Introduction to seed stratification

Perennials are those flowers we look forward to returning in our gardens every year. Whether colorful Irises or the bold Peony, they’re a favorite among gardeners. These flowers seem to magically appear during spring reminding us of the colorful spot they filled the previous year. Long before this reawakening occurs, the seeds have been prepared for growth by the natural cycle of seasons.

Seeds in Nature
Seed germination begins once the conditions for its growth are favorable. This means that a seed can lay dormant until necessary, a neat built in feature for gardeners that pre order their favorite blooms.

Seeds are essentially a case holding the plant embryo. In nature, the seeds absorb moisture through soil causing their outer coat to break. This starts a chain of chemical reactions that allows the plants cells to duplicate and it to begin growing. The root is the first to emerge from its tiny plant cocoon and begins its search for nutrients by growing downward into the soil. This tiny root also serves as an anchor for the seed and a home base for all the oxygen, nutrients, and sunlight that the seed will need to continue its growth.

Once the process begins, the seed needs only three things to survive; water, oxygen, and correct temperature. Mother Nature provides all of these of course, and since the requirements are so necessary at the correct time, gardeners have learned to nurture seeds through the process.

Seeds that germinate in cold winter areas however, have an additional requirement before they will emerge. These hearty seeds ensure survival by developing hardened shells. The shells are softened by the fall rains before being exposed to the low temperatures of winter. It’s not until this two-step process occurs that the seed germination process begins. This process, seed stratification, is how the seeds are signaled to awaken and grow.

What is Seed Stratification?
The term seed stratification can sound intimidating to gardeners, but it’s simply a term used to describe the pretreatment to begin the seed germination process. Think of it as a “jump start” to the rush to spring growth. Other terms you may encounter while reading up on the topic are “cold stratification” or “warm stratification.”

This pretreatment process simulates the conditions that occur in nature and allow us to control when the seeds begin to germinate. Stratification coaxes the seeds embryos from their dormant state and can take months. Since the balance is a delicate one, in nature plants develop a multitude of seeds. Many of them will never reach the maturity for germination and even more end up as bird food or meet another fate.

When gardeners intervene, the process of seed stratification can take over three months. This makes planning ahead essential. Further increasing the need for garden planning is the fact that every species of seed has their own likes/dislikes when it comes to the germination conditions. Some are of course more stubborn than others are and this can be especially true of the seeds of trees and shrubs.

Seed Stratification: The Need
Seeds that grow in the regions that have especially cold winters have greater requirements to coax them from their safe cocoon.  Unlike other growing conditions, these seeds are not happy with simply being placed in good soil with a little light and water. These seeds need an extra boost to get them to open up; this boost is the seed stratification process. Gardeners can intervene by introducing the seeds to the type of conditions they would naturally be found in nature. The seeds from the cold winter areas are built to be especially hearty; they are to endure colder temperatures and so developed a tougher shell. This shell is what seed stratification seeks to break through, to jump start the seed germination process and bring the sleeping flower back to life.

Again, keep in mind that the goal with seed stratification is basically faking winter conditions to prepare a seed to sprout.

The Stratification Process
We know that seed stratification is a way to simulate the conditions that a seed needs to begin germinating. Essentially the goal is to simulate the seasons of fall and winter, then the seed knows that spring is here and nature takes over.

Generally the ideal temperatures for warm stratification are 68 to 86 degrees and cold are 34 to 41 degrees.

Simulating fall starts by soaking the seeds for a period of 24 hours. This is commonly done by placing seeds in a shallow container with water. A small jar is then used to place seeds in water for soaking in cold temperature. This simulates the cold temperature of winter and its end, imagine it as the snow melting.  The seeds are then removed and placed in soil once the required time for seed stratification is met (will vary by seed species.)

