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During the evolutionary course of plant development, the design of seeds has become as diverse and varied as any other group of living creatures. While all seeds have been created with the double purpose of first protecting the embryo within, and then enabling the juvenile seedling to emerge at the most opportune time for optimum growth, the way that some plant species achieve this is akin to a Chinese puzzle box. Without the required periods of dry, cold, wet or heat the protective seed coat will not allow the embryonic seed within to break out into life. However, some seeds will still refuse to germinate! This is known as seed dormancy and - simply put - seed dormancy can be defined as the failure of mature, intact seeds to germinate
under favourable conditions.
So secure is this method of protection offered by some seeds that without the correct environmental responses they can remain dormant for hundreds of years. In fact, ancient magnolia seed retrieved from a Japanese tomb have been germinated after a period of some 2000 years!
While some seeds are extremely easy to germinate, others are clearly not and if somebody wished to bypass the enforced dormancy that many seed coats offer a certain amount of work is needed. Below is a list of common techniques used in bypassing the dormancy process.
Some seeds, e.g. Sweet peas, Ipomoea etc. have hard seed coats which prevent moisture being absorbed by the seed. All that is needed is for the outer surface to be scratched or abraded to allow water to pass through. This can be achieved by chipping the seed with a sharp knife at a part furthest away from the 'eye', by rubbing lightly with emery paper or, with very small seed, pricking carefully once with a needle etc.
In some packet seed instructions you will find a reference to 'pre-chilling'. This is a pre-treatment of the seed which often helps to speed up the germination of otherwise slow to germinate seeds. However, even after pre-chilling some seeds can stubbornly refuse to germinate until a year or more has passed, so never be too hasty in discarding a seed container.
Pre-chilling was traditionally done by standing the pots outside in a cold frame during the winter. It is often quicker to adopt the following technique using a domestic refrigerator and this is of particular value if you obtain your seed outside the winter months.
To pre-chill seeds, first sow the seed on moistened seed compost, seal the seed container inside a polythene bag and leave at 60-65F (15-18C) for 3 days then place in a refrigerator for the recommended period. For convenience large seeds can be mixed with 2-3 times their volume of damp seed compost, placed direct into a polythene bag which is sealed and placed in the refrigerator. However, there must always be sufficient air inside the bag and the compost should NEVER become either too dry or over wet. After pre-chilling these seeds can then be spread with the compost on top of a seed container and firmed down.
The seeds must be moist whilst being pre-chilled, but it will harm them if they are actually in water. During the period in the refrigerator, examine the seeds once a week and remove all the seeds into the specified warm conditions if any of them start to germinate.
Light also seems to be beneficial after pre-chilling, so pre-chilled seeds should have only the lightest covering of compost, if any is required, and the seed trays or pots, should be in the light and not covered in paper.
Soaking is beneficial in two ways; it can soften a hard seed coat and also leach out any chemical inhibitors in the seed which may prevent germination. Anything from 1-3 hours in water which starts off hand hot is usually sufficient. If soaking for longer the water should be changed daily. Seeds of some species swell up when they are soaked. If some seeds of a batch do swell within 24 hours they should be planted immediately and the remainder pricked gently with a pin and returned to soak. As each seed swells it should be removed and sown before it has time to dry out.
Some seeds have a combination of dormancies and each one has to be broken in turn and in the right sequence before germination can take place. For example some Lilies, Tree paeonies, Daphne etc. need a warm period during which the root develops followed by a cold period to break dormancy of the shoots, before the seedling actually emerges. Some seeds need a cold period followed by a warm period and then another cold period before they will germinate.