How To Cold Stratify Seeds (Including Fridge Stratification Method)

Cold stratification is a process where we prepare certain types of seeds for germination before sowing. Done in a fridge at home, this method is beneficial for seeds from cold hardy plants.

After stratifying seeds, see Seed Starting For Beginners for tips on sowing seeds indoors using basic household supplies.

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Cold Stratification of Seeds

Some seeds come with instructions to prepare them before sowing for a better—or any—chance of germination. This may include nicking the seed coat and/or chilling or “prechilling” the seed ahead of time.

Nicking the seed coat, also called scarification, is used on tougher seed coats to help air and water reach the dormant embryo inside. This shows different ways to scarify seeds prior to sowing.

Chilling or prechilling, also called cold stratification, stratification, or cold treatment—along with applying moisture—is required for a number of cold hardy seeds. These are seeds that ripen in fall, stay dormant in the cold of winter, and sprout in spring when conditions are right.

The word stratification comes from strata, meaning layers. The first-known method of stratification by a gardener originated hundreds of years ago using layers of cold, moist soil.

Today, there are several ways to stratify seeds both indoors and outdoors to significantly improve the germination rate of cold hardy seeds.

Contents

• Cold Stratification In Nature
  • Plants That Need Stratification
  • How long does stratification take?
• How To Stratify Seeds Indoors
• How to Stratify Seeds Outdoors
• Seed List
• Resources

Cold Stratification In Nature

Think about how many thousands of species of perennial plants and trees produce seeds in fall right before winter sets in.

What would happen if those seeds went ahead and germinated on a warm fall day—just as the cold winter weather approaches? They’d be sprouting one day and freezing the next.

Instead, to ensure their survival, these seeds have evolved with an extra defense mechanism—the need for a period of cold—that keeps them dormant until spring.

This is why, even when growing conditions in spring and fall are seemingly similar (temperatures, water, oxygen, and light levels), certain seeds will not sprout until spring.

It’s the exposure to cold temperatures over the winter months that makes the difference. Without that stratification, hardy seeds remain dormant.

Plus, by holding off until spring, not only do the seedlings have a much better chance of survival, but they also get a nice, long growing season to get established before winter sets in again.

Plants That Need Stratification

There are exceptions, but, generally, the seeds that need stratification are cold hardy perennials, trees, and shrubs with seeds that ripen in fall. These are plants native to temperate (not tropical or subtropical) regions.

Examples include clematis, milkweed, and maple trees. I’ve provided a more extensive seed list below.

How long does stratification take?

The amount of cold stratification needed depends on the plant’s origins—the conditions it has evolved with.

Many seeds need between 1 to 3 months of chilling. Some hardy tree seeds need 4 to 5 months.

The general rule is, the hardier the plant, the tougher the seed coat, and the longer stratification takes.

The stratification period is like an internal clock which keeps the seed protected until germination conditions are optimum in spring.

That internal clock is also why some unseasonably warm weather in the middle of winter months rarely fools these seeds. Their required stratification period is not yet complete so they are unlikely to be wooed from dormancy prematurely.

How To Stratify Seeds Indoors

The method we’re showing here is basic cold-moist stratification which works for a multitude of hardy seeds.

To prepare for sowing, you stratify the seeds weeks or months in advance.

Unlike dry fridge storage, which is a good way to store seeds for future sowing, cool-moist storage in the fridge mimics the winter conditions that gradually break down the seed coat while providing the necessary cool period.

Be sure to check your seed packet in case they have provided other instructions specific to that type of seed.

Some seed packets neglect to mention the need for stratification. If you have struggled to sprout cold hardy seeds, this may be the missing step.

How-To

Here’s how it works:

• Seeds are placed between a folded sheet of moist (not dripping wet) paper towel, inserted in a food bag or container, and placed in the fridge for the minimum recommended period of time.
  
  You want all parts of the seeds to be exposed to moisture while experiencing this cold treatment but never so damp that the seed could rot.
• Label the bag and note the date you started the stratification process and estimated end date.
  
  Some gardeners close the bag or container; others say air flow is vital. In my experience, there is (apparently) enough air available even when closed as I’ve never had mold or other issues.
  
