Tool
Cold stratification planner
Some seeds need weeks of cold and damp before they will germinate. Work out when to start.
Your result
Start chilling around 1 February 2027
- Seed
- Target sowing date
- Cold period (days)
Start the cold period this many days before you want to sow — then keep the seed cool and just damp, not soaking.
What this calculator does
Many seeds from temperate plants — apples, roses, lavender, many wildflowers — will not germinate straight from the packet. They carry a natural brake that stops them sprouting in autumn, and it is released only by a spell of cold, moist conditions. Gardeners copy this with cold stratification: mixing seed with damp material and keeping it in a fridge or a cold frame for several weeks. This tool counts back from your sowing date to tell you when to begin.
When to use it
- Growing tree and shrub seed such as apple or rose.
- Sowing wildflowers and perennials that list "needs cold" on the packet.
- Working back from a planned spring sowing so the seed is ready on time.
How to use it
- Check the seed needs cold. The packet or a seed catalogue will say. Not all seed does — lettuce, for example, does not.
- Pick the seed from the list for a typical cold period, or enter your own.
- Enter your target sowing date and read when to start chilling.
- Stratify. Mix seed with damp sand or kitchen paper in a sealed bag or jar and keep it in the fridge, checking now and then for mould or early sprouting.
Worked example
You want to sow apple seed outdoors in mid-March and apple needs about ten weeks of cold. Ten weeks back from 15 March gives a start date of around 5 January. Bag the seed with damp kitchen paper, put it in the fridge, and check it every couple of weeks until sowing time.
Typical cold periods
| Seed | Cold needed |
|---|---|
| Apple | 8–12 weeks |
| Pear | 8–12 weeks |
| Lavender | 3–6 weeks |
| Milkweed (Asclepias) | 3–6 weeks |
| Coneflower (Echinacea) | 4–6 weeks |
| Columbine (Aquilegia) | 3–6 weeks |
| Daylily | 4–6 weeks |
| Rose | 8–12 weeks |
Ranges vary by species and source. When in doubt, a longer cold period is safer than a short one.
Limitations
- Neither too wet nor dry. The medium should be damp like a wrung-out cloth; soaking seed invites rot.
- Fridge is not a freezer. Aim for around 1–5°C. Freezing can kill some seed.
- Some seed needs more than cold. A few species need warm, then cold, or light to germinate — check the specific plant.
Frequently asked questions
How do I know if a seed needs cold?
The packet or catalogue usually says "stratify", "needs cold" or "chill for X weeks". Temperate trees, shrubs and wildflowers most often do; most vegetables do not.
Can I stratify seed outdoors instead?
Yes — sow it in a pot and leave it outside over winter, protected from rodents. Fridge stratification is simply a controlled version of the same thing.
What if roots start in the fridge?
Sow it straight away into compost. Early germination in storage means the cold brake is already released.
Related guides
- How to test if old seeds are still good
- Seed germination rate calculator
- How to save bean and pea seeds
Sources and research notes
Cold-period ranges are typical figures from seed-saving and horticultural guidance; the need for cold varies by species and provenance.