Cold stratification: part 1, dormancy

Happy New Year to all again! As the saying goes: out with the old and in with the new, which also means making order in various aspects of one’s life. Website wise, besides deleting older posts with no real significance long term, more important pieces are brought together and pinned on the Germination page.

The cold (moist) stratification as a germination treatment was mentioned many times when talking about germinating various species and how to store your sowing and seedlings over the winter. However, I think one more detailed, only seeds-related post is needed.

First, briefly about seeds dormancy, mostly for those new to germinating seeds. We cannot talk about cold (moist) stratification without a few notes on dormancy.

As we know, many annuals, and also various perennials, will germinate easily in proper conditions related to light, temperature and humidity. These are non-dormant seeds and we wish all were like this!  Examples would fill a whole page: Silene spp., most Dianthus, Bukiniczia, Erinus,some Campanula ssp., Papaver…

On the other hand, in the same conditions, many other seeds stubbornly refuse to germinate; sometimes even after being ‘attacked’ with various so called germination treatments. This is because of the so called seeds dormancy, which we courageously attempt to ‘break’.

The seeds dormancy is commonly described as possible to be: physical, physiological in various levels, morphological and morphophysiological. Gradients between types of dormancy are possible. Many times the seeds may in fact have a combination of these: for example physical + physiological dormancy.

Without going into details (AI can help with that), the physiological dormancy is found in seeds of many species including many perennial plants, also shrubs and trees, and most often it can be broken by the use of a cold (moist) stratification period. It can be: non deep, intermediate and deep, and I found the naming self-explanatory. We can understand why some species will germinate after 1 week of cold (moist) stratification and others after 4 months (those with deep physiological dormancy). One example: Lewisia rediviva with germination after a +/- 1 month of cold stratification and Lindera bezoin with germination after 3+ months. I won’t go into details about the others, will give examples in part 2.

Something else to note is that in some cases even non-dormant seeds can become dormant due to environmental conditions; also dormant seeds can enter a second-dormancy making it even more difficult to ‘break them’.

Another useful bit: in a given lot of seeds, not all need to have the same type of dormancy. For example, part of the seeds can have physiological, while another part can have morphophysiological dormancy.
In translation – not all seeds will always break dormancy and germinate at the same time (examples: Glaucidium palmatum, Sanguinaria canadensis, Hepatica…). 

Back to the cold (moist) stratification which is most commonly required for breaking seeds dormancy in many species, be it as a single treatment or in combination with a warm (moist) stratification (multiple cycles may be required), or in combination with other treatments.

Once upon a time, before we had a million studies about seeds dormancy and its classifications, people would sow seeds in the fall and in most cases they would germinate in some  percentage in the spring. In translation –  the seeds were applied a natural cold (moist) stratification during the winter.

In nature, most often seeds/fruits fall on the ground when they mature, which can happen from early summer to late fall. The ecological approach to seeds germination always takes this into account.
For example, eeeds maturing in early summer will naturally undergo a warm stratification before the fall arrives and the cold stratification starts, so it makes sense to treat those seeds in the same way when we collect and sow them. If we collect and store them under improper conditions in our home until sowing in the fall, is not the same. It should be no wonder that they react differently and maybe will not germinate in the spring. Examples: Corydalis species, also Erythroniums and other early spring flowering species.

Unfortunately, many studies on seeds germination do not follow an ecological approach. Hence, the contradictory germination results for various species. The same applies  for some germination experiments done by gardeners.

Another useful info I would like to share on the subject is that after seeds dormancy was broken, different conditions may be required (for some seeds) to stimulate the germination.
I’ve read it long time ago, but it has taken time for me to really understand the concept. I noticed that some seeds (species) kept in the fridge (vermiculite or moist paper towels) will start growing roots after a while (usually toward the spring) while others not, germinating only after they were taken out the fridge.
Translation 1 – the conditions suitable for dormancy break and germination may or may not be the same, depending on the species.
Translation 2: maybe the seeds are not dormant anymore, but they won’t germinate due to unfavourable conditions.

The general idea I am trying to convey is that some of those little grains we are trying to ‘break’ out of dormancy, may have many reasons for refusing to germinate. There are still many unknowns related to the dormancy and germination, plus there are many differences between the many plant species!

Something to meditate on until part 2 is ready:

“The dormant-seed stage in the life cycle of many plant species should be visualized as a period of time when things are happening in the seed, e.g., physiological/biochemical and morphological/anatomical changes, growth of the embryo, mobilization of food reserves and certainly activation and deactivation of genes.” (Baskin & Baskin, 2014)

Next in part 2 – we’ll get back to the practical approaches for providing the cold (moist) stratification for seeds (boxes, snow and all that…).