A Detailed Guide to Seed Stratification: Methods, Examples, and Practical Tips
Seed stratification It is an essential technique in horticulture and forestry for overcoming the natural dormancy of many species. Its purpose is to facilitate optimal germination even in seeds that, in their natural environment, require very specific environmental or seasonal conditions to activate. In this article, you'll discover techniques, practical examples, and all the information you need to master stratification step by step, integrating advances and methods proven by experts and university departments.
Why do seeds require stratification?
Many seeds present latency, a natural barrier to prevent their immediate germination after being released by the mother plant. This biological mechanism protects the species from germination out of season or in adverse conditions. adverse, increasing the chances of seedling survival.
There are several types of latency:
- Exogenous: Caused by the impermeability, chemical inhibitors or mechanical resistance of the covers.
- Endogenous: Due to underdevelopment (morphological) or internal physiological inhibition (physiological).
- Morpho-physiological: Combination of the above.
Main pre-germination treatments
The objective of these methods is to imitate natural conditions that break the latency and allow the transformation of the embryo into a seedling:
- Scarification: Physical or chemical damage to the cover to allow entry of water and gases.
- Humid stratification: Simulates the passing of the seasons: cold and/or hot, with controlled ambient humidity.
- Soaking and heat treatment: Soften the coating or remove chemical inhibitors by immersion in hot or cold water, or sometimes using water at different temperatures for specific species.
- Other treatments: Use of enamel, sandpaper, passing seeds through animal stomachs, controlled fermentation and acid treatment for very resistant species.
Stratification Types: Methods with Step-by-Step Explanation
The most used stratification method is the cold wet, although there are variations and combinations depending on the requirements of each species. Here we detail the most effective procedures:
Cold stratification
- Pre-soaking: Leave the seeds in water at room temperature for 24 to 48 hours to hydrate them.
- Preparation of the medium: Create a mixture of fine sand and sterilized peat (1:1 ratio) or use vermiculite, coconut fiber, or other substrates that retain moisture well. The substrate should be slightly damp, not soaked.
- Mixing and storage: Mix the drained seeds with the medium in a sealed plastic bag (ziploc or similar), ensuring that there is full contact between the seeds and the substrate.
- Temperature: Place the bag or container in the refrigerator (not the freezer), keeping the temperature between 1°C and 5°C. The stratification period varies by species: from about 3 to 16 weeks, or longer for deeply dormant species.
- Periodic revision: Open the bag every 7-10 days to ventilate, check for humidity, identify signs of germination, and discard infected seeds. If necessary, re-moisten the medium with distilled water.
- Sowing: Once the germination process has begun (radicles are visible), sow immediately in a pot or in the final soil. If the germination period ends and the seeds do not germinate, sow them anyway, as some seeds require a temperature variation to activate germination.
Hot stratification
Some species require warm period before the cold (for example, Prunus avium, Taxus baccata):
- Mix the seeds with the moistened substrate and keep them at a constant temperature between 20°C and 25°C for 2 to 8 weeks.
- Transfer the mixture to the refrigerator and the process continues as cold stratification.
Alternative: Direct stratification in a seedbed or pot
- Prepare one small pot with seedling substrate, moisten and place the seeds on the surface, pressing lightly.
- Cover the container with a clear plastic bag or lid to maintain humidity.
- Place the set in the refrigerator for the necessary period, checking every 10 days.
- After the cold period, move the pot to a bright area away from direct sunlight. Gradually remove the plastic cover to acclimatize the seedlings.
Examples of species and stratification periods
| Gender | Use cases | Duration | Temperature |
|---|---|---|---|
| Abies | Cold | 1-4 months | 1-5 ° C |
| Acer | Hot + Cold | 2-4 months | 20–25°C / 1–5°C |
| Quercus | Cold | 1–3 months | 1-5 ° C |
| Prunus | Hot + Cold | 2–6 months | 20–25°C / 1–5°C |
| Betula | Cold | 1–3 months | 1-5 ° C |
| Fraxinus | Hot + Cold | 2–6 months | 20–25°C / 1–5°C |
| Taxus | Double (hot/cold/hot) | 9+ months | 20°C / 1–5°C / 20°C |
General observations and recommendations
- La planting depth It should be 1,5 times the size of the seed, except for very fine seeds, which hardly need to be covered.
- Always use distilled or demineralized water to moisten substrates and seeds, especially in sensitive species.
- Always label containers with the species name and the start date of stratification.
- Avoid excess humidity that can promote the proliferation of mushrooms: If necessary, add a preventative fungicide.
- Once germinated, gradually expose them to light and outdoor environmental conditions (hardening process).
Common mistakes and solutions in seed stratification
- Sowing too deep: The seeds exhaust their energy reserves sooner and may not reach the surface.
- Lack of moisture: Stops embryo development. Maintain uniform, but never excessive, moisture.
- Inappropriate temperature: High temperatures inhibit germination, while temperatures that are too low without ventilation can cause rot.
- Fungal contamination: Clean the seeds and the substrate, use mild fungicides or oxygenated if you detect problems.
Species that need (or do not need) stratification
- Essential: Cherry trees, apple trees, pear trees, yew trees, ash trees, some Nordic sundews and carnivorous plants such as Dionaea muscipula and Sarracenia.
- Optional/variable: Mulberry trees, birch trees, maple trees, elm trees, carob trees.
- Not required (tropical seeds): Tropaeolum, many subtropical sundews, some Eucalyptus and species of warm-humid climates.
Mastering seed stratification allows you to effectively reproduce difficult or demanding species, ensuring maximum germination and seedling percentages. powerful y healthyThe combination of humid cooling, humidity control, and regular inspection guarantees success for both professional and amateur growers.
