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How to Stratify Seeds Indoors

California poppies with a blue sky background in a pollinator garden

A Simple Guide to Indoor Stratification PLUS Troubleshooting Common Problems when Stratifying

Welcome to the exciting world of seed starting! If you've ever wondered how Mother Nature gets her seeds to germinate so perfectly, you're in for a treat. In this guide, we'll dive into the chillier side of gardening – how to stratify seeds. It may sound like a term straight out of a science lab, but fear not, we're here to break it down in a way that even your grandma's petunias could understand.

PLUS read on to learn how to avoid the common issues that prevent stratification from being successful. If you know what to avoid from the beginning, your chances of successful germination will be much greater.

What is Seed Stratification?

Seed stratification is a fancy term for simulating winter conditions to encourage seeds to sprout. Many plants, especially those from cold climates, require a period of chilling before they'll even think about waking up and stretching their little green arms. Lucky for us, we can recreate these conditions right in our cozy homes.

Materials You'll Need:

1. Seeds: Choose seeds that need stratification, like those from perennials, shrubs, or certain wildflowers.

2. (Optional) Moist Paper Towels: These wrap around the seeds to keep them moist and snug in the fridge or freezer.

3. Plastic Ziplock Bags or Containers: Perfect for capturing moisture.

4. Labeling Supplies: Because even the most seasoned gardeners forget what they planted.

5. Refrigerator and/or Freezer: Our cold allies in this gardening adventure. I recommend using the freezer for seeds that are native to cold climates. The native flower seeds I sell do BEST when stratified in the freezer. This includes penstemons (beardtongues), columbines, coneflowers, and other native flowers. Check the USDA hardiness zone from which the seeds grow native. That should guide you on which is cold enough - the freezer or fridge. For example, one seed I sell is called Desert Penstemon which is native to USDA hardiness zones 5-9. The low temps of zone 5 are -10 to -20 degrees Fahrenheit. Stratifying for 4 weeks in a 45 degree fridge may not be cold enough; these should be stratified in a freezer.

6. Patience and Time: Seeds have their own schedule; we're just here to facilitate.

Step 1: Choose Your Seeds Wisely

Not all seeds need a winter slumber, so make sure you're working with the right ones. Check the seed packet or do a quick online search to find out if your chosen plant requires (or would benefit from) stratification.

Step 2: Soaking the Paper Towel

I like to very lightly moisten a paper towel and loosely wrap the seeds inside. Don't leave too much moisture on the towel; it should be barely damp and not sopping wet. Too much moisture could cause the seeds to mold and will kill the seeds before stratification is complete.

Step 3: Label, Label, Label

Garden amnesia is real. Label your baggies with the plant name and date of stratification. Trust me; future you will thank present you.

Step 4: Ziplock Bag Greenhouses

Place your freshly dampened towel and seeds inside labeled Ziplock bags or containers. This creates a mini-greenhouse effect, trapping moisture and warmth.

Step 5: Into the Freezer or Fridge

For seeds that require cold stratification, like many perennials, pop those bags into the freezer or refrigerator (again, base this decision on the coldest temp of the USDA hardiness zones in which these plants grow native). Be patient; they'll need anywhere from a few weeks to a few months in the chilly paradise. (All seeds available in our Shop require at least 4-6 weeks of stratification using this method. You can also check out our favorite method, Winter Sowing, here. Winter Sowing replaces the need to stratify using this method.)

Step 6: Freezer Tricks

Some seeds, especially those from cold climates, might benefit from a stay in the freezer. Be cautious – not all seeds enjoy this extreme treatment. Check the specific requirements for your chosen plants by checking their USDA hardiness zone. For example, some seeds that grow in tropical areas only need time in the fridge.

Step 7: The Waiting Game

During the chilling period, resist the urge to check on your seeds every day. Let them have their beauty sleep. You're aiming for robust and healthy, not startled and stressed.

Step 8: Thaw and Transplant

Once the chilling period is up, remove your seeds from the refrigerator or freezer. Let them thaw at room temperature for a day or two. Then, transplant the seeds into individual pots or directly into your garden.

And there you have it, a crash course in the art of seed stratification! You're now equipped to give your seeds the winter vacation they never knew they needed.

Shop our beautiful waterwise flowers! Click images below.

Troubleshooting Common Issues with Stratification

Over the years, I've had issues with stratification myself. Here are some tips to ensure your stratification is a success from the start.

Issue #1: The seeds didn't germinate, even though I think I did it right.

Keep in mind that the most likely environment for a seed to germinate is in its native habitat! If you're growing a seed from somewhere else (as many of us do), it's important to 'think' like that plant. The most LIKELY methods for a seed to germinate are as follows:

BEST - Fall sown directly in the ground in a climate similar to the seed's native home. This will give the seeds a very long, drawn-out period of stratification which is actually their preference.

BETTER - Winter sown in a mini greenhouse environment (see directions for this method here).

GOOD - Stratified in a cold environment to simulate the native setting such as a fridge or freezer. Read on to see how to help your seeds succeed with this method!

Always hold onto a portion of your seeds, in case you choose a method that is not successful. That way you can try another method or troubleshoot by yourself.

Issue #2: Stratification temperatures were not cold enough.

Be sure to put your seeds deep in the fridge (such as the back of a crisper drawer) or freezer. Never place them in the door of the fridge or freezer. The variating temperatures when the door opens and closes will decrease your successful stratification. Also keep in mind - heat rises and cold sinks. I recommend tucking them into a low drawer and letting them rest, as undisturbed as possible - just like happens in nature.

Issue #3: Not stratifying long enough.

I get it - it's March, you've ordered some seeds after seeing catalogs or beautiful photos online. Spring has finally arrived, and you're SO excited to have these plants in your garden! Beautiful, sunny days arrive and even though you just got the seeds, there's no time to stratify.

Native seeds require the right conditions to grow and we can't rush what they need to succeed. Be sure to give them at least 4 weeks in a deep cold state. If you can't provide that, then hold onto the seeds until fall and direct sow them. That way, they have the benefit of a cold winter and will come up (hardy and strong) in the spring when they're ready.

If you tend to do most of your seed-buying in the spring, consider (in the future) buying your seeds in the winter instead. I know, it doesn't seem like seed-buying season! But December is a great time to get a headstart on stratification. Better yet! Why not try Winter Sowing in October or November? Read here how to do this simple and very effective method.

Issue #4: They just don't germinate after planting them.

Any gardener's experience will tell you that there are countless reasons why seeds don't germinate. Starting plants from seed ranges from really easy to very tricky. We all know that, dollar for dollar, growing from seeds is cheaper. But not if you can't make it work!

Read what you can about starting plants from seed BEFORE you start this process! That will help you actually save money in the long run by having success growing from seed. I've created a Comprehensive Guide to growing plants from seed. I have a few "tried and true" tools and tricks that have totally changed my success rate in seed-growing! Read the Guide (and bookmark it!) to ensure great success and see how gardeners like me do it. Yes - everyone CAN have a green thumb!

Have you tried the stratification method of gardening? Please send photos and share results on Instagram @planterboxgarden


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