In some seeds, light promotes germination, while in others it can inhibit it. Others require a prolonged period of cold or specific temperatures before they can germinate. These responses help plants ensure that germination occurs under suitable environmental conditions and at the right time.
Germination types and environmental factors: light, dark and cold germinators
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In horticulture, seeds are classified into different groups according to their germination requirements. These include light-germinating, dark-germinating and light-neutral seeds, as well as cold-germinating, frost-germinating and warm-germinating seeds. Fire-stimulated seeds are also described, although less commonly. The following sections explain what these terms mean and their significance for sowing.
Table of contents
1. How seeds respond to environmental factors
- Light-germinating seeds
- Dark-germinating seeds
- Light-neutral seeds
- Cold-germinating and frost-germinating seeds
- Warm-germinating seeds
- Fire-stimulated seeds
5. In summary
How seeds respond to environmental factors
Whether and when a seed germinates depends on several conditions. In addition to water and oxygen, light and temperature in particular can affect germination in different ways depending on the plant species.
The different terms do not all refer to the same process. For light-germinating, dark-germinating and light-neutral seeds, the focus is on how the seed responds to light. Warm-germinating seeds, by contrast, are classified according to their relatively high germination temperature.
Cold-germinating seeds require a prolonged cool, moist period, while frost-germinating seeds require temperatures below freezing as part of the necessary cold stimulus. These cold stimuli help to break existing seed dormancy, technically known as dormancy. In fire-stimulated seeds, dormancy is broken by intense heat or certain compounds in smoke, depending on the plant species.
These terms help to provide a better understanding of the different requirements of seeds. They describe different characteristics and are therefore not mutually exclusive. The same plant species can, for example, be both dark-germinating and warm-germinating, or may also respond to light after a necessary period of cold.
How water, oxygen, temperature and light affect germination and how these factors interact is covered in detail in the article Environmental factors in seed germination: Water, oxygen, temperature and light.
How light affects germination
In light-sensitive seeds, light can promote or inhibit germination. In plant physiology, this light-dependent response of seed germination is known as photoblastism.
Three main responses can be distinguished:
- Light promotes germination,
- light inhibits germination,
- light and darkness have only a minor influence.
The classification into light-germinating, dark-germinating and light-neutral seeds serves primarily as a guide. Under favourable conditions, many species can germinate in both light and darkness. The classification therefore indicates the light conditions under which germination is typically more successful or reliable. The variety, temperature, condition of the seed and other germination conditions can also influence how strongly seeds respond to light.
Light-germinating seeds
In light-germinating seeds, light promotes germination or is particularly important for achieving a high germination rate. This light-promoted germination is known as positive photoblastism.
Light is detected by various photoreceptors, which are light-sensitive proteins in the plant. These include phytochromes, which primarily detect red and far-red light, the latter being barely visible to the human eye, as well as cryptochromes, which respond particularly to blue light. This allows seeds to detect different parts of the light spectrum and adjust their germination accordingly.
For many small seeds, light also serves as an indication that they are close to the soil surface. As their stored food reserves are limited, germinating near the surface is advantageous.
Light-germinating seeds are sown near the surface, gently pressed down and left uncovered or covered only very lightly. As seeds close to the surface dry out more quickly, the growing medium should be kept evenly moist during germination.
Light-germinating seeds do not necessarily require intense sunlight. What matters is the species-specific light stimulus, not the strongest possible light.
Dark-germinating seeds
In dark-germinating seeds, light inhibits germination, while dark conditions promote it. This light-inhibited germination is known as negative photoblastism.
Dark-germinating seeds also have photoreceptors that allow them to detect light. Unlike in light-germinating seeds, however, the light signal inhibits germination. Phytochromes are among the photoreceptors involved in this process.
Dark-germinating seeds are covered with growing medium when sown so that as little light as possible reaches the seeds. However, the required sowing depth still depends on the plant species and seed size.
Dark-germinating seeds therefore do not need to be sown particularly deeply. If a seed is sown too deeply, the young seedling has farther to grow before reaching the soil surface and uses more of its stored reserves in the process.
Likewise, not every seed that is sown covered with soil is automatically a dark-germinating seed. In many species, covering the seed primarily helps prevent it from drying out and ensures good contact between the seed and the growing medium.
Light-neutral seeds
In light-neutral seeds, the difference between light and darkness has only a minor influence on germination. This largely light-independent germination response is known as neutral photoblastism. Light neither clearly promotes nor inhibits germination.
Light-neutral seeds also have photoreceptors and can detect light. However, this light signal is not crucial for the onset of germination. Other conditions, such as moisture, oxygen and temperature, are more important for successful germination.
Sowing depth depends primarily on the requirements of the particular plant species and the size of the seed. Whether light reaches the seeds plays only a minor role for light-neutral seeds.
However, the classification is not always entirely clear-cut for every species or variety. The response to light can also vary depending on temperature, seed age and other germination conditions.
Light-neutral simply means that no specific light stimulus is required for germination. After emergence, the young plants still need sufficient light for their development.
Temperature as a condition and trigger for germination
Temperature influences whether and how quickly seeds germinate. Each plant species has a specific germination temperature range within which its seeds can germinate. Within this range, there are temperatures at which germination is particularly successful.
Temperature affects seeds in different ways. For warm-germinating seeds, it is the temperature during germination itself that matters. Cold-germinating seeds, by contrast, first require a prolonged cool, moist period to break dormancy. For frost-germinating seeds, temperatures below freezing are part of the necessary cold period. Fire-stimulated seeds, in turn, respond to intense heat or other fire-related stimuli.
