What Is Cold Stratification?
Cold stratification is a controlled period of cool, moist conditions that helps certain dormant seeds prepare to germinate. It copies winter outdoors and is most useful for some trees, shrubs, perennials and wildflowers. Done patiently and hygienically, it can improve germination without special equipment or complicated treatments.
Check whether your chosen seed actually needs cold stratification before starting. The packet may say cold treatment, pre-chill, cold moist stratification or autumn sowing. Some seeds germinate perfectly well with ordinary warmth and light, so unnecessary chilling only adds delay. Seeds from many hardy trees, shrubs and perennial flowers have evolved to wait through winter before sprouting. This prevents seedlings emerging during a brief mild spell and then being damaged by cold. Follow the supplier's stated period where available, because requirements vary widely. If instructions are vague, ask the RHS or a local garden centre for guidance on that particular plant.
The simplest method is outdoor autumn sowing. Fill clean pots or module trays with moist peat-free seed compost, sow at the recommended depth and label them clearly. Cover pots with fine mesh if mice or birds may disturb them, then place them outside in a sheltered spot where they receive rain but are not likely to dry out completely. Winter weather supplies the cool period naturally. Check regularly that compost remains lightly moist, not sodden. This approach suits hardy seed in much of the UK, although very wet gardens need good drainage so seed does not rot before spring.
For fridge stratification, begin with clean materials and a small, labelled container. Mix seed with barely moist peat-free compost, vermiculite or clean sand, or place it between damp paper that has been squeezed out well. The material should feel moist but should not drip. Seal it in a bag or container and write the plant name and start date clearly. Keep it in the refrigerator, away from fresh produce where possible, for the period stated on the packet. Check it weekly for mould, drying or early germination. Do not place seed in the freezer, as this is not the same treatment.
When the chilling period ends, sow the seed promptly into clean pots of peat-free seed compost. Cover according to the seed's normal depth requirement, since some need light and should only be pressed onto the surface. Place pots where temperature and light match the seed instructions. Keep compost evenly moist with a gentle watering can rose or by standing pots briefly in shallow water. Germination can still be uneven, so do not discard a tray after only a few seedlings appear. Label every pot with the species, sowing date and treatment, especially if you are comparing chilled and untreated batches.
Handle germinating seed with care. If a root appears in the fridge bag, transfer that seed gently to compost with the root pointing downward, using a clean pencil or label to make a hole. Do not pull at delicate roots or leave germinated seed in the bag until it becomes tangled. Once seedlings have developed enough growth to handle, pot them on individually using peat-free multipurpose compost. Grow them in bright conditions protected from severe weather, then harden them off gradually before planting outside. Young plants can be more vulnerable to slugs and drying winds than established garden plants.
Keep records so the process becomes simpler each year. Note the seed source, date chilled, length of treatment, first germination and final number of seedlings. If a batch fails, consider seed age, waterlogging, excessive warmth after sowing or whether the treatment period was too short, rather than assuming cold stratification never works. Use fresh seed where possible, particularly with species that do not store well. Avoid adding fertiliser to the chilling material. Once seedlings are growing, give them ordinary, gentle care and plant them into a prepared site at the appropriate season.
Frequently asked questions
Can I cold stratify seeds in the freezer?
No. Cold stratification normally requires cool, moist conditions rather than freezing. A refrigerator is easier to control and is usually the appropriate choice when the packet recommends pre-chilling. Outdoor autumn sowing also works well for many hardy seeds, provided pots drain freely and remain protected from disturbance.
How wet should the stratification material be?
It should be evenly damp, never dripping wet. Squeeze a small amount in your hand: it should hold together but release no water. Excess moisture encourages mould and rot, while dry material prevents the seed from receiving the moisture signal that accompanies winter chilling in nature.
What if seeds germinate in the fridge?
Sow them immediately and handle the emerging root as little as possible. Make a small hole in moist peat-free seed compost, place the seed root-down and cover lightly where appropriate. Keep the pot in suitable light and warmth. Early germination is a sign the seed has completed its dormant period.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 18 Sep 2026.