Seed Stratification Timer
Count back from your sowing date to when seed must go into the cold, or forward to when it can come out.
Many tree, shrub and perennial seeds carry a winter inside them: they will not germinate until they have logged weeks of cold, moist conditions, a requirement called stratification. Miss the start date and the whole season slips. This timer takes the species and the date you want to sow, and returns the window to start chilling, in the fridge or in a pot outside, plus the earliest the seed can come out.
Cold means 33 to 41 F, moist but not wet, in a sealed bag of damp medium or a pot of mix. The fridge vegetable drawer is the standard location; a cold frame or a north wall works where winters cooperate.
How stratification works
A dormant seed is not dead; it is locked, and cold moisture is the key its species evolved to count winter with. Inside the seed, germination inhibitors break down and promoters build only while it sits cold and damp, so a warm week does not count and a dry month in a packet does not count. Hit the required total (the range here, since seed lots vary) and the seed will germinate promptly when warmth arrives. Too short a chill gives patchy, weeks-long germination; the full period gives a single flush.
The standard method:
- Mix seed in damp (not dripping) sand, vermiculite or a folded damp paper towel, sealed in a labelled bag.
- Hold at 33 to 41 F: a fridge drawer, or outdoors in a rodent proof pot where winters are properly cold.
- Check weekly. Remoisten if drying, remove mouldy seeds, and sow immediately any that sprout.
- Stones and nuts (peach, walnut) stratify whole in a pot of mix outdoors, protected from squirrels, which is both easier and safer than the fridge.
Worked examples
1. Apple seed for a March 15 sowing. At 70 to 90 days, the bag goes into the fridge between mid December and mid January. Expect seedlings that will not match the parent apple; that is genetics, not a failed chill.
2. Lavender started January 10. At 14 to 30 days, it can be sown from late January to mid February, on the surface with light, and germination still ambles, which is lavender being lavender.
3. Milkweed started in fall outdoors. Thirty days of cold is the requirement and winter provides months of it free: sow in pots outside in November and the timing cannot be wrong in a cold climate.
Which seeds need the cold
| Species | Cold days | Note |
|---|---|---|
| Apple | 70 to 90 | From saved pips; seedlings will not match the parent tree. |
| Pear | 70 to 90 | Needs the full cold period before it will move. |
| Peach and Nectarine | 98 to 120 | Stratify the whole stone, crack it gently at sowing. |
| Plum | 90 to 120 | Slow even after stratification; sow in autumn where possible. |
| Cherry (sour) | 90 to 120 | Birds find outdoor sowings; use a covered pot. |
| Apricot | 60 to 90 | Keep the medium moist, never wet. |
| Black Walnut | 90 to 120 | Sow deep; the shell softens over the cold period. |
| Lavender | 14 to 30 | Light helps after stratification; sow on the surface. |
| Rosemary | 14 to 28 | Slow and patchy even with stratification. |
| Sage (common) | 0 to 10 | Optional; a short chill evens out germination. |
The full list of 20 species is in the calculator, including several honest ‘none’ entries: foxglove, parsnip and asparagus are commonly believed to need stratification and do not, and winter spent chilling them is winter wasted.
Common mistakes
- Dry cold. Seed in a bare envelope in the fridge is stored, not stratified. The cold must be moist to count.
- Warm spells mid-chill. A week out of the fridge on a counter can stall the process in some species. Keep the cold continuous.
- Forgetting the bag. Stratifying seed is the easiest thing in gardening to lose track of. The label gets the species, the start date and the ready date, written when the bag is made.
- Sowing the sprouted and the sleeping together. Check bags weekly near the end; sow sprouters at once and give the rest their full term.
Putting the result to work
Work the start dates into the same winter list as the seed order: stratification for spring sowing begins in December and January for the long chill species, which is precisely when gardeners forget it exists. Once the seedlings are up, their outdoor dates come from the Transplant Date Calculator pattern for vegetables, adjusted with patience for trees.
When do I start stratifying seed?
- Pick the species; its required cold period is built in.
- Choose the mode: count back from a sowing date, or forward from a start date.
- Mix seed in damp medium, seal and label with species and dates.
- Hold at 33 to 41 F for the full period, checking weekly.
- Sow promptly at the end; sow sprouters the moment they appear.
What temperature counts as stratification?
Between about 33 and 41 F (1 to 5 C), moist. A fridge vegetable drawer sits exactly in the band. Freezing solid adds nothing and can damage imbibed seed; room temperature pauses the clock for most species.
Can I stratify outdoors instead of in the fridge?
Yes, and for stones, nuts and hardy perennials it is often better: sow in pots of mix in fall, sink them or stand them against a north wall, and protect from rodents and squirrels with mesh. Winter does the counting. The fridge wins in mild climates where outdoor cold is unreliable, and for small expensive seed you want to watch.
What if my seeds sprout in the bag?
Sow them immediately, at the depth the species wants, under lights or in a sheltered frame. A sprouted seed has spent its dormancy and will not store again; leaving it in the bag twists the root and invites mould. Weekly checks exist to catch exactly this.
Do vegetable seeds need stratification?
Almost none of the common ones. A few perennial herbs benefit from a short chill (lavender, rosemary, sage) and some wildflowers and nearly all temperate tree and shrub seeds require it. Annual vegetables are bred from species that germinate on warmth alone; chilling their seed is storage, nothing more.
How long is too long in the cold?
Most species tolerate weeks beyond the requirement without harm as long as they stay moist and mould free, and some simply sprout in the cold when ready, which settles the question. The real risks of overlong storage are mould and sprouting in the bag, both caught by weekly checks.
Cold period ranges are standard published stratification requirements per species. Temperature band 33 to 41 F follows common seed handling guidance.