When a moist temperature treatment is used to break existing dormancy, this is known as stratification. Depending on the plant species, it may take the form of cold stratification, warm stratification or a sequence of warm and cold periods.
Changes in temperature can also influence germination in some plants. These alternating warm and cool periods are known as alternating temperatures and promote germination more strongly in certain species than a constant temperature. For some seeds, such temperature fluctuations can indicate that they are close to the soil surface, as fluctuations are more pronounced there than in deeper soil layers.
Cold-germinating and frost-germinating seeds
Cold-germinating seeds have a form of seed dormancy that is broken by a sufficiently long cool, moist period. Only after receiving the necessary cold stimulus are the seeds ready to germinate under suitable conditions.
In nature, this mechanism prevents the seeds of many species from germinating at an unfavourable time, such as just before winter. Only after they have undergone a sufficiently long period of cold do they germinate when conditions subsequently become favourable.
Frost-germinating seeds require a stronger cold stimulus. For these seeds, temperatures below 0 °C are part of the necessary seed pretreatment. Frost does not directly trigger germination, but acts on the seed during the preceding dormant period. Germination itself begins afterwards at suitable temperatures.
In certain species, freeze–thaw cycles also play a role. This refers to repeated freezing and thawing.
The necessary cold conditions can be achieved naturally by sowing in autumn or winter, or they can be reproduced under controlled conditions. A controlled cool, moist treatment is known as cold stratification. The interaction between cold and moisture is essential. Simply storing dry seeds in the refrigerator therefore does not constitute cold stratification. For frost-germinating seeds, pretreatment should be planned so that the species-specific temperatures below freezing that are required are actually reached.
The different forms of seed dormancy and how cold, warm and combined stratification treatments are carried out are covered in detail in the article Seed dormancy and stratification: Understanding dormancy and preparing seeds for germination.
Warm-germinating seeds
Warm-germinating plants are those whose seeds require relatively high temperatures for reliable germination. These include many warmth-loving crops such as peppers, cucumbers and pumpkins. At temperatures that are too low, germination is slower or may fail altogether.
Many metabolic processes are controlled by enzymes. Enzymes are proteins that enable specific chemical reactions within the seed, and their activity depends on temperature.
‘Warm’ should always be understood in relation to the requirements of the particular plant species. Warm-germinating seeds also have a specific germination temperature range; excessively high temperatures can also inhibit germination.
Warm-germinating seeds are sown in a sufficiently warm growing medium. Particularly when starting seeds indoors early in the season, the temperature of the growing medium is therefore just as important as the air temperature.
The term warm-germinating therefore refers to the temperature required during germination itself. It should not be confused with warm stratification.
Warm stratification, by contrast, is a warm, moist pretreatment of dormant seeds. In certain plant species, it is used to break dormancy before germination itself begins.
Fire-stimulated seeds
In fire-stimulated seeds, depending on the plant species, intense heat, certain compounds in smoke or both stimuli act on seed dormancy. In heat-dependent species, the seeds often have a hard, water-impermeable seed coat. Dormancy caused by such a seed coat is known as physical dormancy. A sufficiently strong but non-lethal heat stimulus makes the seed coat more permeable, allowing the seed to absorb water afterwards.
In some fire-stimulated seeds, certain compounds in smoke promote germination. Among the known germination-promoting smoke compounds are karrikins, which are compounds produced when plant material burns. In horticulture, fire-stimulated seeds for which intense heat is the main factor are sometimes also referred to as heat-germinating seeds. However, this term is not consistently defined.
In heat-dependent fire-stimulated seeds, germination does not take place during exposure to intense heat. The heat stimulus first breaks the existing seed dormancy. Germination itself begins later, once moisture and temperature conditions are suitable again.
Heat or smoke treatments are only appropriate for plant species whose seeds require such a germination stimulus. These treatments are not necessary for common vegetable and herb species.
Key terms at a glance
| Term | Key factor | Importance for germination | Application when sowing |
|---|---|---|---|
| Light-germinating seeds | Light | Light promotes germination | Sow near the surface and barely cover |
| Dark-germinating seeds | Light | Light inhibits germination | Sow away from light |
| Light-neutral seeds | Light | Light has only a minor influence on germination | Observe the species-specific sowing depth |
| Cold-germinating seeds | Prolonged cool, moist period | Cold helps break certain forms of seed dormancy | Natural cold or cold stratification |
| Frost-germinating seeds | Temperatures below freezing | Frost is part of the necessary cold stimulus | Allow for the species-specific frost period |
| Warm-germinating seeds | Relatively high germination temperature | Sufficient warmth is required for germination itself | Provide a sufficiently warm growing medium |
| Fire-stimulated seeds | Fire-related triggers such as heat or smoke compounds | Depending on the plant species, heat or smoke compounds break seed dormancy | Use only for species adapted to these conditions |
These groups are not mutually exclusive. For example, a cold-germinating seed may also be light-germinating after the required cold period. The terms therefore describe different requirements of the same seed.
Likewise, the classification alone does not automatically determine all sowing conditions. The correct sowing depth in particular also depends on the plant species, seed size and other characteristics of the seed.
How deeply seeds should be sown is covered in detail in the article Optimal sowing depth: How deep to sow seeds correctly.
In summary
The different germination types show how closely germination is adapted to the environmental conditions of a particular plant species. Light conditions, warmth, cold and fire-related stimuli have different effects depending on the species: they may promote or inhibit germination or help to break existing seed dormancy. The terms describe different requirements that can occur together within the same plant species. For successful sowing, it is therefore essential to understand these relationships and treat seeds according to the specific requirements of each species.
That is why our product descriptions provide detailed information on the germination requirements of each plant species.
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