As you learn to garden or grow plants from seed, you may have come across the term ‘Stratify’ or ‘Stratification’. If it is a new term to you it can seem puzzling. So, in this guide I will tell you what Stratification is, why some seeds require it, and several methods you can use to Cold Moist Stratify your seed.
What is Stratification?
Stratification is the process where a seed experiences a set of conditions that will break its natural dormancy. Via evolution process various seeds of plants have evolved to not germinate unless certain conditions have been met. Some examples of this phenomenon include Pale Purple Coneflower, False Sunflower, milkweed, and the Black Walnut Tree.
For example, if a seed just needed moisture and warm temperature to germinate, then this could happen in Autumn. A seed germinating in Autumn will probably not survive a cold winter. This is because it won’t have time to develop sufficient root structure. Evolution has taught seeds from many plants to not germinate until they have spent a certain amount of time in a cold/moist environment. This is the most basic (and common) method of stratification.
Other species of plants require rather unique forms of stratification. For instance, Witch Hazel and Jewelweed require two winters before they will germinate. So, the seeds need to experience a cold period, followed by warm, and again followed by cold before they will germinate. Other species of plants have evolved to germinate after forest fires.
What about seeds that don’t require stratification?
Often seeds that have no stratification period still manage to avoid germination in Fall due to seeds not being produced/distributed until late Fall. Examples of this could include Echincea Purpurea, Spotted Bee Balm, or New England Aster. So, the population survives even though they do not require stratification.
Other seeds with no stratification period may just be early blooming plants that produce seed by July, ensuring sufficient time for the plant to develop roots that can survive the winter.
Guide on How to Cold Stratify Seeds
There are two main categories to cold stratify seeds. The first is winter sowing, which is the easiest in my opinion. The other being simulating winters via the refrigerator.
How to Winter Sow Seeds
Winter Sowing seeds is very easy. You just plant the seeds as you normally would, but place them outside during winter. You want to make sure they are somewhat protected from drying out or from curious squirrels. But, it is very easy to do this. Many people just save milk-jugs, yogurt containers, or any other plastic container with a lid.
- I use a regular plastic garden flat with seed cells.
- Then, I fill with moist soil, plant all my seeds….then cover with a plastic dome.
- I poke holes along the sides and top of the plastic dome to allow for some rain/snow melt to enter the container.
- Finally, I secure the dome cover with a piece of duct tape on each side, and twine wrapped around the entire tray/dome.
- Place it outside in a shady area, or where it would only get morning sun (if any). This is to avoid accidental germination, should a warm day occur in January
To see a short youtube video on how I prepare a regular 10×20 seed flat for winter sowing, click here
How to Cold Stratify Seeds in the Refrigerator
Cold Stratifying seeds in the refrigerator is an option for you as well. You can even do this by placing pots (with your seeds planted) directly in a spare refrigerator, etc. Just make sure they are uncovered or else you may get a white fuzzy fungus. But the most common methods involve sand or a moist paper towel.
How to Stratify Seeds using a paper towel
I’ve used the moist paper-towel method many times to stratify seed, and it is very effective. For smaller seeds it is the best method of stratification using a refrigerator.
- Lay a paper towel out flat
- Fold the paper towel once along the length, and once along the width. Now you should have a quarter of the original surface area.
- Unfold the paper towel, so that the exposed surface area is now half of the original.
- Lightly mist the paper towel with a spray bottle. If you can pick up and squeeze the paper towel, very little to no water should drip out. If this is not the case, keep squeezing it until little to no water drips out.
- Place your seed on one half of the now-moist paper towel (that is already folded in half once)
- Fold the paper towel along the crease you previously made, so that the seeds are sandwiched between two layers of the paper towel.
- Place this into a small zip-lock bag, label and seal it
- Put the zip-lock bag into the refrigerator for the required amount of days
How to Stratify Seeds using Sand
Using a simple zip-lock bag and moist sand or vermiculite, you can stratify your seeds in the refrigerator. This method is effective on larger seeds like legumes. On smaller seeds, it would be difficult to locate the seed within the sand mixture. For smaller seeds, I would suggest the paper towel method.
- In a bowl, add a handful of sand and water, mixing the two together. Only add enough water to moisten the sand. If water drips when you pick up the hand and squeeze, then it is too wet. Add some more sand and try again.
- Then, mix your seed with the sand. You want the seed to have good contact with the moist sand.
- Place the sand/seed mixture into a ziplock bag, and label it with the type of seed and date.
- Put the bag in the refrigerator for the required amount of time, then remove and plant the seed as you normally would.
Well, that’s it! There are many other variants out there that you could try for stratification as well. But, these were just a few of the easiest methods I’ve used on my own seeds. I hope you found it useful, and good luck!
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