What is Stratification?

Many seeds have a natural coating that prevents them from sprouting too early and dying off during the winter. In natural conditions, temperature, moisture, and abrasion act on the seed coating, softening and wearing it off, allowing moisture to enter the seed in early spring to initiate germination. To ensure germination, you must replicate these conditions through a process called stratification. Most milkweed seeds need exposure to both cold temperatures and moisture in a process known as "cold moist stratification".

STEPS FOR COLD MOIST STRATIFICATION:

1. ​Soak milkweed seeds in water for several hours or overnight; tap water is okay.

2. Moisten a paper towel and place the milkweed seeds on one half. Fold in half and place in a labeled Ziplock bag with species name, date in, and projected day out (four weeks). Place in refrigerator. Check your seeds every day or so; some seeds may begin to germinate in the bag, in which case you go to the next step.

3. Place seeds about 1/4 inch deep in damp (not soggy), well-draining seed-growing medium. Mist to moisten surface thoroughly and check daily. As seeds germinate, mist occasionally. Expose seeds to bright, indirect light. A plant heating pad under the seed tray improves the germination rate.

4. After seedlings have sprouted, wait at least six weeks before transplanting them into a larger pot or into the ground.

​Note: COLD-MOIST STRATIFICATION IS NOT NECESSARY for the following milkweed species, most of which thrive in warm and/or wet climates:

• Asclepias angustifolia (Arizona Milkweed)

• Asclepias currassavica (Tropical Milkweed)

• Asclepias incarnata (Swamp Milkweed)

• Asclepias perennis (White Milkweed)

• Asclepias subulata (Rush Milkweed/Desert Milkweed)

• Asclepias tuberosa (Butterfly Milkweed)​​​

Image by woodleywonderworks/Wikimedia Commons, CC BY 2.0