Trees and shrubs

that neutralizes the inhibitor chemical and so allows germination to proceed as soon as environmental conditions are suitable.

If only one of these kinds of dormancy occurred in a seed then overcoming it, although a problem, would be comparatively simple. Many plants, however, exhibit combinations of these dormancy controls so that overcoming them is complicated and time-consuming (see pages 32-3).

Seeds that are obtained from sources other than the gardener's own collections are almost invariably dried; the process of ripening is fully complete and all the dormancy controls are inbuilt, so germination cannot proceed until these problems have been eliminated.

However, for the gardener who is collecting his own seed it is possible to avoid the development of dormancy by collecting immature seed and preventing further drying. To do this, collect the seed when it is green to yellow to buff coloured, and fruit as it just turns yellow. At this stage the seedcoat and fruit are beginning to dry out and so develop

DORMANCY TABLES

Seeds with a hard seedcoat

Seeds having a combination of a hard seedcoat

Acacia

and a chilling requirement

Caragana

Cornus

Colutea

Cotoneaster m

Cytisus

Daphne

Cleditsia

Hornbeam (Carpinus)

Koelreuteria

Magnolia

Laburnum

Maples—Field and Snakebarks '

Robinia

Roses {Rosa)

Ulex

Thorn (Crataegus)

Wisteria

Viburnum

Yew (Taxus)

Seeds requiring chilling

Alder (Alnus)

Seeds exhibiting no dormancy

Apple and Pear

conditions

Barberry (Berberis)

Catalpa

Beech (Fagus)

Ceanothus

Cherry and Plum

Eucalyptus

Clematis

Mulberry (Morus)

Euonymus

Poplar (Populus)

Horse chestnut (Aesculus)

Yucca

Maples—Norway and Sycamore

Oak (Quercus)

Seeds having a hard seedcoat, an immature

Sweet chestnut (Castanea)

embryo and requiring chilling

Vines (Ampélopsis; Parthenocissus; Vitis)

Ash (Fraxinus excelsior)

Walnut (Juglans)

Holly (Ilex aquifolium)

take a long time, or has to attempt to overcome the problem artificially, which may be difficult and complex.

In woody plants there are three different kinds of dormancy.

The simplest is that caused by the seed-* coat, which has thickened and hardened during the maturation of the seed. Its hardness stops water being taken up by the seed; therefore the embryo cannot imbibe and germination is prevented. In nature, this kind of dormancy is gradually reduced by bacteria and fungi in the soil decomposing the seed-coat until it is no longer effective and water can be taken up.

Dormancy may also be caused by an immature embryo, which requires a warm temperature in which to develop to a stage where germination can proceed.

The commonest form of dormancy in the seeds of plants from temperate climates is a chemical inhibition to the embryo development. In nature, this dormancy is broken by normal exposure of a seed in the soil to winter's cold. This initiates a chain of events into the condition suitable for dispersal. It would appear that the dormancy controls develop at this stage.

By collecting the seed when it is anatomically complete with its food reserves finished but before dormancy becomes built in, the hard seedcoat is avoided and the chilling requirement is at a minimum (just enough to prevent germination until the spring). Thus for germination to occur in spring, the fruits of Daphne mezereum should be collected not in September but in early June, when they are small, hard and green. Complicated dormancy patterns which take a long time to overcome or require a complex artificial procedure can then be avoided. However, it is easy to gather seed that is too immature, which will prove disastrous.

Once dormancy conditions are removed, the seed will germinate, provided that suitable conditions are maintained. A change in conditions, such as excessive heat or drying or a continued shortage of oxygen, will cause the development of secondary dormancy, which is extremely difficult to break down.

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