Some seeds germinate as soon as sufficient water, oxygen and suitable temperatures are available, along with appropriate light conditions depending on the plant species. Others initially fail to germinate even under favourable conditions and require specific environmental stimuli or pretreatment before germination can begin.
Seed dormancy and stratification: understanding dormancy and preparing seeds for germination
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Particularly for plants with specific germination requirements, properly preparing the seeds is therefore crucial for successful sowing. The following sections explain which specific requirements play a role and what to consider when pretreating the seeds.
Table of contents
1. Was Keimruhe und Dormanz bei Samen bedeuten
- Physiologische Dormanz: innere Keimhemmung
- Physische Dormanz: Wasser gelangt nicht in den Samen
- Morphologische Dormanz: der Embryo muss sich noch entwickeln
- Morphophysiologische Dormanz: zwei Keimhemmungen wirken zusammen
- Kombinierte Dormanz: Samenschale und innere Keimhemmung
- Kaltstratifizierung
- Warmstratifizierung
- Kombinierte Stratifizierung
What seed dormancy and dormancy in seeds mean
Seed dormancy generally refers to the state in which a viable seed temporarily does not germinate. When the cause lies within the seed itself, even though the external conditions are generally suitable, this is technically referred to as dormancy.
For many plants, dormancy is an important adaptation to their environment. For example, it prevents seeds from germinating during a brief period of favourable weather when conditions may subsequently follow in which the young seedlings would be unlikely to survive.
However, not every failure to germinate is due to dormancy. If water, oxygen, suitable temperatures or other necessary external conditions are lacking, germination is simply prevented by the environment. This state is known as quiescence. Once the required conditions are present and no additional dormancy exists, germination can begin.
A seed that is no longer viable is also not dormant. Dormancy requires the seed to be viable.
If dormancy develops during seed maturation, it is referred to as primary dormancy. If a seed that was originally ready to germinate later develops an internal inhibition of germination again, this is known as secondary dormancy.
Dormancy is therefore not a single, uniform state. The pretreatment required depends on what is preventing germination.
What types of dormancy are there, and what do they mean for sowing?
In seed biology, five basic types of dormancy are distinguished: physiological, physical, morphological, morphophysiological and combined dormancy.
This distinction is important for sowing because not every barrier to germination can be overcome in the same way. Some seeds require a specific temperature period, while in others the seed coat must become permeable or the embryo must first develop further.
Physiological dormancy: internal inhibition of germination
In physiological dormancy, the embryo – the young plant already formed within the seed – is fully developed, but internal processes within the seed initially prevent germination.
Depending on the plant species and the degree of physiological dormancy, this inhibition of germination can be broken by a period of moist cold, a period of moist warmth, alternating temperatures or prolonged dry storage. The breaking of dormancy during such dry storage is known as after-ripening. The conditions required are species-specific.
The pretreatment must be appropriate for the particular plant species. Cold stratification is only useful if a period of moist cold is actually required to break dormancy. With after-ripening, by contrast, the seeds are stored dry. Special treatments should only be used if they are specifically recommended for the particular plant species.
Physical dormancy: water cannot enter the seed
In physical dormancy, the seed coat or fruit wall forms a water-impermeable barrier. As long as water cannot enter the seed, the swelling process required for germination cannot begin.
When sowing, the seed coat of certain species can be deliberately made more permeable. This treatment is known as scarification. The seed coat is carefully roughened or nicked, for example, so that water can enter the seed.
Other methods, such as hot-water treatment, are suitable only for certain plant species and should be used only when specifically recommended for the species concerned.
Scarification should only be used for species in which a water-impermeable seed coat is actually part of the dormancy mechanism. With sufficiently large seeds, for example, the surface can be carefully roughened with fine sandpaper without damaging the embryo inside.
Morphological dormancy: the embryo still needs to develop
In morphological dormancy, the embryo is not yet fully developed when the seed matures. It must first continue to grow within the seed before germination can begin. There is no additional physiological inhibition of germination in this type of dormancy. Above all, the embryo requires suitable conditions and sufficient time to develop further.
Such seeds do not automatically require cold stratification or scarification. What matters is sufficient moisture, a suitable temperature and enough time for the embryo to develop further. A longer germination period therefore does not necessarily mean that there is something wrong with the seeds.
Morphophysiological dormancy: two barriers to germination act together
In morphophysiological dormancy, the embryo is not yet fully developed and also has physiological dormancy. Before germination can begin, the embryo must develop further, while the internal inhibition of germination must be broken either at the same time or subsequently.
