Overcoming seed dormancy

There are many different processes for overcoming seed dormancy, which are outlined below, sorted by the dormancy mechanism the process is used to overcome:

Chemical inhibitors

Collecting seeds ‘in the green‘ (ie before they are fully ripe) can prevent the growth inhibitors building up in the seed (which happens during the ripening process). This is an important method for a lot of tree and shrub seeds.

Some seeds’ chemical inhibitors will be broken down by being soaked in water, acids or alcohol for a period, or through scarification of the seed coat (see ‘hard seed coats’ section below).

Simply storing the seed for a period of time can be sufficient to allow the chemical inhibitors to break down.

Hard seed coats

Scarification is often used to speed up germination where the plant has a hard seed coat. This involves damaging the seed coat to provide an easier entry point for water. This can be done by rubbing the seed with sandpaper, nicking them with a sharp knife, putting them in a food processor where the blades have been covered in masking tape to dull their sharpness, or cracking them gently with nut crackers. When scarifying the seed ensure you do not damage the tip or ‘eye’ of the seed.

Soaking the seeds will also help to break through a hard seed coat. Seeds are placed in hot (but not boiling) water for up to 48 hours, the exact timescale varying from seed to seed. Any seeds which float are dead and can be discarded. The seeds must be sown immediately after soaking.

A few seeds require fire to scarify the seed coat, with the accompanying smoke working to stimulate germination. You can either purchase kits to replicate this, or sow a tray of seeds, cover it with some dry bracken, burn the bracken, then water in the ashes.

Physiological (immature embryo) dormancy

Stratification is a common technique whereby the seed is stored in moist conditions (in growing media or sand) for a period before being sown in order to replicate the environmental conditions the seed requires in order to after-ripen and germinate.

Cold stratification is the most common method, mirroring the winter conditions after which the seed will think it’s spring and, therefore, germinate. To cold stratify seeds they should be kept in a sealed plastic bag with some moist growing media, or sand, in a refrigerator at around 3°C. The seeds will continue to respire in the bag, so it should be opened every now and then to allow fresh oxygen in and carbon dioxide out. After around 12 weeks the seeds should be ready to be sown, although seeds should be sown immediately if they germinate in the bag.

Warm stratification is used for seeds which normally take a season to ripen before they germinate, eg they expect a warm summer to ripen in, before the cold of winter, finally germinating the following spring. For the warm stratification period seeds are sealed in a plastic bag as for cold stratification and then placed in warm conditions at around 20°C, an airing cupboard would be ideal. Alternatively the seeds could be sown in a propagator and kept at the appropriate temperature.

Some seeds can still take over a year to germinate, so check the details for the plant you’re sowing before assuming they aren’t going to germinate!