Cold stratification by species (table)

Cold moist stratification is a pre-germination treatment that mimics the natural winter dormancy-breaking process. Many seeds from temperate climates require a period of cold, moist conditions before they will germinate -- a safeguard against germinating in fall before winter. Per NC State.

—- title: "Cold stratification by species (table)" slug: seed-stratification-guide hub: care category: "Advanced technique" description: "A complete guide to cold moist stratification for seeds, with a species-specific table of temperatures, durations, and special requirements." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 9 —-

Cold moist stratification is a pre-germination treatment that mimics the natural winter dormancy-breaking process. Many seeds from temperate climates require a period of cold, moist conditions before they will germinate — a safeguard against germinating in fall before winter. Per NC State Extension, this physiological dormancy is overcome by holding seeds at 35—40°F in moist medium for a species-specific period, ranging from 30 days to over 6 months.

Without stratification, these seeds either fail to germinate at all or germinate poorly and irregularly.

How cold stratification works

Per Penn State Extension, seed dormancy in temperate species is maintained by germination inhibitors (abscisic acid and related compounds) in the seed coat or embryo. Cold-moist stratification degrades these inhibitors over time, resetting the seed's germination "permission" system. The key requirements are:

• Temperature: 34—41°F (1—5°C); 38°F is the optimal target per most sources. Below 32°F, stratification stops; below 28°F, seeds can be damaged.
• Moisture: Seeds must be hydrated throughout stratification but not waterlogged
• Aeration: Some oxygen exchange is needed; sealed containers with no ventilation can cause anaerobic conditions and rot
• Duration: Species-specific; shortening fails to overcome dormancy; extending is generally safe

Materials for refrigerator stratification

Per UC Cooperative Extension:

• Zip-lock bag or small container
• Moistening medium: clean coarse sand, perlite, peat moss, or moist paper towels — any material that holds moisture without becoming waterlogged
• Refrigerator set to 35—40°F (not a freezer)
• Labels with species and start date

Method

1. Soak seeds in water for 12—24 hours before beginning stratification (recommended for hard-coated seeds; optional for thin-coated seeds).

1. Mix seeds with moistening medium at a 1:3 ratio by volume (seeds:medium). Alternatively, place seeds between two layers of moist paper towels.

1. Check moisture: Medium should feel moist when squeezed but not drip water. Excess moisture causes fungal rot.

1. Seal in bag or container with a small hole for gas exchange; label with species and start date.

1. Place in refrigerator. Check every 2 weeks: remoisten if dry; discard any visibly moldy seeds; remove seeds beginning to germinate (the radicle, or embryonic root, emerging) and plant immediately.

1. Sow after the required period directly into growing medium at the proper soil temperature for germination.

Species table: cold stratification requirements

Per NC State Extension seed germination guides, Penn State Extension, USDA Forest Service Woody Plant Seed Manual, and Missouri Botanical Garden:

Double dormancy

Some species have two dormancy requirements: a warm period followed by a cold period (or vice versa). Per USDA Forest Service Woody Plant Seed Manual:

• Warm-then-cold double dormancy: Ilex, Viburnum, Sambucus, Hamamelis. These seeds need a warm (70°F) stratification period of 60—90 days BEFORE the cold stratification. Sow seeds in a warm place in summer; move to refrigerator in fall; germination occurs the following spring.

• Cold-then-warm-then-cold triple dormancy: Trillium. A 3-year germination process in some populations. Plant where they won't be disturbed.

Scarification

Hard seed coats on species like Baptisia, Cercis, Gleditsia, and most legumes prevent water uptake and prevent germination regardless of stratification. Per NC State Extension, scarify by:

• Sandpaper: rub the seed coat on one side until the inner white layer is just visible
• Nick with a file: small nick in the seed coat at the point opposite the radicle end
• Hot water: pour boiling water over seeds; let cool and soak 12—24 hours (works for some legumes)

Common problems

Frequently asked questions

What if I miss the stratification window? Per Penn State Extension, you can stratify in the refrigerator any time of year. If you sow after the normal spring window, start the seeds in late summer—fall for fall or next-spring germination.

Can I stratify and winter sow simultaneously? Winter sowing (in milk jugs outdoors) essentially replicates natural stratification. Per NC State Extension, species that require cold stratification are ideal winter sowing candidates — the outdoor cold winter provides stratification automatically. You don't need to do both.

What temperature should my refrigerator be for stratification? 35—40°F, per Penn State Extension. Standard refrigerators set correctly hold this range. Do not use a freezer — freezing kills most seeds.

How do I know if seeds are viable before stratifying? Per NC State Extension, a simple viability test: place 10 seeds between moist paper towels at 70°F for the normal germination period. Count germination percentage. Less than 50% viability suggests using more seed per cell or sourcing fresh stock.

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Sources

1. NC State Extension — Seed germination and stratification
2. Penn State Extension — Seed stratification
3. USDA Forest Service — Woody Plant Seed Manual
4. UC Cooperative Extension — Seed stratification
5. Missouri Botanical Garden — Seed dormancy overview