Cold Stratification Of Seeds & When To Cold Stratify Seeds

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One of the things that we’ve run into as we have explored more perennials and cold climate focused flowers has been the need to utilize cold stratification for the successful germination of certain types of seeds.

Cold stratification is the practice of placing your seeds in a cold place, such as a refrigerator or freezer, for a period of time prior to actually sowing them.

This process simulates “winter” for the seed and it can highly benefit specific types of seed.  Essentially, it breaks the seed’s dormancy and assists with improved germination.

This exposure to colder temperatures is typically performed for at least a week and sometimes even several months prior to sowing the seeds.

This article explores the plants that do well with cold stratification and will provide a handy schedule for when you should stratify your seeds so they are ready for your sowing!

We also have a video that covers this topic!

Not All Seeds Need Cold Stratification

It’s important to note that the overwhelming majority of garden seeds, be it flowers or vegetables, do not require any kind of cold stratification to successfully germinate.

However, there are some that will either benefit from or outright require cold stratification for them to germinate properly.

When a seed benefits from the cold stratification process, the direct impact will be to increase your germination rates.  (Or, percentage of seeds that successfully germinate.)

To be clear, even those seeds that supposedly “require” cold stratification can also germinate without any cold stratification process at all!

Essentially, these seeds are “genetically programmed” to expect and experience winter.

The seed’s dormancy, which is broken by cold stratification, is designed so the seed germinates at the appropriate time in the season.  Typically, after winter and right before spring.

In addition, we’ve found that the need for cold stratification of seeds entirely affects flowers.  There really aren’t any vegetable focused seeds that require stratification, at least to our knowledge.

The Process Of Cold Stratification

There are different ways to practice cold stratification and there isn’t one single “right” way.

However, there are differences in the “success rates” of these methods.  Some more more productive, some are less productive.

The following techniques for cold stratification will generally work well:

• Some gardeners simply put the seeds in a labelled plastic bag or container and place them in the refrigerator
• Some gardeners will actually sow the seed into soil and put the whole “kit & caboodle” in their refrigerator
• Some gardeners might sow into vermiculite, sand or peat moss and keep the material lightly damp
• Some will use a lightly damp paper towel, typically enclosed in a Ziplock sandwich bag, to contain their seeds
• Some gardeners sow into soil and simply toss their plants outside early in the season

For the most part, the point is simply cold exposure.  In most cases, you’re not actually trying to get the seeds to germinate during this process!

Adding moisture to the mix during cold stratification can increase germination rates even further.  This is one of the reasons many growers prefer to cold stratify in damp paper towels, soil or other damp mixtures.

We’ve tried many of the methods.  Our current favorite technique is the “damp paper towel” method, illustrated in the image above.

If you are cold stratifying multiple seed types, labelling your seeds appropriately cannot be understated!  You will likely have a hard time identifying things properly if you don’t!

The Different Types Of Cold Stratification

For the most part, the grower has two options for cold stratification.

Refrigeration and freezing.

Refrigeration will typically bring seeds down to about 40 degrees Fahrenheit.  Whereas freezing can bring them down to zero degrees Fahrenheit or even colder.

There is not a “universal agreement” across all the different types of seeds of which method is most appropriate or successful.

Some will say to refrigerate, some will say to freeze.  Both methods will likely work, but most growers tend to gravitate more towards refrigeration.

We have found that some seeds “prefer” freezing.  For example, we had troubles with Larkspur, at least until we utilized freezing.

In general, our recommendation is to use refrigeration, unless you continue to notice low germination rates or there is a clear consensus that freezing is desirable.

How Long To Perform Cold Stratification?

As for the amount of time you will keep your seeds in the cold?  It depends.

Different types of plants will benefit from different amounts of exposure to colder temperatures.  This reflects that some seeds may desire a mild winter, versus some that are expecting a more severe one.

Generally speaking, there is flexibility involved in the timing and this is not an extremely precise process.  Plus or minus a week or two is unlikely to make a massive difference.

But, it’s important to not significantly over-expose those seeds to cold temperatures.  (i.e. multiple months)  Too much cold can be a bad thing, drying the seeds out, which can essentially destroy the seed’s viability.

This guide will discuss those recommended time frames for each of the seeds we talk about.  Try to hit those target time frames and you should be good.

Flowers That Need Cold Stratification

As we mentioned above, there are certain varieties of flowers that highly benefit from the cold stratification process.

For a quick summary, let’s cover all the various types of flowers that will generally see some benefit from the cold stratification process.

• Aster (*some varieties, check seed packet)
• Balloon Flower
• Chinese Lantern
• Columbine (also requires cold germination)
• Cosmos
• Delphinium
• Hollyhock
• Larkspur (also requires cold germination)
• Lavender
• Lobelia
• Lupine
• Marshmallow
• Milkweed
• Poppies
• Primrose
• Scabiosa (Pincushion)
• Tansy
• Thunbergia (Black eyed susan)

There are very likely other types of flowers out there that require cold stratification that we haven’t mentioned.  Always check your seed packets for specific instructions as it will be called out if it’s a requirement!

But, that’s pretty much the majority of them.  If it’s not on this list or on your seed packet, you can safely assume that cold stratification is not required.

When To Cold Stratify Your Seeds

When it comes to cold stratification, you have two important timing events you need to keep track of.

The first timing event you need to be aware of is when you will sow the seed, of course.

This timing is typically defined by your seed sowing schedule, which ensures you have a transplant of appropriate size by the time your last frost comes around.

Then, you also need to know when to start your cold stratification process.  This will typically be some time before that sowing date.

The challenging part here is that different seeds have different recommended timing!

Below, you will find a schedule that defines when to start the cold stratification process for each seed type.

Just like our seed sowing schedule, we’re also using “weeks before last frost” as our guide so you can easily translate this information between different growing zones and last frost dates.

Seed Cold Stratification Schedule

Here are the recommended timelines for when to start the cold stratification process for the seeds that are generally recommended for cold stratification.

Seed Type:	Start Cold Stratification:	Sowing Time:	Notes:
Aster	12-16 weeks	10-12 weeks	Refrigeration, not all varieties call for cold stratification
Balloon Flower	14-18 weeks	10-12 weeks	Refrigeration
Chinese Lantern	8-9 weeks	6-7 weeks	Refrigeration, water soak seeds for 24 hours prior to sowing
Columbine	6-9 weeks	4-5 weeks	Refrigeration, also requires cool germination
Cosmos	8-10 weeks	7-8 weeks	Refrigeration
Delphinium	10-14 weeks	8-10 weeks	Refrigeration
Hollyhock	12-16 weeks	10-12 weeks	Refrigeration
Larkspur	7-8 weeks	5-6 weeks	Refrigeration, also requires cool germination
Lavender	13-18 weeks	10-12 weeks	Refrigeration
Lobelia	13-14 weeks	9-10 weeks	Refrigeration
Lupine	10 weeks	9 weeks	Refrigeration
Marshmallow	7-8 weeks	3-4 weeks	Refrigeration or freezing
Milkweed	12 weeks	8 weeks	Refrigeration or freezing
Poppies	8-10 weeks	6 weeks	Refrigeration
Primrose	7-10 weeks	5-6 weeks	Refrigeration
Scabiosa (Pincushion)	7-8 weeks	5-6 weeks	Refrigeration
Tansy	10-12 weeks	6-8 weeks	Refrigeration
Thunbergia (Black eyed susan)	8-12 weeks	6-8 weeks	Refrigeration

When we say “weeks” here, we’re talking about the number of weeks prior to last frost to reference when you’ll conduct sowing and cold stratification.

This is counting back from your average last frost date, by those numbers of weeks.

The Lazy Approach To Cold Stratification

We illustrated a somewhat complicated approach above.  However, there are ways to “cheat” the process.

One of the things we realized was that at ten weeks to last frost, almost all the seeds we need to cold stratify are 2 to 6 weeks from their planting date.

In recent years, we’ve started cold stratification at ten weeks to last frost.  Then, we just pull the seeds from the refrigerator when we need to sow them.

This offers all these seeds between two and six weeks of cold stratification.  Well within acceptable amounts of time.  Again, this isn’t a precise process, remember that nature doesn’t present exacting conditions either.

This covers 90% (or more) of the seeds we grow that need cold stratification.  Very early sows, such as Hollyhock, might need to be started a bit earlier.

How Do You Know Cold Stratification Has Been Successful?

As we’ve discussed here, the primary benefit to cold stratification of seeds is to increase germination rates of your seeds.

So, you’ll simply see more seeds germinate.

This might be a little hard to gauge, since you’d have to do a controlled experiment to know if you’ve actually seen benefit!  You could certainly do just that!

However, with certain types of seeds (especially things like poppies, milkweed and some others on the list above), you’ll definitely see a difference!

It will be like night and day when it works.  You’ll go from having a few seeds pop to having a significant number of seeds germinate.

You will also observe a small difference in the vitality of the seedlings, too.  The cold stratified seeds will simply be more vigorous than their non-stratified brethren.

Germinating In Cold Temperatures

There’s a fairly obvious difference between cold stratification and actually germinating in cooler temperatures.

For the most part, once the seed is exposed to cold, it should be germinated in normal temperatures.  (i.e. your normal indoor or greenhouse growing process)

While some seeds simply need to be exposed to colder temperatures, a few seeds need to actually germinate in cool temperatures!

To make it easy on you, the only two flowers we’ve grown that require germination in cooler temperatures are:

• Larkspur (and related Delphinium)
• Columbine

Both of these are fairly late sows, typically 4 to 6 weeks to last frost.  You can easily bypass the entire cold stratification process by simply sowing these seeds outdoors and keep the pots outdoors or in a greenhouse during the early season.

Sure, they might freeze and experience frost.  But, that’s the point.  They like it like that!

The cooler temperatures prior to frost should be enough to aid these flowers in their cold germination habits!

If you’d rather control the germination process of these seeds, you can use a lightly dampened paper towel, contained in a Ziplock sandwich bag.

Simply enclose those seeds within the damp paper towel and put them away in the fridge!

You’ll want to check on them somewhat frequently.  Once those seeds have germinated, they need to be transplanted into soil as soon as possible!  They need light, soil and moisture, just like every other seed out there!

Have Fun Chilling Your Seeds!

Hopefully we’ve helped you with understanding the cold stratification process and which types of seeds will benefit from the process.

The schedules provided here will work well with our published seed sowing schedule.

We’d suggest checking this schedule each week during the sowing season.  If any of those seeds are “up” for cold stratification, get them in the fridge!

Then, once the sowing schedule calls for them, take them out of the fridge and pop them!

This should help you with the more difficult task of maintaining two schedules!

Have fun and grow many flowers!