Cold stratification: why and how to germinate seeds?

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You sow plum pits, acorns, lavender seeds, or gentian seeds. Nothing emerges. Yet the batch is not dead: it is waiting for a signal it has not received. These seeds are in dormancy, and only a period of moist cold can break it.

It is not a botanical whim but a survival mechanism. This guide explains what happens inside the seed, how to reproduce winter in practice, and how long it takes depending on the species.

Why some seeds refuse to germinate

A seed that falls to the ground in September faces a problem: if it germinates immediately, the seedling will face winter and die. Species from temperate regions have therefore developed a lock: they can germinate only after experiencing a winter. This ensures that emergence takes place in spring, with the whole growing season ahead.

Acorns, pits, and an empty husk on a forest floor covered with dead leaves and frost — In the forest, dormancy breaks without intervention: the soil freezes, thaws, and the seed waits for spring beneath the leaves.

In practical terms, the embryo is kept dormant by chemical inhibitors, mainly abscisic acid. Prolonged exposure to moist cold gradually breaks down these inhibitors and increases the hormones that trigger germination, the gibberellins. When the balance shifts, the seed becomes capable of germinating as soon as the temperature rises.

The most common mistake

Cold alone is not enough. You need cold and moisture. A packet of seeds forgotten in the freezer, dry, will not be stratified—it will simply be stored. This is the most common misconception about the subject.

Other forms of dormancy

Physiological dormancy broken by cold is not the only type. Two other cases occur, sometimes in combination:

Physical dormancy — an impermeable seed coat prevents water from entering. It is broken by scarification: rubbing the seed with sandpaper, lightly nicking it, or soaking it for 24 hours in lukewarm water. Typical examples include black locust, broom, morning glory, nasturtium, and Judas tree.
Double dormancy — it first requires a warm period, followed by a cold period. This is the case for hawthorn, ash, yew, and Solomon's seal. These species may require two winters before breaking dormancy, which discourages many people—even though the batch is perfectly viable.

The parameters that matter

Temperature — between 1 and 5 °C is the effective range. Below 0 °C, biochemical processes stop: freezing does not stratify the seeds; it puts them on hold. Above 8 °C, dormancy does not break either. The vegetable drawer of a refrigerator is exactly within the right range.
Moisture—the substrate should be as damp as a wrung-out sponge, never waterlogged. Too much water deprives the seed of oxygen and causes it to rot.
Oxygen—the seed respires throughout stratification. That is why you should never use an airtight container without ventilation.
Duration—from 3 weeks to 6 months depending on the species. This is the most variable parameter, and the one you need to check before starting.

Method 1: refrigerator stratification

The most reliable, the most controllable, and the only possible option if you want to sow in spring seeds received in February.

Cold-stratification supplies on a workbench: river sand, perforated zip bag, label, and pencil

Rehydrate the seeds for 12 to 24 hours in room-temperature water. Discard those that are still floating after soaking.
Prepare the substrate—river sand, vermiculite, peat, or simple paper towel. Moisten it, then wring it out by hand: it must not drip.
Mix the seeds with the substrate, approximately one part seeds to three parts substrate.
Place in a zip bag or container, pierced with a few holes for ventilation.
Label with the species and the date they were placed in the cold—you will not remember in four months.
Store in the vegetable drawer, between 1 and 5°C.
Check every two weeks: remove any mold, maintain moisture, and above all watch for early germination. Some seeds germinate in the refrigerator before the planned duration is over: sow them immediately; do not let them become etiolated in the bag.

Acorns mixed with damp sand in a transparent zip bag, first day of stratification — Day 0—acorns mixed with wrung-out sand, condensation on the plastic.

Split acorn with a pointed white radicle emerging, planted in sand — Day 60—the shell opens and the radicle emerges: sow immediately.

David's advice

When they emerge, sow immediately and without thermal shock: first place them in a cool room for a few days before exposing them to warmth. A stratified seed moved back to warmth and then cold can re-enter secondary dormancy—and you will have to start all over again.

Method 2: natural stratification outdoors

Simpler, free, and often more effective for species with long or double dormancy—because the natural alternation of freezing and thawing also works on the seed coat.

Terracotta dish protected by fine mesh, placed outdoors against a stone wall in winter — Against a north-facing wall, sheltered from direct sunlight. Fine mesh protects against field mice, the main risk with this method.