  Others also insist on placing the paper towel in a dark container because most seeds require darkness for germination. I have never worried about this. For most of the time, the inside of the fridge is dark anyways and it’s usually after this process that the seeds will sprout.

I put a reminder in my phone to check on it every few days and re-moisten the paper towels as needed.

The ideal temperature (for most seeds) is around 40°F (4°C). Most fridges run between 35 to 41°F (1 to 5°C). You can use a contactless thermometer to check the temperature—it will vary in different parts of the fridge.

There are also exceptions for some species where below-freezing temperatures are best and that’s the time to use your freezer instead. Some peach varieties need this extra cold.

After Required Time Period

After the required stratification period (weeks or months, depending on the type of seed), remove the seeds from fridge and sow at room temperature without allowing the seed to dry out.

If you are new to sowing seeds indoors, see Seed Starting For Beginners. If the seasonal timing is right, you can also direct sow your seeds outdoors.

Even after stratification, it can still take weeks (or even longer) for germination—when the seed sprouts—if it’s a slower type of seed. Others may pop right away.

The “days to germination” on the seed packet will tell you what to expect if conditions are favorable. “Days to germination” does not include the stratification period.

Other Approaches

Soaking Seeds Prior To Stratification

Some tips suggest soaking or “pre-soaking” seeds like delphinium or sweet peas in water up to 24 hours prior to cold stratification to help soften the seed coat. This is a type of scarification. Some gardeners do this by placing the seeds directly in a container of water.

I’ve never been convinced this really does speed things up since we’re already exposing the seed to consistent moisture throughout the stratification method but you can certainly do side-by-side tests to see if seems advantageous.

I have experimented a fair bit with delphinium seeds trying various scarification and stratification methods. How To Get Stubborn Delphinium Seeds To Germinate describes what works for me.

Other Fridge Stratification Methods

I mentioned some seeds need “dry” stratification instead of “wet.” You will see this recommended for a number of native species. Indoors, dry stratification is essentially the same as storing your (dry) seeds in the fridge for later use.

Some seeds like trillium (Trillium) benefit from repeat cold-warm cycles. One approach is to keep the seeds moist in paper towel while alternating 3-month long shifts between the fridge (40°F / 4°C) and room temperature (approx. 70°F (21°C) for several cycles until the seeds sprout.

Other cold-moist stratification methods use moist potting mix, vermiculite, sand, or other garden media.

I prefer paper towel or fabric scraps because:

• It requires very little space in the fridge.
• I never lose sight of the actual seed.
• I can visually monitor any changes
• It’s easy to locate the seeds for sowing after the fridge process is done. I often can’t find them in dark potting  mix!

No matter what medium you choose, the idea is to keep the seed in contact with moisture and never let it dry out (or rot).

How To Stratify Seeds Outdoors

If you let your cold hardy garden plants go to seed in autumn and leave them for the winter, they will undergo stratification naturally.

While I wholeheartedly endorse leaving perennials (old plant stems, leaves, seed pods, and so on) in place throughout the winter to support all the creatures living in the garden, there are additional, more reliable ways to stratify seeds outdoors.

Direct Sowing

In addition to letting your hardy plants self-seed, you can also intentionally sow seeds in fall too (of course).

This has a list of seeds to sow directly in fall including various annuals and perennials. Some of them require stratification, others just don’t mind overwintering in the cold soil.

The list below suggests additional seeds that benefit from or require stratification to germinate.

Raised Bed Method

Sow your seeds in fall in a raised bed with a mesh screen over top like this one.

The seeds will get their natural stratification period and the cover will protect the seeds and seedlings from hungry birds.

Once the plants are established in late spring or early summer, transplant them to where you want them in the garden.

This has free plans for building raised beds.

Winter Sowing Method

Easy Winter Native Seed Method

Seed List

The plants I’ve listed below are commonly grown in cold climates in the United States and Canada. I am most familiar with hardiness zones 4 to 8.

The stratification times listed come from various sources. I could not find one definitive guide but I have listed some helpful sites in the Resources section.

The standard soil temperature for stratification is 40°F /4°C. This will naturally fluctuate outdoors.

For some, additional pre-treatments like scarification (nicking or sanding the seed coat or soaking the seed) may also be recommended. Others may need warm-cold cycles. Check your seed packet for specific instructions.

