Davidiaceae

 

Taxonomically the genus Davidia has proved difficult to place satisfactorily – it has usually been assigned to the Cornaceae, sometimes to the Nyssaceae but often nowadays to its own Family (although not unanimously), the Davidiaceae.

 

Davidia

 

This (monotypic) genus of medium sized trees is prized for the prolific production of its spectacular flower bracts. It is a native of Yunnan in the South West of China and its introduction to cultivation is the stuff of horticultural legend. It has two variants which are sometimes regarded as separate species – on the basis that they have differing chromosome counts – however the only apparent significant difference morphologically is that one has pubescent leaves (vilmoriniana) and the other has leaves without hairs (laeta). The (variously called) Dove Tree, Pocket Handkerchief Tree or Ghost Tree (Davidia involucrata) is usually propagated from seed – despite its relatively complex dormancy control mechanisms.

 

The fruit of the Davidia is an oval to marginally pear shaped, drupaceous, fleshy structure usually about four centimetres long and three centimetres in diameter, with a longish slender stalk - about the size of a small walnut. It has a strong khaki coloured skin – covered in a pale purple bloom when fresh. It contains a multilocular (longitudinally radial), hard coated, ridged nut containing from two to five seeds.

 

The fruits should be collected when ripe (in the early autumn) although they will usually persist on the tree well after leaf fall into the winter. The nuts are extracted from the skin and fleshy part of the fruit by softening or macerating the pulp in a small amount of water and leaving it to ferment in a polythene bag for a couple of weeks in a warm place. The nuts can then be washed clean.

 

The nuts can be stored dry, in refrigerated conditions for several years and will retain high levels of viability.

 

Germination is initially inhibited by the presence of an extremely hard endocarp (the nut case). This must become sufficiently degraded to allow the seed to imbibe - for germination to proceed. The weak point in the structure of the endocarp is at the suture causing the ridging on the nut, when this has been adequately reduced and the embryo has fully imbibed the power of the expanding radicle will force up the cap of the locule in a hinge like fashion. The only experience to date to achieve this action is to subject the nuts to a warm, moist stratification procedure. How long this will take is very variable but it will reasonably be assumed that the more effective the degrading process (endocarp decomposition) the quicker will imbibition occur. This process might well be accelerated by including a microbial compost activator in the stratification medium and the maintenance of a reasonably high temperature (20 to 25ºC). It would therefore be prudent not to dry the nut after it has been extracted – avoiding any enhancement of its hardness – but to transfer it directly into the warm stratification treatment.

 

Radicle emergence will occur, without further treatment (so the stratification process should be monitored regularly to determine progress) once the embryo has imbibed; however the embryo is now constrained by an epicotyl dormancy condition which is mitigated by a period of chilling.

 

Before further treatment it would be prudent to separate the embryos with their emerged radicles from the fruit case – as the fruit contains from two to five seeds and all may well germinate at once.

 

The separated seedlings are now transferred to a cold temperature, to provide the necessary chill to allow the plumule to develop - when it is subsequently exposed to a warm temperature. The chilling process, when conducted under artificial conditions will require the seedlings to be extended in a suitable, moist medium.

 

There is little reliable evidence for the timing of the various periods of seed treatment. The warm stratification may well require 90 days to achieve imbibition and radicle emergence but this will depend on the degrading agencies provided and the temperature.

 

The chilling period has similarly not been critically determined but the local provenance of this species suggests that it would not exceed 63 days below 5ºC.

 

Although the occurrence of several seeds in the one nut suggests that seedlings will emerge over several years, under natural conditions - the object of the propagator will be to synchronise emergence as far as is possible and achieve maximum productivity.

 

The seedlings will develop when the fully chilled embryo is exposed to warm temperature (20ºC) and, again, how long this process will take to emergence has not been the subject of critical observation.

 

Seedling growth in the maiden year (if germination is achieved in the Spring) is reasonable and seedlings of 50cm tall should be produced by leaf fall.

 

The required programme for achieving reproducible results will initially be developed by standardising a suitable recipe for the warm stratification medium and combining this with a standard temperature. Once this has been determined the length of the chilling period can then be assessed.

 

The blueprint for production will then be derived from the time required:-

a) to harvest and clean the fruits,

b) to warm stratify to radicle emergence,

c) the chilling period and

d) the period of warmth to plumule development.

The total period required, when progressing the process artificially, should permit emergence in the Spring following seed collection.

 

It may well be that an early collection of the fruits will arrest the development of the hard seed coat and immediate extraction and processing allow a much shorter time scale before radicle emergence.