Do you want to grow some perennial seeds that require cold stratification? Many perennials that are adapted to temperate climates with cold winters require undergoing a cold-moist period to unlock their dormancy before they can germinate.

This guide is meant to help gardeners learn why and how to cold stratify seeds successfully. It is thorough enough to give you all the information you need to get going and yet down to earth enough that it should be relatively approachable to most gardeners.

It’s late November at the time of writing and we have had a taste of colder weather to come. The soil hasn’t frozen here yet and it’s a perfect time for preparing seeds to cold stratify through the coldest months of winter ahead.

I’ve been planting seeds that I harvested in our little homestead nursery here and there since August through now. Those seeds have remained moist and fresh and are ready to stratify through winter. Other seeds were recently purchased or otherwise acquired and have been dried for easy storage. I am now planting the remainder of those seeds to re-hydrate and stratify as well.

Whether you have your temperate climate perennial seeds in hand or you are researching ahead for future seasons, this article has you covered for growing perennials from seed that requires cold stratification. Let’s get started!

If you have any questions or if anything is unclear as you read this article, please feel free to ask in the comments and you’ll receive an email notification when we reply.

The Role Cold Temperatures Play in Seed Germination

Many temperate climate perennials have developed a seed dormancy mechanism that requires cold temperatures to germinate. A seed’s dormancy protects the seed until conditions are right for germination.

Seeds don’t want to sprout just before winter because they’d have a very low chance of survival when the freezing temperatures come on. Instead, most want to wait until spring or early summer to germinate so that they can have a whole season to grow and establish themselves, photosynthesize and build strength and reserves to prepare for the winter ahead.

So even though temperature and moisture conditions may otherwise be favorable in the fall when a seed drops to the ground, most temperate climate perennials use this dormancy through the winter.

These seeds have a built in natural technology that allows them to track the length of the cold period and determine when the worst of winter has passed so that they can safely germinate for the best chance of survival.

What is Cold Stratification?

To cold stratify, or stratify for short, is an old term from the nursery trade before the days of refrigeration. Nursery folk of yore would layer tree and shrub seeds between layers of moist sand to overwinter in a protected space.

They would build a wooden box, for example, with a smidge of space between the side panels and bottom slats to allow for drainage and a little airflow.

A layer of sand is spread at the bottom of the box, followed by a layer of seeds with space between each seed. Then another layer of sand, followed by another layer of seeds. Alternating layers of sand and seed are spread out until the box is full or there is no more seed.

Hence stratify or stratification, which come from the Latin word strata, meaning horizontal layers.

The box was either placed outdoors in the winter to experience the natural cold cycle or else in very cold climates it might be protected from the extremes in a root cellar or an unheated building.

Most of us gardeners and homestead scale nurseries don’t need to make the most of space with layers because we’re not working with that much seed. Anyway, regardless of using one layer of seeds or many, this is still a valid way to cold stratify seeds.

I’ll share a few other, perhaps more accessible options too. But first a bit more on the three primary variables we will be working with…

Temperature

Since seeds want to make sure they get through a winter before they sprout they need consistent cold temperatures during the cold stratification period.

In general we are looking for temperatures between 34 – 40° F (1 – 4.4° C), or just above freezing.

To cold stratify outdoors your winter climate will need to have temperatures in this range for at least a few months. Otherwise gardeners in warmer climates that don’t get freezing conditions will need to emulate the temperatures of a colder winter using a refrigerator.

Our zone 7b climate for example has enough cold days to stratify all but the pickiest seeds outdoors during our winters. Our days do get below freezing too but often our coldest months will have many days in the 30’s or low 40’s with freezing nighttime temperatures.

Colder climates with soils that stay frozen for weeks at a time can still stratify seed outdoors (after all that’s what happens in nature) but some seeds may be damaged by harsh freezes if there is not some insulation over the soil like snow cover or mulch. Other seeds are more tolerant and will ride out freezing conditions regardless.

Moisture

Another other key component of cold stratification is consistent moisture.

Moisture is inferred in cold stratification but that’s not obvious.

It wasn’t to me anyway. I initially thought I could just put my packet of dry seeds in the freezer to stratify. Not true. That would help preserve the seed but not stratify it. Seeds need to be moist to cold stratify.

