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Thompson & Morgan®: Seed Germination Guide

Special Treatment of Seeds before Sowing
Some seeds benefit from pre-treatment before sowing or from being sown in a particular way. Here are brief explanations of the techniques mentioned in the following list. In almost all cases it is not critical that you carry out this pre-treatment but if it is not done the seeds will usually take longer to germinate.


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.


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.


In some 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, 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 while 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.

Double Dormancy

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. In all cases the times and temperatures have been provided in the sowing instructions.

Outdoor Treatment

The above mentioned methods accelerate the germination process and help to prevent seeds being lost due to external hazards (mice, disease etc.) but outdoor sowing is just as effective albeit longer. The seeds are best sown in containers of free draining compost and placed in a cold frame or plunged up to their rim outdoors in a shaded part of the garden, preferably on the north side of the house avoiding cold drying winds and strong sun.

Recent tests show that much of the beneficial effects of pre-chilling are lost if the seed is not exposed to light immediately afterwards. We therefore recommend sowing the seeds very close to the surface of the soil and covering the container with a sheet of glass. An alternative method especially with larger seeds, is to sow the seed in well prepared ground, cover with a jam jar and press this down well into the soil so that the seeds are enclosed and safe from predators, drying out etc.

Damping Off

A soil borne disease which attacks seedlings as they germinate, causing them to collapse. It is worse in warm, moist, muggy conditions or where seedlings are sown too thickly. Some plants are especially prone but it is best to take precautions every time you sow. Hygiene at all stages of propagation is essential, the disease is carried in soil and through water stored in the greenhouse. Use only clean pots and seed trays and make sure the greenhouse benches are sterile. Mains water and a proprietary sterilised seed compost which is moist but not over wet should also be used. Sow thinly and prick out as soon as possible, handling the seedlings by their leaves, not stems. Cheshunt Compound or a similar fungicide can be added to the water once a week provided the seedlings are not susceptible. It often affects only part of the seed tray, if this occurs remove the clump of affected seedlings and water with Cheshunt Compound.

Sowing In Outdoor Situations

Where outdoor sowing is recommended, planting in a moist soil which is weed free and has been raked down to a fine tilth is essential. For hardy annuals and perennials sowing can be carried out from late winter onwards as soon as the ground is workable and has warmed up; half hardy annuals after all danger of frost is passed and when the soil has warmed up. Full details can be found on the separate page Sowing Outdoors.

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