Ways to Cold Stratify Seeds

What Is Cold Stratification?
Cold stratification, also known as seed stratification, is the process of exposing seeds to cold and moist conditions to encourage germination. In nature, the stratification process takes place when fallen seeds overwinter underground or beneath a layer of snow. In the spring, the temperatures rise, thawing the ground and breaking the seed out of its dormancy period. The seed sheds its hard seed coat, beginning the germination process. You can mimic this process indoors to prepare seeds for propagation.

Why Do Some Seeds Need Cold Stratification?
Depending on the type of seed, cold-moist stratification may be required to break the seed dormancy period. This is because the hard seed coat needs time to break down before the seeds can germinate and sprout. Many types of wildflowers, perennials, shrubs, and trees produce seeds that must undergo a cold period for some amount of time for seed germination to begin. Generally, native plants grown in cold climates will require cold stratification before you plant them in the early spring. Common seeds that require cold stratification include common milkweed, St. John’s wort, and many coneflower species.

How to Stratify Seeds in the Refrigerator
Follow these steps to cold stratify your seeds in the fridge.

1. Place the seeds in a damp medium. Small seeds can be sprinkled onto a damp paper towel. Larger seeds should be placed in a moist medium such as peat moss or vermiculite. Ensure that the medium is moist but not soaking wet.
2. Store the moist seeds in a plastic bag. Once your seeds are wrapped in a damp paper towel or planted in a moist growing medium, place them in a plastic bag. To prevent excess water from accumulating inside the bag, allow for some airflow by leaving the bag partially open or puncturing the bag with a few small holes.

3. Place the bag in the fridge. Most seeds require about a month of cold stratification to increase germination rates. However, the exact amount of time needed for the stratification process will vary depending on the type of seed you choose. Consult the cold stratification timeframe suggested on the seed packet. Check on your seeds periodically and if they start to sprout, remove them from the bag and plant them.

Cold Stratify Seeds: Why Not Just Sow The Seed?
Many annual varieties, like Zinnias, and Sunflowers, have soft shells and can simply be sprinkled on bare soil in the spring. But some perennials, especially native wildflowers, have a hard coating that helps protect the outer shell from breaking and sprouting too early. We’ve all experienced an unseasonably-warm spell in in the middle of January or February — this mechanism helps prevent the seeds from being tricked into coming out of dormancy until it’s just the right time.

The good news for gardeners is that the natural cold stratification needed for germination can be forced with just a few materials, water, a refrigerator, and patience.

Cold Stratify Seeds: Varieties
There are several perennial and native seed varieties need to be manually broken from dormancy in order to sprout and thrive in your garden. If you’re planting native seeds and aren’t sure, chances are you should at least scarify and soak your seeds before planting.

Learn how to scarify and soak seeds for spring planting.

Common varieties that require cold stratification for spring planting:

Milkweed (Asclepias)
Lupine (Lupinus)
St. John’s Wort (Hypericum perforatum)
Prairie Coneflower (Ratibida)
Prairie Violet (Viola pedatifida)
Pincushion Flower (Scabiosa)
Marsh Marigold (Caltha palustris)
Perennial Sunflowers (Helianthus)
Wild Geranium (Geranium maculatum)
Rudbeckia (most varieties)
Coneflower (some varieties)
Soapwort (Saponaria ocymoides)
Primrose (Oenothera speciosa)
Larkspur (Delphinium)
Shooting Star (Dodecatheon meadia)
Heliopsis
Lavender/Hyssop (Agastache foeniculum)
Catmint (nepeta)
Ironweed (Vernonia gigantea)

Cold Stratify Seeds: Gather Materials
Cold stratification is an extremely easy process and once you’ve done it once, you’ll no doubt get the hang of it. The first step is to gather the materials needed, all of which can be found in your home, tool shed, or with a quick trip to the hardware store.

Materials for Cold Stratification:

Seeds
An all purpose sand mixture and/or Peat Moss
Paper Towels
Water
Plastic ziploc bags
A Sharpie or pen for labeling
Mixing bowls
Refrigerator

Cold Stratify Seeds: Step by Step Process
Now that you have your materials, you can use three different methods for cold stratifying your seeds. All three of these methods work equally well and offer up different ways to basically keep the seeds moist in your refrigerator until it’s time to plant. We’ll go over all three methods:

Sand/Water
Place a 1/4 cup of sand (or more) in a mixing bowl. Slowly add water until you can form a ball with the sand/water mixture.
Add your desired seed amount to the sand. Mix thoroughly.
Place sand/seed mixture in a ziploc bag and seal.
Label the variety and date clearly on the bag.
Place in the refrigerator for 1 month before planting. If seedlings start to sprout in the bag in the refrigerator, remove immediately and either plant in the ground or in pots until it’s time to plant outdoors.

Peat Moss/Water
This method is very similar to the sand method, but uses peat moss (which many of us have lying around somewhere).

Place a 1/4 cup of peat moss (or more) in a mixing bowl. Slowly add water until you can form a ball with the peat moss/water mixture.
Add your desired seed amount to the peat moss. Mix thoroughly.
Place peat moss/seed mixture in a ziploc bag and seal.
Label the variety and date clearly on the bag.
Place in the refrigerator for 1 month before planting. If seedlings start to sprout in the bag in the refrigerator, remove immediately and either plant in the ground or in pots until it’s time to plant outdoors.

Soaking/Paper Towels
This is one of the methods most widely used for native seeds, especially milkweed. It is easy, quick, and the materials are usually right in your kitchen ready for use.

Soak your seeds for 1-2 hours.
Use a paper towel or coffee filter to drain the water.
Spread seeds out in a single layer on the paper towel.
Wrap a dry paper towel around the damp paper towel to help keep things moist but not too wet (which could become moldy).
Place in a ziploc bag and seal.
Label the variety and date clearly on the bag.
lace in the refrigerator for 1 month before planting. If seedlings start to sprout in the bag in the refrigerator, remove immediately and either plant in the ground or in pots until it’s time to plant outdoors.

COLD STRATIFY SEEDS: PLANTING
The time you need to keep your seeds in the refrigerator depends on the variety, but 4-5 weeks should be a sufficient amount of time for most seed varieties. Once there’s no more chance of frost in your area, take your seeds out of the fridge and spread seed on bare soil as normal. The simple, quick process of cold stratification helps the seed germinate quicker and grow more readily in your garden bed.