What is stratification and how is it done?
In simple terms, stratification is the process by which we mimic the natural conditions that help break the ‘dormancy’ of a seed so that it germinates at the time of year it is most likely to survive.
Seed dormancy is nature’s way of delaying germination, either to survive extreme conditions (normally heat or cold), or to increase chances of wider seed dispersal. It is a large and fascinating subject, but we will stick to what is relevant for growing herbs in temperate climates.
Let’s take meadowsweet as an example – a common herb that grows throughout the UK, often in damp ditches that flood in the winter. Meadowsweet’s seed matures in September and starts falling off the plants in November. If the seeds were to germinate in the autumn, the young seedlings would have to cope with extreme cold and wet, very short days and little sunlight – standing little chance of survival. For this reason, the seed becomes dormant, and will only wake up from its dormancy when certain conditions are met.
As herb growers, our challenge is to mimic the natural conditions that break the seed’s dormancy. There are two main ways of doing this: cold-moist stratification and scarification.
This is the process whereby we mimic the cold and wet conditions of a temperate winter either by sowing outdoors in the autumn or winter, or by mixing seed with a small amount of sand or soil and keeping in the fridge for a period of time – normally at least a month. This means you need to work backwards from mid-spring and should ideally be stratifying by mid-February. Here’s how we do it at Earthsong Seeds:
- Gather all your ingredients (see photo).
- Put your seeds in the bag.
- Pour some sand into the bag (a teaspoon is enough for small seed; a few teaspoons may be needed for bigger seed).
- Spray water into the bag and mix it all around. 2-3 sprays should be enough. The sand or compost should be moist, not wet. Squeeze out excess water if needed.
- Label your bag with the name and quantity of the seed, and the date stratified. Add a reminder to sow the seed!
- Put in fridge (and hide behind the old jars of jam if there is resistance from family members).
This is the process by which the hard seed coat of a seed is broken down or softened, allowing water to be absorbed by the seed. This can be done by rubbing seed on sandpaper or a file, or in some cases they can be nicked with a knife. Another method is to soak the seed in warm water for a period of time before sowing.
What are the results?
In this experiment below we sowed meadowsweet seeds that had been cold-moist stratified in the fridge for a month before sowing (tray on left) and meadowsweet seeds that hadn’t been stratified (tray on right). Here you can see the germination rate of the stratified seed is much higher.
Do all herbs seed need stratification?
No, not all herb seeds need to be stratified. If in doubt, check the sowing instructions for your seeds on the website or on the back of the seed pack.