What is Seed Stratification?

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Robert Pavlis

Some seed is difficult to germinate unless you stratify them. If you do that correctly, you can germinate just about any seed with ease. I’ve germinated over 2,000 different types of seed and many needed a cold stratification period before they would germinate. In this stratification guide I’ll tell you everything you need to know about germinating seeds.

3 seedlings just poking above the soil line
Seeds that have been properly stratified, source: Chris Penny

What is Stratification?

Stratification is a way to pre-treat seed in order to help them germinate.

Some seed goes dormant and in order to wake them up in spring they need a special treatment before they will germinate. It is basically a trick to make them think that winter is over. The treatment involves either a cold or warm period in either a dry or wet condition. Each of these is described below.

Then, when we remove the seeds from the cold and transfer them to a warm location, they believe it is spring and germinate.

If you want to understand seed dormancy better have a look at, Seed Dormancy – What is it and How do You Break it?

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Stratification vs Scarification

These are two different seed pre-treatments and because they sound the same, they are commonly mixed up.

Stratification is a treatment that involves temperature and or water. Scarification is a treatment where the seed coat is partially removed or damaged to make it easier for the radicle (root) to come out. This video shows you how to scarify seeds.

YouTube video

Germination Codes

Several organizations have developed germination codes to make it easier for gardeners to know how to germinate seeds. There are several different systems but many of them are similar. For example, a code A usually means a warm germinator where no stratification is needed.

Here are some examples of germination code systems:

The germination codes pictured below are from North American Native Plant Society.

list of letter codes along with the instructions for germinating seeds.

You might also enjoy: 15 Myths About Seeds and Seed Germination

Warm Dry Stratification

Some seed continues to mature as they dry in a warm condition and therefore become easier to geminate after a few weeks or months. This can be useful for some seed but it is not a stratification treatment. It is more correct to think of it as part of the seed maturation and storage process.

Warm Wet Stratification

This is the technique used for most warm germinating seed including all vegetables. Simply moisten seed and wait. Many people simply plant this type of seed in moist soil.

Cold Dry Stratification

In this process, seed is kept cold (see definition below) and dry. Placing dry seed in the refrigerator is a common way of doing this.

WARNING: Many people think that this technique is the same as “cold stratification”, but it’s not. When someone refers to cold stratification they mean “cold wet stratification”. Keeping seeds dry and in the fridge does NOT work for cold stratification.

Plant Science for Gardeners by Robert Pavlis

Does Cold Dry Stratification Exist?

Do any seeds require cold dry stratification? Some references do mention this technique for things like lavender and North American native species, but when you look these up in a reliable germination guide, they are either warm germinators or need cold moist treatment.

I don’t believe any seed requires this kind of treatment and here’s why.

I have germinated over 2,000 varieties of seed and never found one that needed it. The Ontario Rock Garden and Hardy Plant Society has the most extensive Germination Guide on the internet and in their over 8,000 species, they don’t have a single one that requires a cold dry treatment.

The New England Wild Flower Society has germination codes, but not one of them is for cold dry stratification. The germination codes used by Wild Seed Project doesn’t have such a code. I also connected with Prairie Moon Nursery, a good source for native seed, and they don’t use dry cold stratification either.

Sarah Tevlin et al in a study that looked at germination of Asclepias species, one of the seeds that is commonly referenced as needing dry cold treatment, said, “we believe the use of the term “dry–cold stratification” creates confusion. Stratification, by definition, can be conducted only for imbibed seeds (absorbed water). Imbibition restores physiological activity to seeds”.

As far as I can tell, the requirement for cold dry stratification is a myth. I suspect this myth has been perpetuated by a company called Prairie Nursery (not to be confused with the above mentioned Prairie Moon Nursery), who published a list of seeds requiring dry cold stratification. If you disagree with my conclusion, please add a comment below along with a link to reliable information showing that there are seeds that need it.

Cold Wet Stratification

This is the technique that should be used when people use the term “stratify” or “cold stratification”.

The process is relatively simple. Moisten the seeds and keep them cold for a period of time. Then warm them up so they germinate.

How Long Should You Stratify?

That depends on a few things; temperature, type of seed, age of seed. I’ll discuss temperature below.

Each species of seed has a different requirement for cold treatment. In many cases, fresher seed germinates faster than older seed because as seed ages it goes into a deeper dormant stage and needs a longer treatment to wake up.

Some will germinate after a few days while others may require several months. Most seed will germinate after 30 days, and almost all seed will germinate after stratifying for 90 days. The longest I have ever waited was 5 years for Halesia carolina.