Another method referred to as the “paper towel method” is also popular for seed germination. You will need a medium to hold in the moisture, a growing medium such as sand, sphagnum moss, vermiculite, or peat moss. The two requirements are that it has to be sterile and retain moisture. You’ll first soak the seeds overnight to simulate the fall season, then lay them on a moistened paper towel that has been wrung out. Seal the seeds in by folding the paper towel in half and applying pressure. Place the paper towel in a plastic bag and into the fridge they go for an average period of three months, simulating winter. It’s good to check the seeds once a week once you are past the first month to ensure that they are starting to germinate. Any seeds that smell musty or have brown spots should be disposed of, these will not be making it to the year’s garden patch .Another fast method to jump start the seed stratification process is to use the refrigerator for placing seeds in jiffy pots. These are just kept cool for a short period of time to get the process started.

You can tell by now that a certain amount of planning is necessary to be successful with seed stratification. It starts not only with the species that you wish to germinate but also with the time that you would like it to grow. The process will vary depending on fall, winter, or even snow planting.

Though the variety of techniques for seed stratification can seem endless, the basics are the same. The goal is to simulate the fall and winter seasons, softening the seed before exposing it to cold temperatures to start germination. Undoubtedly, one method will work for a certain species and fail with another, keeping a gardener’s journal will help you to nail down which works best for your favorite seeds.

As a note, some seeds require both a warm and a cold stratification period for them to germinate. In these cases, the warm seed stratification process is done first followed by the cold. It should also be noted that this is the general outline of a seed stratification process. Each plant and growing region has specific requirements and may respond very differently to conditional variances. Specialized information should be obtained for each type of perennial, tree, or shrub seed you wish to germinate.

How does seed stratification work in the nature, in the wild?

Seed stratification in nature is the process where seeds are out in the open and subjected to low and high temperatures, which, in turn, breaks dormancy. It has been said that the seeds of perennials, evergreen trees, shrubs, and herbs will not germinate after being harvested, unless their seeds were pre-treated, which is known as the stratification process. Seeds like these are often times incomplete and would require a bit more time in order to develop completely. Some of them would even have what we call a built in dormancy mechanism that would prevent them from germinating

What types of seed stratification are there in nature?
Basically, there are two types of seed stratification in the wild: cold stratification and warm stratification. The process of cold stratification means exposing the seed to cold temperatures, i.e. in the winter season, when the seeds lie on or in the ground. The process of warm stratification takes place during – you guessed it – the warmer season, when the seeds have fallen off the plant or flower, usually in the autumn. Regardless of it is a cold or warm stratification process, the overall purpose is to awaken the seed from its sleep so that it can start to grow.

As nature can be seen to be unpredictable, humans intervened by making use of modern methods of seed stratification, which is to mix the seeds in a moist like material such as perlite, peat moss, sawdust, or other special potting soil that would help stimulate the type of condition the seed would normally experience out in nature. For the stratification process, polyethylene bags are used to store the seed mixture in. Depending on the seed, the bags would either be stored at room temperature to coincide with the warm stratification process in nature, or it would be placed in a refrigerator where cold and moist conditions are required for seeds that would normally do better under the cold stratification process.

If the natural way of cold stratifying is preferred, then the seeds can always be sown directly into a nursery bed during the Autumn season, or placed into a pot during Spring season as it would satisfy the requirements of the particular seed. These methods would mean that you can skip the pre-treatment process as the natural method would accomplish the same end goal. The only difference being, that the natural stratification process takes a little longer than what would be the case if humans intervened.

To let mother nature takes is course with regards to the stratification process, it is suggested that gardeners or hobbyists just plant the seed in a natural seedbed during fall, and at a time when the first frost is expected. It is best to cover the planted seed with some mulch to help protect it against the elements of nature such as snow, rain, or the wind. Once this has been done, it is a case of displaying the needed patience and let the natural cold stratifying procedure run its course. All these seeds need are time to overcome its dormancy and germinate when the time is right for it. Also take care not to overwater the seeds that needs germination to take place.