Depending on the plant species, this requires periods of moist warmth or cold, or a specific sequence of both temperature ranges.
Many peonies are a clear example of this type of complex dormancy. In species with this type of dormancy, the embryo initially develops further during a warm period before a subsequent cold period is required for further seedling development.
The sequence of temperatures and the duration of treatment should be followed precisely according to the requirements of the plant species. Specified warm and cold periods cannot simply be shortened or reversed. For suitable species, sowing outdoors can be an alternative if the necessary temperature periods occur there naturally.
Combined dormancy: seed coat and internal inhibition of germination
In combined dormancy, physical and physiological dormancy occur together. The seed has both a water-impermeable covering and an internal inhibition of germination.
A commonly cited example is the eastern redbud (Cercis canadensis): in horticultural propagation, the hard seed coat is first made permeable, after which the seeds are stratified under moist, cool conditions.
Both barriers to germination must be overcome. A single treatment is therefore not sufficient in cases of combined dormancy. The steps required and the order in which they are carried out depend on the particular plant species.
Stratification: breaking dormancy through moist temperature periods
Stratification is a moist temperature treatment that can break certain types of dormancy or allow an embryo that is not yet fully developed to develop further.
What matters is the interaction of temperature, moisture and time. Sufficient oxygen must also remain available during the treatment; the material should therefore be moist but not saturated with water. A cold treatment without sufficient moisture also does not constitute the moist stratification described here.
Cold stratification
During cold stratification, seeds are kept moist and cool for a specific period of time. This replicates the conditions they are exposed to in nature during a cold season.
In cold-germinating seeds, this cold period helps to break the existing dormancy. How cold the conditions need to be and how long the treatment must last depend on the plant species.
The term frost-germinating seeds is also used in horticulture, but it is not a distinct scientific type of dormancy and is not used consistently. It therefore does not automatically mean that temperatures below 0 °C are essential. In some species, frost or freeze–thaw cycles can contribute to natural pretreatment, whereas for many others, a prolonged period of moist cold above freezing is sufficient. The species-specific requirements are always what matters.
Further information on germination types can be found in the article Germination types and environmental factors: Light, dark and cold germinators.
For controlled cold stratification, the seeds can, for example, be placed in moist growing medium, sand or vermiculite and stored in a suitable container or bag in cool conditions. The material should be evenly moist but not saturated with water. The temperature and duration depend on the particular plant species. During the treatment, the seeds should be checked regularly for drying out, excessive moisture, mould and signs of germination. Labelling them with the plant species and the start date of the cold period makes monitoring easier, particularly during longer treatments.
Temperatures below 0 °C should only be used in controlled pretreatment if they are specifically recommended for the plant species concerned.
If individual seeds begin to germinate during the cold period, they should be carefully removed and grown on under suitable conditions.
Warm stratification
During warm stratification, seeds are kept warm and moist for a specific period of time.
This period may be necessary to allow an embryo that is not yet fully developed to develop further or to break certain physiological barriers to germination. Some plants require only a warm period, while others also require a subsequent cold period.
The seeds and growing medium must remain evenly moist but should not stay continuously wet. As warmth and moisture can also encourage mould, the seeds should be checked regularly. If a cold period is required afterwards, the change should only be made after the period specified for the plant species.
Combined stratification
Some seeds require several periods of moist conditions at different temperatures in succession. This treatment is known as combined stratification.
It should not be confused with combined dormancy. Combined dormancy refers to two barriers to germination that are present at the same time, whereas combined stratification simply refers to a sequence of different temperature periods. Depending on the plant species, for example, a warm period may follow a cold period, or vice versa. In more complex cases, several changes between temperature periods may also be required.
The individual periods should be carried out in the specified sequence. For longer treatments, it is useful to note the start and end of each period. There is no universally applicable duration.
Natural or controlled stratification
A required stratification can be carried out either under natural weather conditions or under controlled conditions.
When seeds are sown in autumn or winter, they experience seasonal changes in temperature and moisture directly outdoors. This is simple and close to natural conditions, but offers less precise control. Mild winters, dry conditions, excessive moisture or damage from animals can affect the sowing.
With controlled stratification, temperature, moisture and duration can be maintained more precisely. A cold period can, for example, be carried out in the fridge, while a suitably warm location is required for a warm period.