Sow in autumn, from October to December, in pots or a seed tray filled with well-draining potting soil.
Place them outdoors, against a north-facing wall, sheltered from direct sunlight and drying winds.
Protect them from rodents — this is the main risk. Place fine mesh over them, or improvise a cage. Voles and squirrels will find acorns and stones in just a few days.
Let the rain do the work — just make sure the substrate does not dry out during rainless periods.
Wait until spring. And if nothing emerges, don't throw them away: species with double dormancy will emerge in the second spring.

This is also the method we recommend for trees, shrubs and bonsai: it exactly replicates what happens in the forest.

How long, depending on the species

Indicative durations for moist cold stratification, between 1 and 5°C.

Duration	Species
3 to 4 weeks	Lavender, thyme, clary sage, yarrow, primrose
4 to 8 weeks	Pine, spruce, larch, bellflower, columbine, delphinium
8 to 12 weeks	Oak, beech, chestnut, maple, linden, gentian, hellebore
3 to 4 months	Apple, pear, plum, cherry, peach, almond, medlar
Double dormancy (2 winters)	Hawthorn, ash, yew, Solomon's seal, some species roses
Scarification followed by cold treatment	Black locust, broom, Judas tree, ceanothus

These durations vary depending on the batch's origin and harvest year. If in doubt, extend rather than shorten the period: stratification that is too long has no adverse effect, while stratification that is too short will not break dormancy.

Common problems

White, filamentous mold on acorns in an overly wet stratification bag — White fuzz indicates that the substrate is too wet or poorly aerated. Remove affected seeds, rinse the others, prepare a drier mixture and add more holes.

Symptom	Cause	Correction
White mold in the bag	Substrate too wet, insufficient aeration	Remove affected seeds, rinse the others, prepare a drier medium and add more holes
Seeds germinated too early	Dormancy lifted ahead of schedule	Sow them immediately—it is a good sign
No emergence after stratification	Insufficient duration, or double dormancy	Return them to the cold, or wait until the following spring
Soft, brown seeds	Rot caused by excess water	Batch lost; reduce moisture next time
Seeds gone (outdoor sowing)	Rodents	Fine mesh from the outset

After stratification

Young oak seedling removed from its deep pot, with a long visible taproot — The taproot drops down during the first few weeks. A pot that is too short makes it spiral, and the tree will bear the mark for the rest of its life.

Sow in deep pots — most of these species develop a taproot within the first few weeks. A shallow pot makes it spiral, and the tree will retain the resulting mark.
Well-draining substrate — seed-starting compost lightened with sand or perlite.
Patience when germinating — germination is often irregular and spread over several weeks. Do not throw away a pot because only one seed has sprouted.
Do not repot too early — wait until the second year for trees, when the root system is established.

Frequently asked questions

Does the freezer work?

No. Below 0 °C, the biochemical reactions that break dormancy stop. Worse, freezing can damage a rehydrated seed through crystal formation. The refrigerator, between 1 and 5 °C, is the appropriate tool.

Can vegetable seeds be stratified?

It is unnecessary in the vast majority of cases: annual vegetables do not have cold dormancy. Stratification mainly concerns trees, shrubs, perennials, and certain woody herbs.

How long does a seed remain stratified?

Sow promptly after removing them from the cold. A stratified seed that is then returned to dry, warm conditions may enter secondary dormancy, which is more difficult to break than the first.

Do pits from store-bought fruit need stratification?

Yes, they have the same dormancy. But be aware that store-bought fruit almost always comes from grafted trees: sowing will produce a seed-grown tree with fruit different from that of the parent, often of poor quality. The experience remains fascinating, and the resulting tree will make an excellent rootstock.

How can you tell whether a seed needs stratification?

General rule: if the species originates from a region with a pronounced winter and its seeds ripen in autumn, there is a strong chance. The information appears on our packets, and the species concerned are identified among our trees and shrubs.

Key points

The essentials in three lines

Cold and damp, between 1 and 5 °C, for a period specific to each species. If you have no scheduling constraints, sow outdoors in autumn and let winter do the work — it is the simplest method, and the one nature uses. Above all: never throw away a pot that has not sprouted by the first spring.

Going further

Discover our tree, shrub, and bonsai seeds and our perennial plants — species requiring stratification are identified on each product page.