Understanding Chilling Hours

Plants have a built-in mechanism for dealing with cold winters. Fruit trees won’t start to grow in spring until they have accumulated a certain number of “cold hours”. That is why some fruit trees won’t produce fruit in warm climates. Bulbs such as tulips won’t bloom until they get properly chilled.

Some seeds also use this mechanism which is called “chilling hours”. Until a seed has accumulated a certain number of chilling hours, it won’t germinate. This keeps seed from germinating in the middle of winter. Each species of seed requires a different number of chilling hours and most tropical seed doesn’t require any.

Chilling hours (Utah model) are accumulated when the temperature is between 3 °C (37 °F) and 9 °C (48 °F). The ideal temperature is  7 °C (45 °F) which matches closely to the temperature of a fridge. Temperatures below or above this this range do not add chilling hours, and temperatures above 16 °C (60 °F) actually reduce the number of chilling hours.

It is important to note that freezers have temperatures below 3 °C (37 °F), which means that placing seed in a freezer does not add chilling hours. Therefore freezing seed does NOT stratify them.

Seeds of a certain species are fully stratified and are ready to germinate when they have accumulated enough chilling hours.

Cold Wet Stratification Methods

There are several ways to carry out this technique and each method has pros and cons as listed below.

  • Direct fall sowing
  • Winter sowing
  • Direct planting in pots
  • Vermiculite baggy method
  • My improved baggy method

Direct Fall Sowing

This is the method most beginning gardeners prefer because it is easy. Experienced gardeners almost never use this method because it is very unreliable.

Simply spread the seed on the ground in fall and let nature take care of things. If the method work you will have seedlings in spring. The problem is that natural sowing results in a lot of seed death either before germination or after. Animals come and eat the seeds. Some are too wet and rot. Others are too dry and die. Even if germination happens the little seedlings need to fight off predators and the weather.

Pros

  • Easy.
  • No transplanting or hardening off seedlings.

Cons

  • % Germination is low.
  • Survival rate is low.

Winter Sowing

Winter sowing is a perfect technique for home gardeners because it is easy, produces good results and it allows you to garden in the middle of winter. Sow seed as normal in pots or clear containers containing soil. Put them outside and wait. Nature automatically does the cold stratification for you and in spring you have lots of tough little seedlings. This method is fully described in Winter Sowing.

white milk jugs sitting on a deck, some open showing seedlings
Winter sowing in spring time, plants are ready for transplanting

Pros

  • Easy.
  • Seedlings don’t need to be hardened off.

Cons

  • Can’t watch germination because it happens under the soil.
  • If seed does not germinate before spring, you have to wait a whole year to give than a second cold treatment.

Direct Planting in Pots

In this method you plant seeds in pots of soil and then expose them to cold, usually in the refrigerator or in a cold garage. After the cold treatment, the pots are put under lights or on a sunny window to see if they germinate. If not, the cold treatment is repeated. Make sure your seeds are still viable before using this method and use a good potting mix.

This method works quite well and is similar to the way most people germinate seeds so it seems as a good choice. When it works it is great, but what happens if the seed does not get enough cold treatment? Soon it is too warm outside to give them this treatment. So you need to use the fridge or wait a whole year to try again. Who wants dozens of pots containing soil in the fridge? This is the main reason for using the baggy method instead.

Pros

  • Relatively easy.
  • A method you are familiar with.

Cons

  • Managing a lot of pots is difficult.
  • Hard to continue cold treatment in summer.

See also: Germinating Seeds Quickly: Complete Guide

Vermiculite Baggy Method

This method is similar to the baggy method, but instead of using paper towels, it uses moistened vermiculite, sand or peat moss. It is important to keep the right of moisture in the bag or the seed either rots or dries out and this is the reason I don’t use the method, although some people have good success with it. If it looks wet, there is enough, and if water runs out of the bag, you added too much.

vermiculite and germinating seeds mixed together in a small baggy
Seeds being stratified in vermiculite, source: BotanyCa

Pros

  • Fairly easy to do.
  • Takes up a small space in the fridge.

Cons

  • Can’t see smaller seed to tell when they germinate.
  • Getting the right moisture level is tricky.

My Improved Baggy Method

The video below will show you everything you need to know about using the baggy method. In short, you place seeds on paper towels (not inside them), you wet them and place them inside a plastic baggy. It can then placed in the fridge, or or stored warm as needed. If seeds don’t germinate after the first cold treatment, just put them back in the fridge for another 30 days.

YouTube video

Once seed starts to germinate, plant them as usual. A big advantage of this method is that it take almost no room in a fridge. A couple of small boxes will hold 200 seed packs. It is also the only method that lets you see the germinating seeds.

