Hippocastanaceae

 

This small Family is another which is nowadays often included in the Sapindaceae, on the basis of DNA analysis and assessment, but debate, among taxonomists, continues as to whether it should remain as a separate entity. It contains three genera, Aesculus with a circumpolar distribution in the temperate climates of the Northern Hemisphere, Billia – from sub-tropical and tropical America and the Chinese, monotypic Handeliodendron; of these only Aesculus has a temperate distribution.

 

Aesculus

 

The most satisfactory practical and economic method of producing the species of the horse chestnuts and buckeyes (Aesculus), and their naturally occurring variants, is undoubtedly from seed. This production process should ensure normal and realistic growth patterns and the production of typically developing specimens – without any of the rootstock influences which can be encountered if grafting is employed.

 

Grafting should only be employed where the production of particular clonally propagated individuals is required. Reasonable experience has shown that the most satisfactory grafted plants will be produced by using a rootstock of a closely related species with similar growth patterns and rate of growth. Fortunately the wood in the hypocotyl/stem region of an Aesculus seedling is soft and even, cuts readily and makes an efficient union if the carpentry and environmental aspects are dealt with effectively.

 

The trees are characterised by their substantial compound palmate leaves. Mature trees produce an inflorescence which is an erect panicle described as a ‘candle’ - during late spring or early summer. The fruit is a largish capsule, which can be spiny, warty or smooth surfaced; it may contain up to three seeds which, if there is more than one in a capsule will have flattened sides. The capsules split into three, during the autumn, to release the seeds which fall to the ground. The seed coat is brown (different shades according to species) with the characteristic roundish pale scar like hilum.

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The production of seedlings is a relatively uncomplicated procedure as none of the species develop any difficult or complex dormancy problems and the seeds are big enough to handle individually. The only technical issues involved are associated with maintaining the viability of the seed until it is sown and in the sowing process itself - to ensure the development of an acceptable proportion of vigorous, straight, upright seedlings.

 

Seeds of all the species mature in the late summer to autumn and are shed when the fruit capsule splits open and liberates the seed. The seeds can then be readily picked up from the ground.

 

Not all of the species experience sufficiently warm and bright enough summers in the UK to produce seeds with a fully developed food reserve. This condition, however, is often associated with young trees which are still growing vigorously and producing extensive vegetative growth – as the tree matures and annual growth rates decline more food can consequently be diverted into seed production.

 

The viability of all species is limited to a short term and under natural conditions rarely exceeds six to eight months.

 

At dispersal the seeds have a relatively high water content (about 50% on a fresh weight basis) and this internal moisture is conserved by the shiny, waxy surface coating present on the seed. However once this deteriorates the seed coat is not efficient at preventing water loss.

 

Food reserves in the seed are mainly of non-carbohydrate materials. These are chiefly fats, waxes and protein and when these lose water, by the drying out of the seed, they will not rehydrate successfully (because these substances are naturally water repellent). Thus the seeds cannot be reconstituted by re-wetting and this therefore makes these food materials unavailable to the embryo and dramatically influences the potential germination of the seed.

 

The first essential therefore in the successful production of a crop of seedlings is to collect the seed as soon as it is shed and maintain it under conditions which will reduce water loss to a minimum – ie maintain the water status as at dispersal. In the short term this can be achieved by placing the seeds in a sealed polythene bag. However seeds of this size when kept in bulk will soon heat up and degenerate rapidly unless kept cool – hence the sample should be transferred to cold storage (c3ºC) as soon as possible. To prevent the potential for heat build up as a result of metabolic activity the seed would be best stored in small units.

 

Successful storage therefore is dependent on the collection of the seeds immediately they are shed, conserving their moisture content and transferring them quickly to cold temperature storage. Under these conditions useful viability will be maintained for six to nine months and usually for as much as a year. This scenario is dependent on understanding that these optimal storage conditions will only be effective in reducing the rate of degeneration on the initial viability of the sample when it enters the storage regime.

 

Before storing the seed it would be prudent to surface sterilise the seed with a mild antiseptic (such as hypochlorite) in order to prevent the development of fungi (especially Powdery Mildew) externally on the seed coat.

 

The most reliable guide to the internal viability of a sample of seed is shown by the condition of the external surface of the seed coat – while it still has a shiny and polished appearance viability is normally high – thus the internal reduction of viability is progressively associated with a loss of shine to a dull but still, taut appearance and then the development of a wrinkled seed coat. Internally the deterioration can be seen by cutting the seed in half and observing the level of the development of brown necrotic spots which become progressively more invasive until they can be seen within the embryo tissues.

 

Although many species require a short chill to break embryo dormancy this is normally provided by the above storage regime and seeds can be satisfactorily sown in the early Spring – if so required.

 

The seeds are normally sown in open ground seedbeds in the autumn. However the seeds of those species which germinate almost immediately – such as A. californica, which germinates normally and A. parviflora, which produces a radicle but has a marginal epicotyl dormancy - will need to be provided with protection over the winter as the young shoots are not hardy.