From a scientific point of view, the seed stratification process starts when certain levels of endogenous inhibitors have been lowered. These inhibitors keep the seeds asleep, and the stratification process stops that by lowering these levels. For a discussion about this, see this excellent 2008 article by Bentsink and Koornneef (click on image):

Cold stratification, seed stratification, warm stratification, methods of seed stratification, seed stratification in nature

Bentsink and Koornneef (2008)

To conclude, note that some seeds need to be exposed to warm temperatures, e.g. humid, moist conditions, and some need to be exposed to cold conditions, in order to be able to start growing.

What is seed stratification?

Have you ever wondered about what the term seed stratification means? Within the circles of horticulture, stratification would be considered as the process of actually pretreating seeds in a similar way that mother nature would. In the process the seed would be subjected to cold as well as moist conditions where the temperature would vary between 34 degree Fahrenheit to 41 degree Fahrenheit (1-5 degrees Celcius).

When controlled, the stratification process is nothing more than ensuring that the seeds in question are subjected to cool, but not freezing conditions for a period that is long enough for the relevant plant species. The time period could be anywhere between one to three months.

Well, in short, it is a term that describes a process. The significant thing about this process is that it is beneficial for the seed because it makes it easier for it to germinate. Which, in turn, makes it possible for it to become a plant, and, eventually, to grow into a vegetable, a tree, a beautiful flower or whatever it is striving for. An example of a plants that needs to be stratified, is Atropa belladonna.

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Atropa belladonna (a perennial plant) does require stratification. Photo by Peganum.

Most gardeners become skilled in the art of stratification by effectively copying the set requirements for a particular seed, but doing so indoors. They do this by placing the seeds in a moistened paper towel that gets put in a sealed container, and in the case of cold stratification, they would place it in refrigerator. For warm stratification, the seeds would just be stored indoors in an environment where the temperature does not exceed 85 degrees Fahrenheit (30 degrees Celcius).

Another way to explain cold stratification. It is the process where one would fool seeds into thinking it is winter, when it is not.

When would cold stratification be needed? Generally, cold stratification are applicable to seeds that need time in the ground during winter time to get them ready for the germination process.

How does it work? In short, the process of seed stratification means that the seed goes from one state to another; it leaves the dormancy and starts to germinate. The seed stratification process may follow different steps depending on what seed we are talking about. For example, it may be a cold process or warm process. Further, the process may occur in your garden or in your home, if it is something that you are working with, or in the nature if it is a wild plant. Seed stratification is described in many books about flowers and plants, for example, Zimdahl’s book Fundamentals of Weed Science, Third Edition, published by Elsevier. There is also plenty of information and advice on the Internet, for example on this web site. We have tried to make this site accessible to as many as possible, to the benefit of gardeners and their plants everywhere.

What seeds would normally need to undergo the cold stratification process for them to break dormancy and then germinate? The plants that would normally require stratification would include Fuchsia, Hardy Hibiscus, Evening Primrose, Sedum, Ironweed, Hen-and-Chickens, Chinese lantern, Perennial Sweet Pea, False sunflower, Foxtail, Butterfly bush, Catmint, Black Eyed Susan, and Lavender.

Gardeners taking it upon themselves to speed up the stratification process would need to take much needed care in ensuring that any additional material such as seed-pod fragments, cone scales, fruit pulp, and the like are carefully removed from the seeds without removing the shell of the seed they want to stratify. The stratification process can be shortened further by soaking seeds in cold water for at least 6 to 12 hours before it gets placed into a sealed bag or container. Other seeds would need warm stratification which are then followed by cold stratification. This is accomplished by planting the relevant seeds in a mulched garden bed that would be ready for germination during the following Spring season. Some gardeners have a knack for stratification, while others would have to practice to get it right.

Do all plants require stratification? No. You do not have to stratify, for example, anemones:

seed stratification flower

Anemones (a spring bulb) does not require stratification. Photo by Özgür Mülazımoğlu.

In summary, seed stratification is a process that you will have to master, eventually, when you progress as a gardener. Browse this web site with its texts, images, and links to learn more and to become inspired. Good luck with your gardening.