Cold/Moist Seed Stratification (for Milkweed)

Cold/moist stratification is giving seeds a period of cool, moist conditions before planting. This process mimics the natural winter conditions that some seeds experience outdoors and helps break their natural dormancy.

In nature, seeds often fall to the ground in autumn and remain cool and damp throughout the winter. After experiencing this period of cold, the seeds are better prepared to germinate when warmer spring temperatures arrive.

For gardeners, cold/moist stratification can be done by placing seeds in a moist growing medium and keeping them in the refrigerator for several weeks, usually at about 33–41°F (1–5°C). After the chilling period is complete, the seeds can be planted in warm conditions where they can begin to germinate.

In simple terms, stratification gives a seed a simulated winter so it knows when it is time to grow. Not all seeds need stratification, but for species that do, this extra step can improve germination and produce more consistent results.

For milkweed, the easiest method is the refrigerator method.

Refrigerator method

  1. Moisten a paper towel so it is damp but not dripping.
  2. Place the seeds on the towel.
  3. Fold the towel over the seeds.
  4. Put it inside a resealable plastic bag or container.
  5. Label it with the species and date.
  6. Place it in the refrigerator at approximately 35–40°F (2–4°C).
  7. Leave it for the recommended period—generally 3–4 weeks for the species where I recommend stratification.
  8. After stratification, plant the seeds immediately.

An even easier nursery method

Winter sowing: Sow the seeds in trays filled with a well-draining seed-starting mix in late fall or winter. Keep the trays outdoors in a protected location where they experience natural winter temperatures and moisture, then allow them to germinate naturally as temperatures rise.

This has the advantage of providing natural temperature fluctuations rather than trying to reproduce winter conditions in a refrigerator. USDA Forest Service guidance also recognizes outdoor winter exposure as a form of cold stratification.

Important: Don't bury the seeds deeply

Milkweed seeds are generally small, flat, and equipped with silky hairs for wind dispersal. For species such as Butterfly Milkweed, extension recommendations call for sowing the seed on or near the soil surface and keeping it moist during germination.

Sow seeds on the surface of moist, well-draining seed-starting mix. Press gently into the soil but do not bury deeply. Keep evenly moist during germination.

Not all milkweed species require cold/moist stratification

No stratification generally needed:
A. asperula, A. latifolia, A. subverticillata

Stratification recommended:
A. speciosa, A. tuberosa

Stratification may improve germination:
A. angustifolia, A. engelmanniana, A. involucrata, A. linaria, A. nyctaginifolia