 

If the seeds are variable in size (they usually are) it is as well to grade them so that the small sizes can be discarded or sown separately - as the size of the seed will govern the size (grade) of the ultimate seedling. If plenty of seed is available it would be as well to grade the seed before storage.

 

As the seeds are relatively large they can be individually station sown at a predetermined density with an expectation of maximum productivity. They can, of course, be sown individually in suitable sized containers.

 

The seeds will be sown at a density compatible with the expected size of the seedling and the productive use of the seedbed – large seeds (eg A. californica) at 15cm x 15cm (40/m²) and down to 7cm x 7cm (200/m²) for the small seeded types (eg A. sylvatica).

 

The depth of sowing should be sufficient to prevent the seed drying – even if the surface layer of soil dries out.

Thus the seeds should be sown with at least their own depth of soil covering them.

 

Topping off the seedbed with a thin layer of grit helps to prevent drying of the sowing medium and mitigates any surface panning - as well as easing the removal of weeds.

 

Many species of trees which produce large nut-like seeds develop unsatisfactory seedlings if they are not sown with their correct polarity – ie ‘the right way up’.

 

In those species of Aesculus which have been assessed the position in which the seed is sown can have a significant effect on the eventual size of the maiden seedling. Seeds sown with the hilum scar underneath produce seedlings with the earliest emergence and the straightest upright axis through the shoot, hypocotyl and initial tap root – by the end of the season these have developed into the tallest, most substantial, grade of seedlings.

 

Table 1

A trial to determine if the polarity/orientation of seeds of A.x carnea at sowing subsequently influenced the size and quality of the seedling.

 

Autumn sown 1976 – assessed at leaf fall 1977

 

Position of the hilum  Total  Development of stem/hypocotyl

scar when sown         sown      Straight    Angled/bent

 

Scar on top              84      65 (76%)     20 (24%)

 

Scar underneath         112     110 (99%)*     2  (1%)

 

Scar to the side        134     124 (93%)     10  (7%)

 

* This group not only produced the ‘straightest’ seedlings on the root/hypocotyl/shoot axis but also developed a significantly greater stem and root length.

 

 

Horse Chestnuts and Buckeyes

- occurrence and potential for seed production in the UK

 

The genus consists of twelve to twenty species (depending on the taxonomic distinctions attributed to particular variations). It has a circumpolar distribution throughout the Northern Hemisphere in temperate climate regions.

 

The eastern American A. glabra, A. octandra, A. pavia and A. sylvatica, if collected from pure stands, develop true to type. They are best obtained from relevant sources in native stands and dispatched, by air, immediately. None of these set reliable seed crops in the UK largely because the summers are rarely hot and bright enough. However as more trees mature in British collections and summers ‘improve’ it is possible that local supplies may become more satisfactory. A. glabra has an extensive clinal distribution westward and at the farthest extreme is known as A. arguta.

 

Aesculus californica is a native of California in areas with hot, bright, dry summers but normally by water courses; it produces huge seeds and really does not consistently set satisfactory crops in the UK.

 

Similarly A. parviflora, which is from the mid western United States, experiences a continental type of climate and crucially flowers in late summer (August). It has only set satisfactory seeds in the UK in hot, late summers such as 1959 and 1975.

 

Aesculus indica, which has been in cultivation since 1851, is a not uncommon tree in Parks and the larger Gardens and generally sets seed regularly - although in dull summers the crops tend to be of poor size and quality. Its local relative - the Himalayan A. assamica was rarely encountered in cultivation but seed collections within the last forty years have been planted and although most are still ‘young’ (having been derived from these relatively recent introductions from the Himalayas) they are beginning to produce good crops of seed. Very similar to A. assamica (and taxonomically often lumped with it) is A. wangii from the other side of the Himalayas in southern China and Vietnam. This plant has only been introduced in very recent times and as yet has not reached a seed bearing maturity.

 

Of the Chinese species A. chinensis has been in cultivation for a long time but has rarely flourished - seed has sometimes been produced (at Kew) but this is not usual.     However A. wilsonii, which has also been around for a hundred years, does produce good seed in quantity - at least in Cornwall. Both of these species are nowadays subsumed into a portmanteau A. chinensis by Chinese botanists who attribute a broad clinal variation in the wide geographic spectrum of distribution.  Other Chinese species are little known and any trees in cultivation are very new.

 

The ubiquitous Horse Chestnut (A. hippocastanum) was introduced to the UK in the seventeenth century and is readily recognised, however its use as a rootstock for virtually all other grafted Aesculus has not proved the best option for successful mature tree development. Although satisfactory for its own variants, other hybrids and variants should be grafted onto much more closely related species which have similar and relevant growth patterns.

 

The Japanese A. turbinata is very similar in appearance to the European Horse Chestnut and is occasionally seen in cultivation. It has the largest leaves found in the genus. Some majestic trees can be found in cultivation - which produce seed freely.

 

The hybrid A. x carnea (A.hippocastanum x A. pavia) is (unusually) tetraploid, it develops viable seed and produces offspring which are effectively identical to the parent. It sets seed, often prolifically, in the UK.

 

Aesculus parryi (from northern Mexico) is probably not in cultivation in the UK and in any case would be unlikely to set good seed even in a hot summer.