Stratification is the use of cold, moist conditions to prime seeds into germinating.
The process of stratification occurs naturally for plants where the seed is ‘self sown’ in the garden.
It can be copied by the gardener either to force the seeds into germinating out of season when sowing indoors or in order to improve the germination rate of seeds indoors that would normally experience stratification in the wild.
Not all seeds have detailed research into how, what conditions and when they will best germinate under. Sometimes it is anecdotal research by individual growers that informs how best to germinate seeds. For example many people would automatically soak larger seeds such as peas and beans prior to sowing so as to soften the hard coat and to allow the seed to take up moisture.
Methods for stratification
1. Sow outdoors in pots or the ground in autumn and put in a shady protected place. For some seeds sowing in January or February will also give them time to stratify before spring arrives.
- Protect from pests such as mice or slugs.
- Cover with either a propagator lid or a plastic bag to keep the seed moist but not saturated.
- Check regularly for germination and moisture levels.
3. The bag method allows the seed to be stored in a refrigerator and takes up less space than pots.
- Use a piece of kitchen towel cut to fit inside the bag.
- Sprinkle the seeds on one side of the paper and then moisten lightly with a spray. Label the bag.
- Close or partially close the bag. Place the bag in the fridge with the paper towel above the seed. This allows to the root to grow down instead of into the paper towel.
- Check the moisture weekly and remove any seeds showing roots using tweezers or a moist toothpick and transplant into pots, leaving the ungerminated seeds to continue the chill period. Once the chill period is over allow the bag to gradually come to the recommended germination temperature with the bags still face down. Continue to transfer to pots or trays the seeds that have germinated. If the seeds need a further cold stratification cycle it is then easy to return them to the fridge.
As different varieties of plants have different cold, moist stratification requirements it is important to check how long to chill them for. If no information is available try germinating at room temperature and if there is minimal success then place in a fridge for 2-4 weeks before returning to room temperature. This cycle can then be repeated.
Examples of seeds we frequently are asked about that require cold, moist stratification are Nepeta cataria (Catmint) and Primulas.
Suggestions for germinating Catmint
Nepeta cataria is related to the mint family and often bought dried as entertainment for cats.
Nepeta is a small seed which requires light to germinate. If sown outdoors in autumn protect in a shaded cold frame. If sown indoors in January to April then germination will be improved by cold, moist stratification followed by temperatures of 15-20oC (60-68oF) Germination usually occurs in 7 to 21 days although it can be erratic.
Primulas are hardy perennials that grow well in moist well drained soil. They will self seed naturally in colder climates. Treatment and length of time to germination depends on the individual variety.
Most Primulas need light to germinate. Keep compost moist but not waterlogged and keep at a cool room temperature around 12-15oC (54-60oF). Air every few days to prevent damping off. Germination takes varying lengths of time depending on the variety:
Primula candelabra - 2-3 weeks warm followed by 4-6 weeks at 2-4oC. Keep cool once germinated.
Primula denticulata - sow at 18oC. Germination takes 20-30 days
Primula veris (Cowslip) - Sow in situ March/April and Sept/Oct. Surface sow. Germination slow and may need freezing and thawing several times if sown indoors to break dormancy.
Primula elatior (Oxlip) needs 2-3 weeks warm followed by 4-5 weeks at 2-4oC.
Primrose - Mardi Gras. Sow on surface. Temperatures between 15-20oC (59-68oF)
Echinacea are hardy perennials of North American prairies which have cold winters and hot summers.
In the UK sow February, March or August/September under cover. These seeds need light and can be started at temperatures of 18-21oC. Germination could take between 30 and 90 days. If germination is poor then cold, moist stratify for 2-3 weeks.