Cornus

 

The Dogwoods (Cornus) are a relatively small genus of some thirty to sixty species. They are natives of the Northern Hemisphere chiefly in temperate climates - with a preponderance of species occurring in Eastern Asia. The divergence in the number of species cited indicates the uncertainties currently existing in the taxonomic opinions which determine the rank of many of the entities described. The number of species is broadly divided into four sub-genera (although this is also a matter of debate, as is the opinion that they constitute separate genera). All the species are more or less suitable for cultivation somewhere in the UK – although not necessarily producing fruit successfully or consistently – especially those from high light intensity, continental climate summers.

 

The four groupings (although there are some authorities which cite six) are:-

a) Benthamidia is a small grouping which contains the species usually described as Flowering Dogwoods and includes C. capitata, C. florida, C. kousa and C. nuttallii – these are generally summer flowering, large shrubs to small trees, with conspicuous bracts subtending the flowers;

b) Chamaepericlymenum consists of the two species of Dwarf cornels – C. canadensis and C. suecica – these are very similar to each other and are low growing, creeping, ground cover subshrubs;

c) Cornus is a small group of species which are described as the Cornels – typified by the Cornelian Cherry (C. mas) and including C. chinensis, C. officinalis and C. sessilis – all with small yellow flowers produced in profusion during the late winter before the leaves and

d) Swida contains the shrubby, sometimes creeping, dogwoods often with coloured bark, this is the largest grouping which includes C. alba, C. sericea (which includes C. stolonifera) and C. sanguinea as well as the alternate leaved, larger growing C. alternifolia and C. controversa.

 

The fruit, in this genus, is a one or two seeded fleshy drupe; the flesh of which contains an inhibitor to germination. In nature the pulp of this fruit is usually removed as the fruit is digested during passage through the gut of a bird. The seed is protected by a hard seed coat which is sufficiently resistant to the digestive processes and so protects the seed tissues during this passage, but, as a result, the coat is sufficiently reduced to allow the pregermination processes to continue on expulsion - as water permeability is no longer impeded. When the seed is treated artificially to remove the pulp, the seed coat is not reduced by the digestive activity and in many species the seed coat remains impermeable to water. When dealing with mature fruits and seeds in which the seed coat is hardened, it will be necessary to degrade the condition for water permeability. This can be achieved by warm stratification, scarification or inorganic acid digestion.

 

Where it is feasible, it is therefore prudent to collect fruits well before the ripening process is complete – ie when they are just changing colour from green. The seeds can then be extracted and processed into the treatment protocol without allowing any drying to occur (which may induce development of a hard seed coat in some degree). It is unlikely that any significant seed coat impermeability will have developed by this stage.

 

Thus as a general rule of thumb - seeds, of any species in the genus Cornus,

a) should be collected before the fruit is completely mature in order to avoid hard seed coat development;

b) should be extracted from the fruits by maceration, fermentation, decanting the dross and washing the seeds thoroughly to remove all traces of the flesh;

c) process into the chilling phase (84 days at 3˚C) without delay and certainly avoid any drying as this may well cause hardening of the seed coat;

d) germinate at reasonably warm temperature (c18˚C)

 

Where only mature or stored seed is available, which has been extracted artificially, it is probable that some degree of warm stratification will be required to reduce the seed coat. This is especially true of the Cornels and Dwarf Cornels, which have a capacity to develop a hard seed coat of considerable effectiveness.

 

It would be prudent when dealing with the Cornels and Dwarf Cornels to i) harvest early and extract without allowing any further drying then ii) warm stratify the seeds for as much as 70 days before iii) transferring them to a chill of 3˚C for 84 days; they can then be germinated with an acceptable level of productivity at 18 to 20˚C.

 

C. capitata is from a warm temperate environment in the foothills of the Himalaya, it produces largish fleshy amorphous fruits – these are developed from the fusion of several drupes – so that they contain several seeds. The seeds are extracted by maceration of the fruits which are then allowed to ferment for a couple of days - after decanting the dross the seeds are washed to remove any traces of flesh. The seeds will then require a chill of 42 days at 3˚C without delay – so that drying is avoided. The seeds will then germinate readily and prolifically at 20˚C.

 

It would appear that all the species in the section Swida have the capacity to develop a hard seed coat in some degree – depending on the environment to which they are subjected and the generalised protocol suggested above should be used.

 

Alangium

 

This genus contains probably about twenty species of shrubs and small trees, which, principally, have a tropical and sub-tropical distribution. However several of the species have extremely extensive territories and, inevitably, are variable across this spectrum and there is, also, much debate about species determination. Two species have at least part of their provenance in temperate climates and are cultivated in the UK (albeit not commonly) – Alangium chinense which occurs from South East Asia to West Africa and A. platanifolium which is found from Eastern Asia to Russia and so both have an extensive geographical distribution as well. Neither is common in cultivation and it is difficult to identify each, with clarity, without experience and flowers and fruit. Although listed in nursery catalogues both of these species have probably been produced vegetatively (over generations) and it is doubtful if they produce seeds in cultivation in the UK.

 

The fruit is a drupe which contains a woody endocarp and this contains one seed. There is no evidence from the literature which suggests that there is any constraint to germination.