Cold Stratification: The Gardener's Trick for Healthier Plants

Have you ever wondered how to unlock greater seed potential to achieve more vibrant plant growth? Cold stratification is the answer. Cold stratification is a cool gardening trick and is the key for certain crops to grow well. This technique sounds complicated until you learn more about plant germination and winter sowing.

Cold stratification for seeds is simple and a great boon to the garden, but many gardeners fail to do it. This post covers what cold stratification is, why it is vital, how to cold stratify, which plants need it and which ones don’t.

Successful Gardening

Gardening is my passion, and I love inspiring others to grow a garden, live healthier lives, and have fun doing it. I want everyone to enjoy an organic garden with amazing, fresh produce right at home!

Gardening doesn’t have to be labor-intensive, overwhelming, or time-consuming. That is why I share my tips and tricks and create resources like this Seed Starting Guide, how to make DIY organic potting soil, how to make compost, raised bed garden ideas, and this post about cold stratification to help you have ultimate success in the garden.

What is Cold Stratification

Cold stratification is a process that mimics natural winter conditions where frost and weather soften the seed coat. It is essentially tricking a seed into thinking that it went through a period of cold winter weather.

Certain types of seeds germinate only after a period of cold weather, like a start-up sequence. The first step in the ordered sequence must be completed before the seed can move on to the next step. 

Biologically, these plants are geared with a safety mechanism that causes them to wait through winter weather before germinating to protect them from dying. 

Cold Stratification Benefits

A good cold stratification method benefits your garden by causing more consistent seedling emergence and higher germination success.

A common problem people encounter is that their seeds won’t germinate and so they think they have bad seeds. The first thing I will ask them is if the seeds were cold stratified. 

The difference between seeds that have gone through cold stratification or not can have a huge difference of 90% to 95% improved germination! That is one or two seeds out of a hundred that germinate compared to around 95 seeds out of a hundred that will germinate after cold stratification. 

Cold stratification also leads to more vigorous, resilient plants better adapted to their growing environment. They have strong roots and fight off diseases better. The consistent and robust stand of plants that results will enhance garden aesthetics and productivity.

Crops that Require Cold Stratification

You may wonder if you should cold stratify things like peppers, onions, lettuce, and tomatoes. The answer is no. Plants that naturally grow in a climate that never experiences cold winters will not benefit from cold stratification.

Mainly seeds of perennial plants require a cold temperature exposure to germinate, while many annuals do not. Pro-Tip: Here is a good rule of thumb for knowing which crops need cold stratification; if the plant is a perennial that grows naturally in an area that experiences cold winter weather, you should cold stratify the seeds.

The following list is by no means extensive, but a good starter list of plants that I recommend cold stratifying:

• St. John’s Wort

• Artichokes

• Strawberries

• Asparagus

• Milkweed

• Catnip

• Lemon Balm

• Lavender

• Milk Thistle

• Marshmallow

• Poppies

• Coneflowers

Deciduous trees like oak, maple, walnut, and fruit tree seeds require cold stratification too. If you are unsure whether a seed needs it, do some research. A simple Google search should get you the answer quickly. 

Properly Timing Cold Stratification

It is important to properly time your cold stratification. Once the cold stratification is complete, you want to be right on time to start your seeds.

To do this, first determine the best time to start your seeds and then plan to begin cold stratification 2 to 3 weeks prior to that date.

For example, if I want to plant Mary Washington Asparagus, I would look at the back of the seed packet or at Clyde’s Garden Planner to determine when to start the seeds based on my last average frost date. These asparagus seeds need to be started 8-12 weeks before my last frost date. 

I then add an additional 2 or 3 weeks to that time to allow for cold stratification. So for Mary Washington Asparagus, I would begin cold stratification 10 to 15 weeks before my last frost date.

Instructions to Cold Stratify Seeds

There are several methods used to cold stratify seeds, but I want to share a simple, effective method of cold stratification that I have been using successfully for years. 

Materials Needed

• Seeds - Gather any seeds that need to be cold stratified like asparagus, poppy, or lavender.

• Paper Towel - The paper towel will be used as a moistening medium.

• Water - A small amount of tap water is needed to wet the paper towel.

• Resealable plastic bag - Use a bag large enough to fit a folded paper towel inside.

• Marker or Pen - If you plan to cold stratify more than one type of seed, it's a good idea to label each bag. 

• Refrigerator - You will need a standard refrigerator to stratify the seeds.

Step-by-Step Guide

1. Prepare Your Medium - Fold one sheet of paper towel in half and lightly dampen it with water. Do not soak or completely saturate the paper towel. Too much water may cause the seeds to rot or mold.

2. Place the Seeds - Sprinkle the seeds onto the paper towel in a single layer. I recommend using a different paper towel and plastic bag for each type of seed.

3. Seal in a Plastic Bag - Put the paper towel and seeds into a plastic bag. Make sure to remove as much air as possible before sealing.

4. Label - Label each plastic bag with the type of seed. 

5. Refrigerate - Place the plastic bag in the refrigerator where it will not be disturbed for 2 to 3 weeks. I place my seed bags in the crisper bin. Pro-Tip: Seeds left in the fridge for an excessive amount of time, even 5 to 7 weeks, can have issues with germination because the moisture will cause rot and mold over time. Be sure to set yourself a reminder!

6. Plant - After 2 to 3 weeks, take the plastic bags out of the refrigerator. (Don’t leave them in the plastic bag!) Plant the seeds in a good sterile seed starting mix with a slow-release fertilizer right away.

And that's it! I know a lot of gardeners start to feel like gardening is hard when they hear terms like stratification. But when we break it down, it's really simple!

I hope you learned something new to add to your gardening tool belt.