more effectively if the seed is soaked in a solution of commercial ‘liquid smoke’ prior to sowing although this is more practically achieved if the imbibed, sown, seed is watered in with the material.

 

The received wisdom suggests that there are no dormancy constraints to germination in the cold temperate Eurasian species and that they will germinate successfully at suitable temperatures with no pretreatment; however the germination response for these species will also benefit from smoke treatment and will promote a greater proportion of seedlings to emerge.

 

As would be anticipated, light is necessary for speedy and synchronised emergence.

 

Bryanthus and Elliottia, Epigaea, Kalmia, Kalmiopsis, Phyllodoce and Rhodothamnus

This group of closely related genera - each of which has only a few species or is monotypic; are all native to temperate to cold temperate climates in the Northern Hemisphere. They all produce dry fruiting capsules in the autumn, these should be collected prior to dehiscence and dried in a paper bag and then shaken to release the seeds – this produces a relatively clean and usable sample. Sometimes capsules persist until the following autumn but the current season’s crop should be collected and only fresh seed used.

 

All the species will show some germination without pre-treatment; however there is widespread evidence and much practical experience that demonstrates that a chill period of 35 days at 3˚C enhances the speed of germination and increases the number of seedlings and the synchronisation of seedling emergence at 20˚C.

 

Rhododendron

This very large genus, with a virtually worldwide distribution, consists of around one thousand species which vary in size from substantial trees to procumbent shrubs. They can be evergreen or deciduous and terrestrial or epiphytic. There are very many ornamental species which are suitable for cultivation in temperate climates.

 

They produce dwarf seeds which are generally quite small and these are produced in dry, locular capsules.

 

They do not exhibit any dormancy controls although there are some cold temperate species which show improved germination after chilling.

 

The dry (brown) capsules are collected just prior to dehiscence; drying is continued in a paper bag (of a relevant size) until the seeds can be liberated by shaking - when a clean sample will be produced.

 

Germination is generally most prolific at 22-25˚C, light is required for a least part of the day – there will be no germination in continuous darkness.

 

This genus is generally obligate in relation to the development of a mycorrhizal association.

 

Styphelioideae

Epacris, Cyathodes & Leucopogon

These three genera mostly have a tropical to warm temperate distribution – the relevant, sufficiently hardy, species suitable for cultivation in the UK are mainly found in the Antipodes. They are closely related and previously were attributed to their own Family – Epacridaceae.

 

They are evergreen generally small, low growing, heath-like shrubs. The fruits of Cyathodes and Leucopogon are colourful berries while those of Epacris are five-valved capsules containing numerous seeds.

 

The brown-black seeds are small and the available reports on successful germination tend to be eccentric and erratic.

 

In the absence of any immediate knowledge or experience of the germination of the seeds of this group of plants, the only general trends to be extracted from the available reports suggests that the seeds of all of these plants benefit from  a period of warm stratification and those at the temperate extreme from a short period of chilling.

 

Vaccinioideae

Andromeda & Zenobia

These two monotypic genera are both natives to damp bog type conditions – Zenobia pulverulenta limited to the Eastern USA (North and South Carolina and Virginia) and Andromeda polifolia with an almost circumpolar distribution in the northern parts of the Northern Hemisphere. Both are evergreen shrubs although in cold conditions Zenobia may be semi-evergreen.

 

The fruiting bodies are brown, five locular, dry capsules. Germination occurs most effectively in the range 18-22˚C although the speed and synchronisation of emergence was improved in Zenobia by chilling for 56 days at 3˚C; Andromeda shows little response to chilling other than to speed emergence by a couple of days.

 

Chamaedaphne, Gaultheria, Leucothoe & Pernettya

The Leather Leaf is the sole species of the genus Chamaedaphne (C. calyculata). It has a nearly circumpolar distribution in northern latitudes of the Northern Hemisphere. It is a small wiry shrub with white flowers. The fruits are round, dry capsules which are developed in the late summer.

 

Fresh seed should be used and is sown in the usual way, imbibed in the warm and then chilled for 28 days at 3˚C before germinating at 20˚C.

 

The genus Gaultheria contains about 170 to 180 species which are found in Asia and North and South America. They tend to be small shrubs – often low growing or prostrate - although Salal (G. shallon) from the Western fringes of the Snited States is an upright medium sized shrub. In Australasia the genus is currently separated as Pernettya but there is apparently no meaningful reason for this distinction. They are chiefly evergreen and occur in temperate to sub-tropical climates.

 

The fruit is a drupe-like berry usually with a bright red skin but also white, blue, purple, pink and black. The fruits are edible.

 

The seeds are extracted by maceration. The seeds of all the species would appear to benefit from a short period (28 days) of warm stratification and those from a colder provenance exhibit a faster and more synchronised emergence after a period (35 days) of chilling: a few species from northerly provenances (eg G. procumbens) require a chill of as much as 63 days to induce an adequate level of emergence.

 

The genus Leucothoe is one of those genera (viz Wisteria and Magnolia) which have a disjunct distribution in Eastern Asia and the South Eastern United States – which purports to demonstrate the shift of Tectonic plates in the earth’s crust. They are evergreen shrubs of no more than medium stature which produce white flowers. Many of the temperate species are cultivated as ornamentals.

 

The fruit is a dry, valved capsule which contains many tiny, winged seeds - generally the viability is not high. There are no constraints to germination which will occur prolifically at 20˚+C.

 

Craibiodendron, Lyonia, Pieris and Oxydendrum

This group of relatively small genera produce fruits which are dry, five locular, more or les globular capsules.

 

The genus Craibiodendron consists of about five species of relatively large, evergreen trees occurring in China and Vietnam, which are not often seen in cultivation in the UK. The capsules contain fewer ‘large’ seeds than most of the Family.

 

The genera Lyonia and Pieris are evergreen shrubs from Eastern Asia and North America. The capsules contain many small seeds – the viability of which is often low, it is therefore necessary to grade the seed - selecting plump seed for processing.  There are no constraints to germination, which will occur satisfactorily at 20˚C.

 

Oxydendrum is a monotypic, deciduous, genus from eastern North America. It is a native of wet places and it requires damp conditions for good germination – which occurs most prolifically at 20-25˚C.

 

Agapetes, Gaylussacia & Vaccinium

These three closely related genera all produce edible (in some degree) berry-like fruits which include the Blueberries, Bilberries, Huckleberries etc. Many species are native to cold and cold temperate climates and are cultivated extensively as both ornamentals and fruit crops. However many are also native to tropical and sub-tropical areas.

 

As all have similar berries the extraction of the seeds is the same ie maceration of the ripe fruits in running water so that the detritus is floated off.

 

Agapetes (aka Pentapterygium) – contains about ten species from the Himalayas. They are scrambling, evergreen shrubs with thin and sparse branches; they mostly occur in warm temperate climates  but the spectacular, almost hardy, A. serpens can be grown outside in mild areas in the UK, although it is  more satisfactorily grown in a conservatory.

 

The fruits – fleshy berries - are edible but with a bland flavour. Good germination is best achieved with fresh seeds – which are quite tiny. Emergence at 20˚C will happen but erratically - better synchronisation and rapidity follows a brief chill of 35 days at 3˚C.

 

The genus Vaccinium consists of about 450 species which are distributed chiefly in temperate climates of the Northern Hemisphere although some species are found in places as widely divergent as the Azores, Madagascar and Hawaii. They are chiefly large bushy to prostrate or dwarf shrubs and are evergreen or deciduous.

 

The edible fruits (Blueberries, Bilberries, Cranberries, Whortleberries etc) are brightly coloured berries with distinct flavours and are used for a variety of domestic products.

 

Treatments to achieve the best synchronisation and speed of emergence are particular to individual species.

The seeds of northern species require chilling for 12 wks at 3˚C and a high threshold temperature for germination (20˚.C)

 

Gaylussacia

The Huckleberries are very closely related to Vaccinium and consist of about 50 species – 8 occur in the Eastern United States (mostly in temperate climates), 7 occur in the Andes and the remaining 35 species are found in the mountains of South Eastern Brazil. They can be deciduous or evergreen.

 

They are distinguished from Vaccinium chiefly by the development of an ovary which contains ten ovules and hence the berries contain ten seeds (as opposed to five). The seeds are relatively large, bone-coloured nutlets.

 

The fruits are fleshy, juicy berries but the number and size of the seeds limit interest as an edible commodity. The seeds are extracted by maceration and usually exhibit a high viability.

 

The bony endocarp, although ‘hard’, is not impermeable to water and the seeds will imbibe. However they still require a warm stratification of 30 days at 20˚C to ‘mature’ seed before chilling for 60 days at 3˚C. Germination is then prolific at 20˚C.