Cold stratification or seed stratification is the process of giving seeds a cold dormant period that is needed for some seeds before they can germinate. Not all types of seeds need this process to germinate, but some do need that cold period for successful germination.
In some plants natural process, the seeds they drop for the next season’s plants have a hard seed coat that prevents them from being able to germinate right away. The cold stratification process helps to break down that coat so the seed will germinate in the spring.
Can you start these seeds without the stratification process? Sure can! However, seed dormancy is an important part of some plant life cycles, and cutting corners will likely you spotty germination.
This cold treatment mimics what cold hardy perennials typically experience in cold winters. When you’re starting a new plant from scratch, you should consider the plant’s natural environment. Does it grow in your zone? Is it perennial? What type of winter do you have? Is it very cold? Very wet? Dry? There are many factors that will determine your appropriate approach.
What Seeds Need The Cold Stratification Process
Let’s talk about what types of seeds require this process. There are quite a few medicinal herbs that should have a period of cold stratification for a length of time. The length of stratification varies depending on each plant’s requirements.
Some require a 2+ month period before they’re started, and some are ready to be started after only 2 weeks. Some seed packets will give you the instructions right on the package. My top choice for medicinal herb seeds and education on how to grow them is Strictly Medicinal Seeds.
Here is a list of some of the more common herbs that either benefits from or require a cold stratification period:
Elecampane
Lavender
Hops
Skullcap
St. John’s Wort
Catmint
Marshmallow
Mullein
Echinacea
Plantain
Valerian
Blue Vervain
Licorice
Lobelia
All of these plants are wither perennial or biennial and need time in cold weather to come back to life the following year and be fruitful. Because of the natural life cycle of these plants, as a general rule of thumb, you can assume that their seeds need cold stratification. If you live in a milder climate and you do not experience snow, ice, and prolonged cold weather, don’t worry, you can mimic these conditions. But first, let’s talk about the types of cold stratification…
Types of Cold Stratification
Cold Stratifying seeds is not a one size fits all technique. There are different stratification needs for different seeds. As a general rule of thumb, if you consider the plant’s natural conditions, you can usually figure out what type of stratification you should use.
Cold Moist Stratification
Plants that need cold moist stratification are plants that experience snow, ice, melting, and soggy conditions into the spring when they are starting to come back to life in the new season. When you plant seeds in the fall, as we will discuss soon, you are mimicking a cold moist stratification process.
Cold Dry Stratification
Plants that need cold dry stratification should be those that grow in climates that experience cold winter months, but not necessarily snow, ice, melt and repeat. Cold dry stratification is achieved by storing the seeds in cold conditions under 32 degrees Fahrenheit and is considered a type of stratification pre-treatments.
Ways To Cold Stratify Seeds
Direct Sow
Depending on your climate, you may be able to direct sow your seeds into the grounds and let Mother Nature and her natural winter conditions do the cold stratifying for you! To do this, plant your seeds directly into the soil in late fall when temperatures are hovering around freezing at night. Be sure to water deeply and mulch heavily. In the spring, after the appropriate period of cold temperatures, you should see your seeds germinate with the warmer temps.
Refrigerate
This technique is the most common and the most appropriate for a small scale set up. Using a damp paper towel, place the seeds inside it and place them into a plastic bag. Put them into the fridge and remove the seeds when the required time has passed. From there, allow them to warm naturally, start them in your seed starting soil, and you’re off!
Soil Blocks
This is by far my favorite approach. Make dampened seed starting soil into soil blocks. Here’s the soil blocker I use. Then place your seeds in them. Put your blocks in a container and keep the container in a cold place like the fridge or outside if you live in a cold climate. After the required amount of time, let the seeds warm naturally, and place the container on a seed starting mat or under lights, to start the germination process.
Soil blocks are my favorite because I can put my soil blocks in trays and stack multiple trays in a tote and place them outside. By choosing this method, I have the potential to cold stratify 150 new plants. This is an excellent option if you are looking to start a substantial amount of plants.
How To Use Cold Moist Stratification
If cold moist stratification is the approach you’ll be using, follow the steps listed above in the ways to cold stratify seeds section. Direct sowing, refrigeration, and soil blocking techniques will all be effective for this approach.
How To Use Cold Dry Stratification
For the cold dry stratification approach, you’ll still be using moisture. I know, why do they call it dry then? The damp paper towel method can be used with the cold dry stratification approach, just be sure that you are cold storing your seeds in an area where they will not thaw and refreeze, or warm the re-cool. When repeated thawing and cooling happens, that is more typical of a moist stratification and it will likely affect your germination rate.
If you’d like to see exactly how I used the cold stratification method to start 150 seeds, check out this video:
