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Seed dormancy and how to overcome it

Seed dormancy

A seed is said to be dormant when it is unable to germinate, even though the environmental conditions are suitable.

Dormancy is a natural defence mechanism developed by the plant to ensure its survival, particularly at the seedling stage. It makes sure germination doesn’t take place when the environmental conditions are unsuitable (or about to become unsuitable) for the seedlings to survive. As gardeners, we can usually provide seeds with favourable conditions for germination all year round but even so, at times, we need to overcome dormancy to achieve germination when we want it.

Scarification

Scarification is used for seeds with a hard or waterproof outer seed coat. They need to have this covering damaged slightly to allow water and gases to enter the seed and trigger germination. This should be done before sowing and usually involves using an abrasive material, such as sandpaper, to wear away some of the outer seed coat.

A form of scarification known as “chitting” involves partially damaging the seed by using a knife to remove a small amount of the seed coat to allow water to enter into the seed.

A much safer way to chit the seed is by soaking it in a shallow container of warm water (so the seed is just covered) for up to 24 hours. This is particularly useful for seeds which have a waterproof layer, but only a thin seed coat which may result in the seed being damaged if a knife or similar implement is used. This is sometimes referred to as “warm water scarification”.

Stratification

The term “stratification” can be traced back to at least 1664, where seeds of forest trees were alternated between layers of moist soil. These stratified (layered) seeds were then exposed to winter conditions.

It is the process by which seeds are artificially exposed to certain conditions to encourage germination, a technique which mimics the conditions these seeds would naturally encounter in the wild.

Most commonly, this is a period of low temperatures similar to those experienced during the winter, used for seeds that require periods of cold temperatures to overcome dormancy. For these treatments to work, the seed must be kept moist or the chemical changes will not take place.

*Some seeds display multiple dormancies and only germinate in their second spring after periods of both cold and warmth, but these conditions must occur in a set sequence.

Cold stratification

Place the seed in a container, such a clear plastic bag, glass or metal container filled with moist - but not wet - peat or sand (for dark-coloured seed, use sand as it is easier to spot the seeds in sand than it is in brown peat). Label the contents clearly, including the date, and close but do not seal the bag. If you use a glass or metal container do not close the lid too tightly.

Place the container in a refrigerator at a temperature of 2-3C (36-38F) and chill for four to 20 weeks, depending on the species. The aim is to chill the seed, NOT freeze it. If the seed is frozen it will not speed up the process and may actually inhibit it.

Inspect the seed regularly, especially towards the end of the allotted time, and sow the seed immediately at the first sign of germination.

*Soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place.

Warm stratification

Place the seed in a container as for cold stratification and label.

Put the container in a warm place at 18-24°C (65-75°F) for up to 12 weeks, before giving the seed a period of cold treatment.

During the period of cold stratification, inspect the seed regularly, especially towards the end of the allotted time, and sow the seed immediately at the first sign of germination.

NB. The treatments required will vary from one variety to another and you need to bear in mind that some seeds (especially trees) naturally take up to three years to germinate.



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