What is Cold Stratification
Stratification is the conditions a seed is exposed to to get them to germinate. When people think about this stratification process, they often think of the moist and warm temperature needed to get the job done. These conditions are not always the case. In the wild particular plants flower in late fall. These plants produce and drop their seeds around winter time when the temperatures are usually cold and wet. This process of exposing seeds to cold, moist temperatures is known as cold stratification. Not all plants need the same conditions during the cold stratification process. Some plants like damp and humid conditions, while some plants prefer cold and dry conditions. The process of exposing the seed to cold temperatures primes the seed. How do you know which seeds need to go through this process? By paying attention to the plant’s flowering times. Plenty of trees, shrubs, and perennials need cold stratification to germinate.
Here is a list of plants that fall into that category:
Milkweed
Butterfly bush
Fuchsia
Rhubarb
St Johns Wort
Shooting Sar
Prairie Coneflower
Certain marigolds
Lupine
False sunflower
Hardy hibiscus
Catmint
Evening primrose
Perennial sweet pea
Rudbeckia (black-eyed Susan)
Sedum Hen-and-chicks
Ironweed
Chinese lantern
Lavender
Verbena
Cold stratification is a survival tactic engrained in individual plants. This tactic ensures that plants do not germinate at inappropriate times. Sudden changes could trick a plant into thinking it is time to grow too early in the spring or too late in the summer. Without the cold and moist conditions during winter and early spring, individual seeds will not even sprout.
In the wild, nature subjects these seeds to these conditions for months. Luckily when recreating this process at home, that time length is not necessarily needed.
Here are different ways to copy nature’s cold stratification easily in your own home.
Materials
Seeds
Plastic Ziplock bags
Refrigerator
Water
Paper towels
Peat moss
Sand
Cold Stratification Methods
Paper Towel Method
Soak seeds in a cup of water for 2-24hrs. Seed size and determine the soaking time.
Moisten a paper towel or coffee filter
Pour seeds into paper towels
Fold the damp paper towel over the seeds and place it in a ziplock bag
Label the start date and place the bag in the refrigerator for a minimum of 3 weeks
After a minimum of 3 weeks, remove the bag and plant the seeds as you would normally.
Sand method
Place sand in a mixing bowl and moisten with water. The sand has the proper moisture content when you can form it into a ball
Add your seeds to the sand and water mixture
After mixing, place sand and seed mixture in a Ziplock bag and place in the refrigerator for a month
After a minimum of 3 weeks, remove the bag and plant the seeds as you would normally.
Coco Coir Method
Pre-moisten the coco-coir. To check for the proper moisture consistency, I grab a handful of this mixture and attempt to squeeze it. If I can squeeze water without clinching a tight fist, I know I have the proper amount of moisture in my medium.
Next, place seeds flat and lightly press them into the coco coir. Cover with a light layer of moistened coco coir. The seeds should still be visible—cover the container with Ziplock or something similar to keep moisture level consistent.
Place this mixture in the refrigerator. After the designated time, remove your seeds from the fridge and now plant your seeds as you would normally.
To be honest, the sand method or the coco coir method both work, but that’s not my go-to. I prefer the paper towel method because I can remove individual seeds from the plastic more efficiently than sand, peat moss, or any other medium. And to me, when we’re urban gardening, efficiency is essential.
So, if you have ever attempted to start certain seed varieties and wondered why you could never be successful or successfully get them to germinate. We may have just solved that issue. Maybe those particular varieties required a cold dormant period.
For seeds that need a cold and dry stratification period, simply place seeds in the refrigerator. You can put the seeds in mason jars, ziplock bags, Tupperware, or in the seed pack. Just make sure they stay dry.