That’s why some seed companies say something like “seed requires a moist stratification period”. Otherwise gardeners sometimes get confused and put dry seed packets in their fridge.

Moisture and temperature combined are the two key components a seed needs to unlock during a cold stratification period. Seeds need to be kept moist through the entire cold stratification period and until the seeds germinate.

Some people pre-soak seeds before stratifying, but when planting in soil or potting soil this is not necessary since the seeds will quickly absorb moisture from surrounding moist soil.

Time

Seeds need to be moist and cold for a minimum duration of time to successfully unlock dormancy.

The time frame is different for everyone. For some seeds it can be as little as one week but most seeds need at least a few months of cold stratification. Typically many seeds require somewhere between 2 and 3 months of cold stratification.

Most seed companies will indicate how long any given species’ seeds need to cold stratify. When in doubt give them 3-4 months of cold stratification.

For me, I typically cold stratify my seeds outdoors in the nursery. I try to get my seeds potted up by late November (if not earlier) so that they have the full 3+ months of coldest temperatures. I do this with pretty much all my seeds that need stratification regardless of their minimal stratification time requirements.

An exception, for me anyway, is with seed that only needs a short period of cold stratification, such as somewhere in 1 week to 1 month. Then I might pot up my seeds to stratify outdoors between January and early February. Colder climates might be able to extend that a bit later.

Or sometimes seeds, like valerian for example, don’t require cold stratification to germinate successfully, but will have a higher germination rate with cold stratification. This is usually noted by seed companies as “benefits from some cold stratification.”

Either way I have more flexibility with those last two scenarios and I can start stratifying earlier or later.

1. As a general rule of thumb, stratifying earlier is often better than later, when possible.
2. The second rule of thumb, which is even more noteworthy, is that fresh seed often germinates better than dry seed, when possible. Coincidentally this also implies earlier sowing, hence the first rule of thumb. (More on that in Fresh Seed vs Pre-Dried Seed below).

What Seeds Need Cold Stratification?

As alluded earlier, seeds that require cold stratification come from plants that evolved to live and sexually reproduce in climates with cold winters. So we can say:

Many, but not all, temperate climate perennials require some amount of cold stratification to break their dormancy.

Plants who evolved to live in warmer (southern) parts of temperate climates and whose range don’t extend north into zones with colder winters probably don’t require cold stratification. Some plants that handle cold weather but have other mechanisms for unlocking and require scarification (such as fire or acid in nature) may or may not also require cold stratification.

As mentioned above, some plants don’t need stratification but can benefit from it. In my experience temperate climate short lived herbs may require less stratification time and longer lived trees and shrubs may require more stratification time. I am not sure if this is an evolutionary feature or a result of faster domestication of plants with shorter lifespans. Perhaps both!

Most quality seed companies will indicate details about a seed’s cold stratification requirements if any. However, if I am collecting my own seed, working with a new species, or if I am in doubt for any reason I consult third party resources.

My favorite resource for researching perennials is Plants for a Future (PFAF). If a plant is in their database they will provide a relatively brief snapshot of the plants growing requirements, including whether cold stratification is necessary. I suspect entries in PFAF are not always based on first hand experience, but they always appear well researched at the least. I typically consult PFAF as well as googling and looking for reputable seed companies, blogs or forums that offer hints as to the cold stratification requirements of any given species.

Fresh Seed vs Pre-Dried Seed

When we grow annual vegetables and save our seed, we are used to drying out the seed for planting next spring. Seeds that require cold stratification on the other hand will typically have their best germination rate when the seed is not dried out before cold stratifying. In other words, keeping seeds fresh is always best when possible.

However there is a big gradient here. Some temperate climate seeds, like apple seeds, can be dried and still have really good germination rates after soaking and then cold stratifying them. On the other hand some seeds, such as nuts like Chestnuts, will simply not germinate after being fully dried.

Many species lie somewhere in the middle and drying the seed out to store them for some time before cold stratifying them will result in either somewhat lower or significantly lower germination rates. But often we can enjoy at least some germinating plants from dried seed, which allows us the luxury of purchasing or acquiring less common seed from non-local sources.