Pros

  • Fairly easy to do.
  • Easily moved in and out of the fridge.
  • You can see when seeds germinate.
  • If a seed does not germinate, you have not wasted a pot, soil and space for it.

Cons

  • Takes a bit more time.
  • Two step process; germinate in the baggy and then plant.

Stratifying in the Freezer

Lots of people try to stratify seed in the freezer – it does not work because such low temperatures do not add chilling hours. Freezing can also damage some seed.

Double Dormancy

Double dormancy is a term used to describe some seed, such as peony and trillium, that requires more than one cold warm cycle before green growth shows above ground. In nature you don’t see the seedling until the second spring. I’ve made the argument that double dormancy does not really exist, because these seeds germinate after the first cold period. They just don’t make an above ground shoot until after the second cold period. You can read more about this in Double Dormancy In Seed – Does It Exist?

Cold – Warm Cycles

Some seed requires cycles of either cold-warm-cold or warm-cold-warm. These cycles are generally repeated until the seed germinates. Sometimes it will germinate in the cold, but most of the time germination happens once seed is warmed up. Such cycles are always done moist.

The cold period for this kind of treatment are periods of cold stratification as described above. The baggy method makes the movement of seed much easier than trying to use pots.

Seed Stratification Chart

The following is a list of common seeds that need cold stratification.

Common NameBotanical Name
Balloon FlowerPlatycodon grandiflorus
Catmint Nepeta spp.
ColumbineAquilegia vulgaris
Coneflower some varieties
False SunflowerHeliopsis helianthoides
GentianGentiana spp.
GoldenrodSolidago spp.
HollyhockAlcea spp.
Hyssop Agastache foeniculum
Ironweed Vernonia gigantea
Joe Pye WeedEutrochium purpureum
Larkspur Delphinium spp.
Lupine Lupinus
Marsh Marigold Caltha palustris
MeadowsweetFilipendula ulmaria
MilkweedAsclepias
MilkweedAsclepias spp.
PenstemonPenstemon spp.
Perennial Sunflowers Helianthus
Pincushion Flower Scabiosa
Prairie ConeflowerRatibida
Prairie Violet Viola pedatifida
Primrose Oenothera speciosa
RosemarySalvia rosmarinus
Rudbeckia most varieties
Shooting Star Dodecatheon meadia
Shooting StarDodecatheon spp.
SkullcapScutallaria lateriflora
Soapwort Saponaria ocymoides
St. John’s Wort Hypericum perforatum
ThymeThymus vulgaris
Wild Geranium (Geranium maculatum) Geranium maculatum
WormwoodArtemisia absinthium
Common NameBotanical Name

Answers to Seed Germination Questions

Q1: How do you know if seeds need stratification?

Seed packages usually tell you if they need to be stratified but if you don’t have the seed package you will have to look them up. I suggest you use the best germination guide on the net, by ORGS&HPS.

Q2: What happens if you don’t stratify seeds?

If they need to be stratified and you skip that step, the seeds won’t germinate.

Q3: How do you cold stratify seeds quickly?

Use the right temperature and a fridge is perfect. Make sure the seeds are moist. Other than that you can’t speed up the process. The normal advice is to treat the seed for 30 days. It might actually take less so you can try to warm them up after 15 days to see if they germinate. If thy don’t, just return them to the cold for another 2 weeks.

Q4: Do you need to soak seeds before cold stratification?

All of the above methods automatically expose the seed to water so there is no benefit to soaking the seeds first. At most it will speed things up by a few hours which is insignificant compared to the 30 to 90 days needed for cold treatment.

Q5: Do coneflowers need cold stratification?

Some varieties need cold treatment and others don’t. Either look up the species or give them all a cold treatment to make sure they germinate.

Q6: Do poppy seeds need cold stratification?

Some species do, but most poppy seed will germinate without cold treatment.

Q7: Do rose seeds need cold stratification?

It is best to cold treat roses for 3 months.

Q6: Does lavender need cold stratification?

Lavender usually germinates warm, but if it does not germinate, give it a cold stratification period.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

5 thoughts on “What is Seed Stratification?”

  1. I have found if I plant Columbine seeds right away they will germinate. Seeds here in Texas are produced in late spring. If the plants self sow, I see the seedlings soon after, not the next spring. Cold stratification is not necessary here.

    Reply
  2. I am aways confused when I buy Delphinium seeds. The package never mentions cold stratification. Would this be because they are already stratified?

    Reply
  3. I have had trouble with germinating seeds sending their radicle into the damp paper towel when they germinate. Using a piece of tightly woven cotton material prevents this. It can also be washed and used repeatedly.

    Reply

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