Aquifoliaceae

 

Ilex

 

The Hollies (Ilex) are a large genus of between 400 and 600 species. They have a world wide distribution but are found chiefly in the sub-tropics (where 300 or so of the species occur) and the tropics. This single genus is the only remaining representative of the Family Aquifoliaceae in the present day. The Family is very old and appears early on in the evolutionary Angiosperm tree. This fact is given credence by the structure of the seeds which, at dispersal, have only a rudimentary embryo of a primitive amorphous constitution.

 

The Family contains trees, shrubs and climbers and can be either evergreen or deciduous. The evergreen leaves are invariably glossy, long-lived and ‘tough’, the margins often serrated or with spines.  The plants are dioecious. The flowers have four petals, are usually white and are small and inconspicuous, they are generally insect pollinated. The prominent fruits are drupes (usually described as berries) which have a highly coloured skin – red, yellow, brown, blue, purple or black and these contain usually four seeds, each with a very hard seed coat. The fruits of the temperate climate species usually mature during the winter.

 

Dispersal of the seeds is achieved by various animals who take the berries as a food resource. The chief vectors are birds (in the UK – Mistle Thrush, Fieldfare and Redwing – although not exclusively) they take them from the tree or bush but also rodents who pick up off the ground. This accounts for the hard seed coat condition which protects the seed in the passage through the gut; presumably in pre-history the vector had a strong digestive process which caused the development of the extreme nature of the condition of the seed coat. The berries are poisonous to humans but many animals are capable of dealing with or tolerating the toxins.

 

The Common Holly (Ilex aquifolium) has an extensive distribution in the Northern Hemisphere and is native from the western seaboard of Europe across Eurasia to China and also occurring in North Africa. It is the only species native to the British Isles.

 

There are some 30 or 40 species with a sufficiently hardy temperate climate provenance which are possibly suitable for cultivation in the UK. Some of the most commonly seen are:- the Chinese I. cornuta, the Japanese I. crenata and I. latifolia, the Inkberry - I. glabra, the American Holly - I. opaca and the deciduous I. verticillata all from East and Central North America.

 

Most of the literature, other posted comments and the perceived wisdom seem to give up on the controlled seed treatment and germination of holly seeds – indicating that there is little more to do other than sow the seed in an open ground seedbed, leave the elements and the seasons to do their stuff and wait for at least two or three years to get a partial germination at best!

 

However it would be possible to hasten the process and achieve a productive result if the nature of the problem is analysed and understood. Certainly the various contributory factors individually can require a lengthy resolution but the proposed treatments can each be reduced to the required minimum for the elimination of the particular constraint and then transferred to the next treatment in the correct succession without delay.

 

Holly species from temperate climates have three conditions which control the sequence of events to germination:–

a) a very hard seed coat (which will have developed significantly when the fruit is mature),

b) at the dispersal stage it still has an underdeveloped (immature) embryo which cannot be developed until the previous condition has been dealt with and imbibition is possible and

c) when the embryo has been developed and matured it has an endogenous dormancy which will require a period of chilling to mitigate.

 

The following practical implications can be drawn from known experience and the available evidence:-

a) collect the seeds as early as possible to reduce the level of hard seed coat development;

b) extract the seeds from the unripe fruits;

c) treat the sample with a 2% citric acid solution for 5 days;

d) warm stratify (@20˚+C) for about 60 days - to continue to reduce this condition and reach water permeability and hence the pathway for imbibition.

e) continue the warm stratification of the now imbibed seed to mature the embryo (@ 20˚+C) to a level at which it is capable of germination – at dispersal the embryo consists of an amorphous aggregation of cells which have to develop to maturity both morphologically and physiologically – hence this period is relatively substantial – about 70 days.

 

(This warm stratification can effectively be merged into one treatment. This allows a routine to be established in order to monitor progress and ensure that moisture does not become limiting or that oxygen becomes deficient. Eventually when sufficient time has elapsed it will be necessary to make a regular assessment of the seeds by a cut test. This will provide an indication of the development of the embryo so that the end of the process can be recognised.)

 

f) once the stage of embryo maturity is reached it requires a chill to mitigate the endogenous embryo dormancy – a period of 90 days at 3˚C is sufficient.

g) transfer to warm temperature (20˚C) to achieve an enhancement of the rate of germination.