Stratification: treatment given to certain seeds by exposing them to a period of chilling to help them out of their dormancy.
“Vernalization” is sometimes used as a synonym.
Many plants acclimatized to northern climates require a period of cold temperature before they can germinate. In their natural habitat, this characteristic prevents plants from germinating before winter. Stratification, with its exposure to cold followed by warmth, makes the seed believe that spring has arrived and it’s time to wake up.
Some plants for which stratification is recommended: alchemilla, milkweed, pale echinacea, strawberry, nigella, cherry, mulberry, apple.
For seeds requiring stratification, several options are available.
Direct seeding in autumn
Rather than simulating winter, it’s of course even simpler to sow your seeds in the ground in autumn and let them experience the real cold season. There’s just one drawback: sometimes we see an early spring (which can hasten germination) followed by late frosts. Unfortunately, seedlings are sensitive to these frosts.
Sow seedlings in aerated containers outdoors
Seedlings can be planted in aerated containers, but with wire mesh (or other rodent protection) placed outside. Containers must receive rain (and have drainage holes), or you’ll have to make sure you water them from time to time.
Here’s how to do it:
- Soak the seed for 24 hours;
- Put 1 cm of sand or a mixture of sand and seed starting potting soil in the bottom of the tray;
- Put the seeds on top;
- Cover with 2 mm of sand;
- Sprinkle with a light rain;
- Protect access to rodents, for example by laying a wire mesh over it;
- Check the seeds frequently and transplant them into potting soil as soon as they begin to germinate.
Germinating seeds in the refrigerator
Very simple to use: use the refrigerator to chill the seeds. In this technique, we use a substrate: often sand or seedling soil, alone or mixed with perlite or vermiculite. Another option is to use blotting paper. This makes it easier to spot sprouted seeds, but the rootlets get caught in the paper fibres and can be damaged during sowing.
A refrigerator set between 0 and 10°C is suitable for most seeds, but some seeds prefer colder temperatures and should be stratified in the freezer. In the tisanji plant cards, if there is no special indication, it means that it is not necessary to use the freezer.
- Soak the seed for 24 hours;
- Moisten the substrate or blotting paper (damp, but not soaked);
- Place seeds on substrate or damp blotting paper;
- Place in an airtight bag or container.
Keep your seeds visible to avoid handling them during stratification.
Stratification time
Depending on the species, stratification generally lasts between 1 week and 4 months. In most cases, this is considered a minimum, i.e. if you leave your seed in the cold any longer, it will think it’s still winter and remain dormant without causing any damage. Nevertheless, it’s a good idea to check regularly (at least once a week) whether your seedlings have germinated, so that you can quickly transplant them into a seed starter mix.
The cold snap is over and my seedlings haven’t sprouted.
This is quite normal: seedlings generally wait for the heat to settle in after stratification. So, once stratification is complete, the seeds can be sown at the optimum depth, heat and light conditions for germination.
Each seed has its own germination time (from a few days to 18 months).
Some seeds are particularly difficult to germinate and require more than one cold period. These are identified in tisanji, and you are warned that several hot-cold cycles may be needed to germinate these seeds. If you’ve gone well beyond the expected germination time and the seeds still haven’t germinated, a 2nd stratification might be the solution, unless your seeds are simply no longer viable (too old, badly stored, etc.).