While many seeds germinate easily, others need a bit of help before they can be sowed. These seeds may require one or more pretreatments to break the dormancy that prevents them from germinating. Stratification, scarification, and soaking are the most commonly used methods.
Information on specific seed germination requirements can be found on seed packets and on seed providers’ websites.
What is seed dormancy?
A viable seed is considered dormant if it does not germinate despite environmental conditions favourable for germination. This temporary inability to germinate can be caused by various factors, such as:
- a seed embryo that has not fully developed
- germination inhibitors inside the seed or seed coat (testa)
- a seed coat that isn’t letting water and/or oxygen in, both of which are essential for germination
- a seed coat that is too hard, which prevents the embryo from expanding
Dormant seeds require specific conditions in order to germinate, such as a cold, humid period, or conditions that alter their seed coats.
The advantages of dormancy
Dormancy is an evolutionary adaptation with several advantages:
- It allows the seeds to germinate at the right time.
- It ensures that all seeds of a species don’t germinate at the same time, which would be risky in adverse climatic conditions or if competition for resources is high.
- It allows plants to have better seed dispersal, such as when seeds must make their way through an animal’s digestive tract before being able to germinate.
Scarification: damaging the seed coat
One method that helps break seed dormancy is scarification, which consists of slightly damaging the seed coat, allowing water and oxygen inside.
In natural environments, several factors can alter seed coats:
- climatic conditions (such as alternating periods of drought and humidity, as well as freeze and thaw cycles)
- microorganisms in the soil
- seeds rubbing against sand and rocks
- seeds travelling through an animal’s digestive system, etc.
Small seeds can be scarified by rubbing them between two pieces of sandpaper until they lose their sheen.
Larger seeds can have their coats weakened with a file, a blade, or nail clippers, but it's important not to damage the embryo.
Soaking: softening seed coats
Soaking is used to soften seed coats and can also remove germination inhibitors. Follow these steps for successful seed soaking:
- Soak your seeds in lukewarm water for 8 to 24 hours. Remember to label the soaking containers if you’re soaking different types of seeds.
- When seeds swell up, it’s a sign that the soaking process has worked. Pick out these seeds and sow them right away, leaving the other seeds continue to soak until they have grown in size.
- If some seeds are not getting any bigger and were scarified before soaking, their seed coat may not have been sufficiently weakened. Try scarifying them again (without letting them dry out) and then let them continue soaking.
- Sow seeds promptly after soaking.
Cold stratification: replicating winter
Many cold-hardy trees, shrubs, and perennials have seeds that require a cold, humid period to germinate. This is true for many plants that are indigenous to Quebec.
When their seeds drop to the ground at the end of summer or in the fall, they stay dormant until the spring. If they germinated in the fall, the seedlings would probably not survive the winter.
Cold stratification replicates winter conditions. Here’s how to do it:
- Check the recommended cold stratification period so you can plan when to start. It’s often between 4 and 6 weeks but can vary from 2 weeks to 4 months.
- Place seeds in moist—but not overly wet—substrate (vermiculite, sand, peat, potting soil). Soggy soil can lead to mould growth.
- Transfer the substrate and seed mix to a plastic bag. Label the bag with the name of the species and the stratification end date.
- Place the bag in the fridge (at around 4 degrees Celsius).
- Check on it often. If the substrate is too dry, use a spray bottle to mist it with water. Remove any mouldy seeds. If some seeds germinate prematurely, sow them in containers.
- Once the stratification period is over, remove the bag from the fridge. Sow seeds directly in the ground, weather permitting, or start your seedlings in containers indoors.
Alternative option: If you have enough space in your fridge, you can sow your seeds in containers filled with moist soil and cover them with a plastic bag so the soil stays humid.
Opting for a natural stratification by sowing outside in the fall
Seeds can be sown outside in the fall, either directly in the ground or in containers that are buried in the ground. The seeds will undergo a natural stratification process.
If there are a lot of squirrels in your area, cover your seeds with wire mesh to protect them.
Warm stratification with heat and humidity
Certain seeds need to be exposed to heat and humidity (warm stratification) for their embryos to fully mature.
Other seeds require warm stratification followed by cold stratification, which roughly replicates fall and winter conditions.
Still other seeds require multiple stratifications: cold, warm, cold, warm, etc.
Warm stratification requires the same initial steps as cold stratification, but the bag or container must then be stored at room temperature (between 20 and 24 degrees Celsius) rather than in the fridge.