What is cold stratification? I was reading up on different types of native and perennial seeds to plant in a bee garden on my property. Many of these seeds need a “cold stratification” treatment. So what is cold stratification and which seeds need cold stratification?
Cold stratification (actually cold moist stratification) is when we simulate a cold, wet season for certain native and perennial seeds. These seeds need a cold moist period before they will germinate. Cold stratification is accomplished by putting seeds in a plastic baggie with moist media such as sand or sphagnum moss. For small seeds, folding them in a damp paper towel works well. A refrigerator is a good place to keep the seeds cold.
Let’s look more at why cold stratification helps some seeds to germinate. I’ll also tell you exactly how you can cold stratify seeds, and which seeds need it.
Why Some Seeds Need Cold Stratification
Certain plants, commonly natives, perennials, and most trees, produce seeds that are dormant until environmental conditions are favorable for growth.
Let’s say a plant drops a seed in the summer or fall. A week of warm weather and afternoon showers follow. If the species was a warm-weather variety, and the climate was favorable, such as in Florida, the seedling might survive.
But what if the seeds we are talking about are from native perennial plants that grow in the north, the midwest, or the Rocky Mountains? Or even some places in the south that experience a cold season? If the seed germinates and begins to grow, it will certainly not survive the winter.
To overcome this danger, and give the seeds the best odds of survival, they are designed to be dormant until winter has passed. Seed dormancy is an important mechanism to delay germination until the environmental conditions are favorable.
The exact way that cold moist conditions awaken the seeds from dormancy is complex and involves biochemical processes within the seed. Certain plant hormones have been identified as instrumental in regulating dormancy and germination.
Seeds That Need Cold Stratification
The seeds of some perennials and many native plant species need cold stratification. If you have seeds in a packet, it should say on the packet if they need cold stratification.
Some common seeds that need cold stratification are Milkweed, St. John’s Wort, most Coneflowers, Catmint, Penstemon (beard-tongue), Black-Eyed Susan, Globe Flower, Iron Weed, Lavender, Lilac, Violets and Perennial Sunflowers. The seeds of many trees and woody shrubs usually also need cold stratification.
You can also lookup your seed on the internet to see if they need cold stratification. My favorite website for this is Prairie Moon Nursery. They have germination codes listed for each seed. The code tells you information about germinating the seed, including any cold stratification requirement and the length of cold stratification needed.
Fall or Early Spring Planting
Seeds that require cold stratification can be planted in the fall or sometimes in early spring. Wildflower seed should not be covered but will benefit by being pressed into the soil. You can accomplish this by walking on it or using a seed roller. By planting in the fall or early spring, nature takes care of the cold treatment for you.
Fall planted seeds don’t need a lot of water, but you should keep the soil from completely drying out for the first month they are planted.
Cold Stratify Seeds in the Refrigerator – Coffee Filter / Paper Towel Method
If you didn’t plant seeds that require a cold moist period in the fall, you will need to give them a cold treatment artificially. I will show you how this can be done in the refrigerator.
Step 1 – Soak the Seeds
Soak your seeds in a jar or glass of water in the refrigerator. Opinions vary on the soaking time, from one hour to twelve hours. I soak seeds for about eight hours. Soaking the seeds in cold water will help start the process.
Make sure to label your soaking jars with the type of seeds you have in them.
Step 2 – Strain Out the Seeds
After the seeds have soaked for several hours, I use a coffee filter held over the top of the jar to strain out the seeds.
Step 4 – Prepare Seeds and Media to go into the Baggie & the Refrigerator
You can just fold up the seeds in the coffee filter or you can fold a moist (not dripping wet) paper towel around the coffee filter for added moisture retention. If the coffee filter and/or paper towel are too dry, spray them with some water. If they are too wet, gently squeeze out excess water until they no longer drip.
Seal up the seeds in a baggie and place it into the refrigerator (not the freezer). Make sure to label the baggie with the type of seed and date the cold stratification began. A month is sufficient for most seeds, although some require more time. Research the specific seed or look on the seed packet for cold stratification times.
Cold Stratification – Sand or Sphagnum Moss Method
Cold stratifying seeds in sand or sphagnum moss works also and may help inhibit mold growing on the seed. Sphagnum moss is naturally anti-fungal.
Some people suggest cold stratifying in peat moss. I prefer to use sphagnum moss. Peat moss is made up mostly of decaying sphagnum moss, harvested from ancient bogs (not a sustainable practice). It is very acidic and high in tannins and contains other decaying organic materials. Sphagnum moss is pure moss, is sustainable, and is PH neutral.
The only problem when using sand or sphagnum is if you are using small seeds, like most flower seeds. It can be nearly impossible to find small flower seeds in sand or moss when planting time comes. I like to plant seeds individually and properly space them.
If you are using larger seeds, such as milkweed seeds, shrub seeds, and tree seeds or nuts, sand or sphagnum moss works well.
Soak seeds for eight hours or so, as you did when using the paper towel method. Strain out the seeds through a coffee filter.
Dampen some fine sand or sphagnum moss in a bowl. It’s best to let sphagnum moss soak for a while. If you add too much water, squeeze out the media so it doesn’t drip. Don’t use beach sand from the ocean shore, the salt content will be too high.
Place the seeds inside the moist sand or moss. I like to place a layer of sand (or moss) on a paper towel, put the seeds down, and cover with another layer of moist sand (or moss).
You can just put the sand or moss containing your seeds straight into a baggie if you prefer.
Fold the paper towel around the sand or sphagnum moss with your seeds. Spray the paper towel with water, place into a baggie, and put it into the refrigerator.
Some Important Tips
Check your seeds at least once a week. If your media is getting dry, spray water into the baggie to dampen, but you don’t want them soaking wet. You also do not want seeds to dry out; they will die if they do.
Look for mold on your seeds. You should remove any seeds with mold. Alternatively, you can try rinsing moldy seeds in a diluted hydrogen peroxide solution. Use one part 3% hydrogen peroxide (the kind you get in the first aid section of the store) to ten parts water. Spray seeds and let sit for a few minutes and then rinse with water.
If any of your seeds have sprouted, immediately remove it from the bag and plant. If your last frost date hasn’t passed, plant indoors in a seed starting mix.
By cold stratifying seeds that require it, you will drastically increase your germination rate for these seeds.
For seeds requiring cold moist stratification, fall planting is usually considered the best. However, life happens and we don’t always get around to everything we’d like to. So if you didn’t get those seeds outside in the fall, fear not! You now know how to fool those seeds into germinating!