So if you are collecting your own seed, or have access to local fresh seed, it is always advised to plant it as soon as possible after it is ripe and then keep it moist through the cold stratification period.

On the other hand when buying seeds, it is the rare seed company that will send moist pre-stratified seed to their customers, because of extra expense required for refrigeration and the risk of spoilage during shipping. I have purchased seed from a few such vendors and really appreciate the high quality and receiving fresh seed.

If a seed company is selling dried seed, it is ethical to only sell dried seed that will have some reasonable amount of germination, or being transparent and indicating the difficulty of germinating the seed, or ideally both.

In practice there are a lot of seed companies that will still sell seed that will be very difficult to germinate or have extremely low germination after the seeds dry without providing any indication, so we need to be aware as gardeners and consumers what we are purchasing.

It really is up to the gardener to know how to grow what they are buying. These days many of us plant enthusiasts are re-learning old ways, such as growing perennials from seed whose plants we’ve never seen, smelled, touched or tasted before, so we are creating new relationships with seeds and plants learning as we go.

This kind of education is a big part of the motivation for this article, because we are all helping each other.

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Cold Stratify Seed Outdoors in Pots

My favorite way to cold stratify seeds is outdoors through the entire winter.

Stratifying outdoors is working directly with the natural rhythms of the climate and winter season and it’s easy and fun. I am grateful our climate is appropriate for cold stratifying outdoors.

What I do is plant the seeds in nursery pots with good quality potting soil with plenty of drainage and good moisture retention.

I could plant one or a few seeds per pot and sometimes do with larger seeds especially. But personally I usually prefer to put several or many seeds in one appropriately sized pot and then divide the seedlings in the following spring, summer or fall after they have germinated and grown to an appropriate size.

I try to space the seeds out within the pot. For large seeds like nuts I keep at least an inch or two between seeds. For smaller seeds which will grow slower initially I space them out maybe a quarter inch to a half inch at minimum. I’m keeping in mind that the more dense my spacing, the earlier I have to divide them the following season. With more generous spacing I could hold off on dividing and up-potting seedlings for a whole season if needed.

I cover the seeds with soil per the requirements of each seed. The rule of thumb is to cover the seed with about 2-3x the width of the seed using potting soil. Some seeds require light to germinate and shouldn’t be covered much or at all, and simply pressed into the soil. Especially very tiny seeds don’t have the strength to push through much soil depth.

Label each pot carefully with the plant name and month and year they were planted so you don’t lose track. Keep the potting soil consistently moist until they germinate.

Cold Stratify Seed Outdoors in the Ground

Another natural way to stratify seeds outdoors in a climate with cold enough winters is to plant the seed directly in the ground as early as possible before winter. For big seeds like nuts this is relatively easy. Smaller seeds require a finer prepared seed bed.

Follow the same instructions above except instead of using potting soil in pots you are planting directly in the soil. It’s crucial that the soil is well draining. Raised beds will also work well.

Mark your bed carefully so that you don’t disturb the soil later, for at least one season and up to two or three seasons for slower seeds. Keep good track of what was planted where and what date they were planted.

Cold Stratify Seed in the Fridge

The less natural but sometimes more convenient way of cold stratifying seed is using a refrigerator. Coincidentally refrigerators are often set to the perfect temperature for cold stratifying seeds. Somewhere between 34 – 40° F (1 – 4.4° C) is perfect. But use a thermometer to be sure.

I always prefer cold stratifying outdoors when possible, but the fridge can be a convenient alternative. So when do we want to use a refrigerator to cold stratify seeds?

• In warmer climates when winter is not cold enough to stratify outdoors.
• When winter is too cold and doesn’t provide a reliable snow layer of insulation to protect seeds from freezing too hard. (Some seeds can handle harsh freezes while others can’t.)
• When stratifying out of season because we got a late start (or early depending how you want to look at it!)
• When a seed requires double periods of cold stratification (see Double Cold Stratification toward the end of this article) and we want to speed things up rather than waiting for two winters.
• When we want to otherwise have access to pre-stratified seed for sharing, selling or planting as needed throughout the winter.

Stratifying in the fridge is pretty much the same concept as outdoors during winter but the biggest challenge is that mold is more likely since we are creating a more artificial environment, rather than planting seed outdoors in a healthy ecosystem.