If you don’t know how long to cold stratify, between 2 to 3 months is considered adequate for most plants other than very hardy trees which take longer. Too short a time may prevent germination—going longer is not an issue.

As always, before adding any plants or seeds to your garden, be certain they are suitable for your hardiness zone, growing conditions, non-invasive, and beneficial to the local eco-system.

Plant Common Name (Botanical Name) | Estimated Stratification Period

• Allium (Allium) Flowering Onion | 4 to 6 weeks
• Arisaema (Arisaema) | 6 weeks
• Asters (various) | 2 to 4 weeks
• Bachelor Buttons, Cornflower (Centaurea cyanus) | 10 to 30 days
• Bee Balm / Bergamot (Monarda) | 2 weeks
• Blazing Star (Liatris aspera, L. pycnostachya, L. spicata) | 30 days
• Black-eyed Susan (Rudbeckia) | 2 to 4 weeks
• Bugbane (Cimicifuga) | 6 to 8 weeks, repeat warm-cold cycles if needed
• Butterfly Weed (Asclepias tuberosa) | 4 weeks
• Candytuft (Iberis) | 2 to 4 weeks
• Cardinal Flower (Lobelia cardinalis) | 30 days
• Clematis (Clematis) | 3 weeks
• Columbine (Aquilegia) | 4 to 6 weeks
• Coneflower (Echinacea) | Some 2 to 4 weeks, others up to 9 weeks
• Cranesbill Geranium (Geranium spp.) | 4 to 6 weeks
• Delphinium (Delphinium) | varies, days to weeks
• Dogwood (Cornus) | 3 to 4 months
• False Indigo (Baptisia) | 1 week
• False Sunflower (Heliopsis) | 2 to 4 weeks
• Foxtail Lily (Eremurus) | 11 to 12 weeks
• Hellebores (Helleborus) | 6 to 8 weeks
• Joe Pye Weed (Eupatorium maculatum, E. purpureum) | 2 to 4 weeks
• Lady’s Mantle (Alchemilla) | 4 to 6 weeks, may need below freezing temperatures
• Lavender (Lavandula angustifolia) | 2 to 4 weeks
• Lupine (Lupinus) | 12 weeks, may need warm-cold cycling
• Magnolia (Magnolia) | 2 to 4 months
• Marsh Marigold (Caltha palustris) | 4 to 6 weeks
• Marshmallow (Althaea officinalis) | 2 weeks
• Masterwort (Astrantia) | 3 to 5 months
• Milkweed (Asclepias syriaca) | 1 to 3 months
• Penstemon, Beard Tongue (Penstemon) | varies
• Perennial Sunflowers (Helianthus) | some need 4 months
• Pincushion Flower (Scabiosa) | 2 to 3 months
• Prairie Coneflower (Ratibida) | 2 weeks
• Prairie Violet (Viola pedatifida) | 2 to 4 weeks
• Primrose (Primula) | varies, 2 to 6 weeks
• Poppy (Papaver) | some need 2 to 4 weeks
• Rock Rose (Helianthemum) | varies, 1 to 4 months
• Sage (Salvia officinalis) | 2 weeks
• Shooting Star (Dodecatheon meadia) | may take months
• Soapwort (Saponaria ocymoides) | 2 to 4 weeks
• St. Johns Wort (Hypericum perforatum) | 2 to 4 weeks
• Trillium (Trillium) | several cold-warm cycles, 3 months each
• Turtlehead (Chelone) | 2 to 4 weeks
• Veronica, Speedwell (Veronica) | varies, 2 to 12 weeks
• Wallflower (Erysimum) | 1 to 2 weeks
• Wild Geranium (Geranium maculatum) | varies, weeks to months
• Yarrow (Achillea) | 4 weeks

Resources

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Databases

• Seed Germination Database – Perennials | Tom Clothier at hort.net
• Prairie Nursery Seed Propagation | prairienursery.com
• Ontario Rock Garden & Hardy Plant Society | Tips for germinating seeds by species

Notes

What is vernalization?

While the word “vernalization” is sometimes used interchangeably with “stratification” but the two words have different meanings.

Vernalization is “the artificial exposure of plants (or seeds) to low temperatures in order to stimulate flowering or to enhance seed production.” [Britannica.com]

~Melissa the Empress of Dirt ♛