Natural stratification is suitable when the conditions at the growing location meet the requirements of the particular plant species. Controlled treatment is particularly useful when specific temperatures, periods of time or temperature sequences need to be maintained reliably. Species with particularly complex requirements may also require several seasons or repeated changes in temperature. Such requirements should always be followed on a species-specific basis.
Common mistakes when preparing seeds for germination
Problems with seed pretreatment arise mainly when the moisture, temperature, duration or treatment do not match the requirements of the plant species. The following guidance will help you quickly identify and correct common mistakes.
Seeds fail to germinate despite cold treatment
Incomplete pretreatment: Not every type of dormancy is broken by cold. Some seeds instead require a warm period, after-ripening, scarification or several successive treatments.
→ Solution: Check the type of dormancy and the species-specific germination requirements, and carry out only the pretreatment required for them.
Seeds fail to germinate after dry storage in the fridge
No moist stratification: For cold stratification, the seeds must be able to absorb sufficient moisture during the treatment. Dry storage in the fridge does not meet this requirement.
→ Solution: Stratify the seeds in an evenly moist, well-aerated material suitable for the plant species.
Mould or rot during stratification
Too much moisture or insufficient air exchange: Water-saturated material reduces the oxygen supply and encourages mould and rot.
→ Solution: Keep the growing medium evenly moist, avoid waterlogging and check the seeds regularly.
Seeds dry out during stratification
Too little moisture: If the seeds dry out during a required moist stratification period, the conditions necessary for breaking dormancy are no longer present.
→ Solution: Check the moisture regularly and, if necessary, carefully add moisture without making the material waterlogged.
Seeds germinate during cold stratification
Dormancy already sufficiently broken: Individual seeds may begin to germinate during the cold period.
→ Solution: Carefully remove germinated seeds and grow them on under conditions suitable for the plant species.
Hard seeds do not absorb water despite stratification
Water-impermeable seed coat: In physical dormancy, water cannot enter the seed. Cold stratification alone does not remove this barrier.
→ Solution: Only for suitable species, carry out the recommended scarification or another species-specific treatment of the seed coat.
Seeds fail to germinate despite scarification
Another barrier to germination is present: In combined dormancy, making the seed coat permeable alone is not sufficient.
→ Solution: In addition, carry out the treatment required for the plant species to break physiological dormancy.
Seeds fail to germinate even after prolonged pretreatment
Treatment or development period not yet complete: Some species require several weeks or months before the conditions for germination are met.
→ Solution: Check the species-specific treatment and germination times and do not discard the seeds prematurely. If the specified periods have been significantly exceeded, the moisture, temperature and seed viability should also be checked.
Seeds fail to germinate despite warm and cold periods
Incorrect sequence, temperature or duration: In some species, the individual temperature periods must occur in a specific sequence and for a specified length of time.
→ Solution: Follow the temperature sequence, temperature range and duration required for the plant species, and note the start and end of each period.
Dormancy types and their treatments at a glance
| Dormancy type | What prevents germination? | What does this mean for sowing? |
|---|---|---|
| Physiological dormancy | Internal inhibition of germination | Depending on the species, cold or warm stratification, alternating temperatures or after-ripening |
| Physical dormancy | Water-impermeable seed coat or fruit wall | Make the seed coat permeable in suitable species through scarification or another species-specific treatment |
| Morphological dormancy | Embryo not yet fully developed | Keep moist and allow the embryo to develop further at a suitable temperature |
| Morphophysiological dormancy | Underdeveloped embryo and physiological inhibition of germination | Species-specific warm and/or cold periods in the required sequence |
| Combined dormancy | Water-impermeable seed coat and physiological inhibition of germination | Overcome both barriers to germination using the required treatments |
Even within the same type of dormancy, plant species can differ in the temperature, duration and sequence of the required treatment. Species-specific sowing instructions are therefore more important than general standard treatment times.
In summary
Dormancy prevents a viable seed from germinating immediately despite generally suitable external conditions. The pretreatment required depends on what is inhibiting germination.
Stratification, scarification and after-ripening each serve different purposes and should only be used when they are appropriate for the particular plant species and type of dormancy. Other treatments may be useful for certain species, but do not generally replace these methods. Temperature, moisture, duration and, where applicable, the sequence of several treatment steps are crucial to success.
To ensure that seeds can be treated according to their actual germination requirements, our product descriptions provide detailed information on dormancy, pretreatment and sowing for each plant species.
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