Potting Soil in Pots

When I only have a few kinds of seeds to stratify, or if space in the fridge isn’t otherwise an issue, I will plant seeds in nursery pots and stratify those pots and their seeds directly in the refrigerator. I pot up the seeds as described above in Cold Stratify Seed Outdoors in Pots.

To get around the mold issue, when I cold stratify seeds in my fridge I use or re-use healthy potting soil or raised bed soil that’s been in the garden for some time and is already full of life (i.e. not straight from a bag).

If the soil is not already moist, I make sure to fully saturate the potting soil before planting and then again moisten any additional soil that was added after planting the seed. Allow plenty of time for the soil to drain excess water (since we are using well draining potting soil!).

Then I will place a yogurt lid (or plate) under the pot to keep the surface of my fridge dry and clean. I will place another yogurt lid (or plate) on top of the pot to keep the soil from drying out. (See photo below.)

Label each pot carefully with the plant name and month and year they were planted so you don’t lose track. I set a reminder on my calendar when they are done stratifying so that I can pull them out to the garden or nursery to warm up and hopefully germinate.

I love stratifying seeds this way because its so easy. I don’t have to re-plant my seeds later. When the stratification period is done I just take the pot out to the garden and continue to keep it moist until the seeds germinate.

I also periodically check, maybe once a month to make sure the soil doesn’t dry out, but I’ve never had it dry out this way.

Potting Soil in Bags

Second to using nursery pots, my next favorite way to stratify seeds in the fridge is using healthy, living potting soil in plastic zip bags appropriately sized, such as quart or sandwich size, sometimes gallon for larger seeds and nuts (see photo above).

I like to have enough soil in the baggy to surround the seeds on all sides. This is to prevent mold from forming. If part of the seed is outside the soil there is more likelihood that mold can develop. Some seeds will be more prone to mold than others, and its also a factor of how clean the seeds are from when they were collected and processed.

Since plastic bags do not drain, it’s even more important to make sure the soil is moist but not sopping wet. If you squeeze the soil you don’t want to see water running between your fingers and you don’t want to see water gathering at the bottom of the bag. If so, remove the soil, let it drain longer and then try again.

Label each bag carefully with the plant name and month and year they were planted so you don’t lose track.

Other Mediums

Other mediums can be used in place of potting soil. Moist sand, coconut coir or sawdust for example work well for some gardeners.

Many websites, nurseries and seed vendors unfortunately recommend using moist peat moss, and while effective this is not an ethical choice because peatlands are being unsustainably harvested and their carbon sequestration is important for the health of our planet. So please avoid peat moss and use a medium that is more local or sustainable.

Sterility

Some sources call for sterile medium to stratify seeds to prevent mold. As I mentioned above I prefer to use healthy living potting soil (or sand or what have you) and let soil biology do what it is supposed to, rather than trying to create a sterile environment which is even more artificial. But it is possible and it works for some people, it’s just not my bag of seeds.

Checking for Germination

When stratifying in the fridge it’s important to periodically check for germinating seeds. When the seeds germinate they will need to be moved into soil with light as soon as possible so that the plants can grow. If they germinate and are left in the fridge too long they will perish.

Protecting Your Seeds

When your seeds are stratifying outside through the winter it’s important to consider protecting them from rodents and birds.

Rodents are hungry during the winter and will seek out seeds for food. The colder the climate, the more protection your seeds need.

In my climate and with our rodent pressure, covering the tops of the pots with hardware cloth is enough to keep squirrels and birds from digging in the potting soil.

In colder climates where food is more scarce during winter it’s advised to completely encapsulate your nursery pots or your seed bed with quarter inch hardware cloth on the top, bottom and all sides to prevent rodents from accessing the seeds and digging or chewing through plastic pots. Make a rectangular bottom and top that can be wired together or overlaps in such a way that mice and voles can’t get in. Make sure the top is easily separated for maintenance or as needed.

Root cellars and unheated buildings are advantageous in colder climates because they are cold, humid and additional protection is not necessary, assuming they are rodent free.

After the Stratification Period is Over

When the stratification period is over the seeds will theoretically be ready to sprout but unless they germinate in near freezing temperatures they will still need additional conditions to be favorable before they will sprout. Typically change in temperature and light will eventually trigger the seeds to start sprouting.

