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 all 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, most of 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.

Cold-moist Stratification

This is the process by which we mimic the cold and wet conditions of a temperate winter. There are two main ways to do this; the first method is to sow seed outdoors in the autumn or early winter, and to keep it exposed to the winter conditions until it germinates in the Spring. The second method is to mix seed with a small amount of damp sand, compost or coconut coir and keep it in the fridge for a period of time; a couple of weeks can be enough, but for best results you should aim for at least a month. This means you need to work backwards from mid-spring and should ideally be stratifying by mid to end-February. If you miss this window, don’t worry – you can still stratify your seed to sow later in the Spring.

At Earthsong Seeds we generally prefer to stratify in the fridge; here’s how we do it:

1. Gather all your ingredients (see photo).
2. Put your seeds in the bag.
3. Pour some sand into the bag (a teaspoon is enough for small seed; a few teaspoons may be needed for bigger seed). Yellow building sand is easier to work with and retains more moisture than the grey horticultural sand.
4. 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.
5. Label your bag with the name and quantity of the seed, and the date stratified. Add a reminder to sow the seed!
6. Put in fridge (and hide behind the old jars of jam if there is resistance from family members).

Scarification

Strictly speaking, scarification is a form of seed-treatment rather than stratification, but they share the same objective of improving germination time and rate. There are two main forms of scarification:

Scarification through Abrasion
This is the process by which the hard seed coat of a seed is broken down, allowing water to be more easily 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. Rubbing with sandpaper is a fiddly job and often results in seeds flying all over the place so it can be helpful to do it in a large pan, bucket or sheet.

Scarification through Soaking
Another method of scarification is to soak the seed in water for a period of time before sowing, which softens the seed coat, again helping the seed to absorb water more easily. We use these method for our leguminous seeds such as licorice and astragalus, soaking them for 24 hours before sowing, usually with very good results.

In conventional (non-organic) growing systems people sometimes use sulfuric acid to help break down hard seed coats; this works in the same way as digestive juices in the gut of a bird (our pokeroot seeds germinate far and wide after being spread in bird droppings). Apparently there are organic alternatives, including soaking seeds in vinegar or even cooled chamomile tea, which is mildly acidic. We haven’t experimented with these methods yet – do let us know if this is something you have tried, with or without success…!

Which herb seeds need stratification?

Below is a list of which of our herb seeds benefit from stratification or scarification. Please note that we have compiled this list based primarily on the results of our own experiments and that it may differ to other sources of information on the Internet. Our experiments are ongoing and we will update this list as and when we can.

Cold-moist stratification highly recommended:
– Angelica
– Arnica, Meadow
– Meadowsweet
– Motherwort
– Vervain, Blue
– Vervain, European

Cold-moist stratification improves germination, but is not essential:
– Culvers root
– Echinacea angustifolia
– Echinacea purpurea
– Echinacea pallida
– Gentian, Tibetan
– Henbane
– Horehound, White
– Marshmallow

Scarification recommended:
– Lavender
– Pokeroot (this often self-seeds in different parts of the garden having been scarified in the gut of a bird!)

Soaking recommended:
– Astragalus
– Licorice

How to sow stratified seeds?
One thing you do need to be aware of when sowing stratified seeds is that it is very hard to separate individual seeds once they have been mixed in a damp medium, which means that it becomes very difficult to sow seed in individual plugs. So for this reason we recommend sowing stratified seeds into open trays and spreading the sandy/soily mix as evenly as you can on the surface of the tray. You can read more about what to sow your seeds in here.

Stratification Experiments

Every year we do a few different experiments to see how different stratification methods affect germination. For example, in the experiment below, we sowed two trays of Good King Henry; the seed on the left had been cold-moist stratified in the fridge for a month before sowing, the seed on the right had not been stratified. In this case, the germination rate is so much higher from the stratified seed that there is a risk that the seedlings on the left will be too closely spaced if they are not potted up soon. Good King Henry is an erratic germinator (i.e. they don’t all germinate at once) and our records show that more of the seeds on the right continued germinating, but the overall germination rate was still much lower than the stratified seed.

In some cases, stratification will speed up germination time, but not significantly affect the final germination rate. For example, the photograph below shows two trays of Echinacea angustifolia; the seed in the tray on the left had been stratified in the fridge for 3 months, the seed on the right was not stratified. This photo shows the trays 14 days after sowing; the stratified seed germinated more quickly and vigorously, but within a few weeks the second tray had more or less caught up.

This next experiment shows the results of an experiment with meadowsweet. Again, the seed on the left had been stratified in the fridge for 1 month prior to sowing, the seed on the right was not stratified. The stratified seed started germinating after 12 days, whereas the unstratified seed started germinating after 19 days. Meadowsweet is an unpredictable seed; some years we get no germination from unstratified seed. On this occasion we had some germination, but nowhere near as good as the stratified seed on the left.

And finally, another stratification experiment, this time with blue vervain. The tray on the right was sown with seed that had been stratified in the fridge for 3 weeks prior to sowing. Whilst the germination rate is clearly excellent, you can see that we sowed WAY too much seed for the little tray! This was a mistake we made at the time of mixing the seed in the damp sand, after which it is hard to see how much seed has been mixed in. Our records show that the non-stratified seed did eventually germinate, but at a much lower rate, which in this case was a very good thing!