Many seeds will sprout in the spring or summer, in warmer temperatures. Others that are more cold hardy, like valerian or blue elderberry for example, will germinate earlier in colder weather.

If you are stratifying seeds in pots outside or in the ground, there is not much else you need to do except keep them moist, other than waiting patiently for them to sprout as the weather warms up.

If you are stratifying seeds in the fridge you will need to sow your seeds when the stratification period is over. If it is already spring you can sow them in pots and put them straight outside.

Otherwise if it is still winter and you live in a climate where the ground is still frozen solid, you can:

• Keep them in the fridge a little longer if they haven’t germinated,
• Sow them in pots inside somewhere with plenty of light and then harden them off when the weather warms up outside,
• Or if the soil is not frozen outside you can sow them outside immediately in nursery pots or in a prepared bed.

Spread the seeds out and sow them as described above in Cold Stratify Seed Outdoors in Pots. It’s easier to do with large seeds. The smaller the seeds get, the harder it is to see them in your potting soil. Therefore the smaller the seeds are, the less soil you need to use to stratify them in your fridge and using minimal soil helps sow effectively.

Some people like to stratify very small/fine seeds in a small amount of sand. Then when it’s time to sow the seeds, simply spread the sand out on the surface of the nursery pot’s soil and the seeds will also be spread out.

Otherwise if the seeds are already germinating, carefully plant each seedling in its own pot.

Multiple Seasons

Some seeds don’t germinate in their first season after stratifying. Often this is by design (see Double Cold Stratification below).

I think its also a feature for some seeds that are dried. Fresh seeds may germinate more reliably, but also sooner, than dry seed. If the seeds don’t germinate in the first season don’t give up on them. I’ve had some seed germinate after going through two or three winters outdoors.

I usually don’t keep my pots of seeds after 3 or 4 seasons if they haven’t germinated by then. With larger seeds I can actually excavate and see if they are still there and alive, or if they have rotted.

Other times I’ll have some seeds germinate in the first season and then if I don’t disturb the soil more seeds will germinate in the next season or two. Sometimes seeds like to hedge their bets and spread out their germination across multiple seasons in case one of the seasons presents harsh conditions and limits survival rates.

Here’s a telling story: I tried to sow skullcap seeds one year and they didn’t germinate but I kept the pot undisturbed and moist. The following year I purchased skullcap seed from another two sources and still no luck. The third year I purchased seed from yet another source. The following spring I had four pots of skullcap germinating! Each time I sowed it and stratified them the same way. I can only conclude that the final source of seeds were younger and higher quality. But they all eventually germinated!

So don’t give up on your perennial seeds after one season! Wait at least a few seasons.

Double Cold Stratification

Some species absolutely require going through at least two winters to unlock their dormancy. This is often referred to as “requiring two periods of cold stratification”.

This also implies that the seeds will need to go through a warm and moist period between the two periods of cold stratification. Sometimes they need a warm and moist period before the two cold periods as well.

The easiest way to do this is leave your seeds planted outside for several seasons at least, until they germinate, keeping them moist the whole time.

Some gardeners will speed the process up with the help of a refrigerator and moving the seeds between the fridge and a warm space or warm garden for the required time and get it all done in the better part of one season.

Wrapping Up

Today we learned about cold stratification. In a nutshell we are talking about planting seeds and allowing them to go through winter, or winter like conditions, so that they can germinate successfully.

When we bring seeds away from their natural rhythms of dropping to the ground or being eaten by birds and mammals and then pooped to the soil, we need to be mindful of their germination requirements if we want them to grow well consistently.

Seeds and nature are amazing and have so many intricate ways to live and move through this world. And yet with all their inherent sophistication they are also incredibly simple. What a paradox!

The more I work with seeds the more I am humbled, amazed and grateful to be a part of this beautiful world. This world has everything figured out and here I am a tiny human trying to learn how to work with seeds.

Well, seeds already know what to do—seeds and plants are the true gardeners. I’m just over here playing with seeds in the sandbox while I watch the seasons go by.

I hope this guide was helpful to you. If you are giving cold stratification a try, let us know how it went in the comments!