Cold-Moist Stratification

A seed is a wondrous thing; each one consists of a tiny plant embryo with its own food supply encased in a protective coat. Certain conditions need to be met for a seed to germinate. Although these conditions are found in nature, we can mimic them for germination success.

After a milkweed disperses its seeds in the fall, the seeds sit on the ground through winter. Often, they are covered by a blanket of snow. Here they remain, waiting for the warmth of spring to germinate. In cases where milkweed seeds require this process to break the embryonic dormancy seed phase, these conditions of coolness and moisture can be duplicated by a process called cold-moist stratification.

How to cold/moist stratify seeds:

  1. Place seeds between two paper towels and moisten.

  2. Place the mixture in a labeled plastic bag (species name, date in, and scheduled date out). Squeeze out excess air and seal.

  3. Store in a refrigerator (33–38°F) for 30 to 60 days (depending on seed type). Some seeds may sprout in the storage bag if moist stratified too long. If sprouting occurs, plant immediately.

  4. After required time, remove seed bag from refrigerator. Place paper towel with seeds on a towel let dry for a day or two. 

  5. When dry, place into a large bowl. Carefully peel apart the paper towels and shake and/or scrape the seeds into the bowl. They are now ready to plant into germinating cells or your garden.

We recommend germinating milkweed seeds outdoors when the danger of frost has passed. To germinate, the seeds can be placed in germinating cells filled with moistened peat pellets or quality organic soil. In nature, seeds fall to the ground and sprout with little cover, so don’t put more than 1/8” – 1/4” of soil over the seeds. Water gently and place where sunlight is available. As the seedlings become established, keep the soil moist but not saturated. In a few weeks, when the seedlings are three to six inches tall, they are ready to be transplanted into the garden. (Make sure they have at least four true leaves). Place them a minimum of 12 inches apart in light, well-draining soil in a sunny place, or into larger containers filled with quality organic soil. Milkweed plants have long roots, so give them plenty of room to spread downward.

Please Note: 

Most milkweeds require cold-moist stratification for 30 to 60 days prior to germinating. While some seeds may germinate without this process, best practice calls for cold-moist stratification. 

Milkweeds that DO NOT require cold-moist stratification include pine needle milkweed (A. linaria), rush milkweed (A. subulata), sand milkweed (A. erosa), and tropical milkweed (A. curassavica). 

Photo: Justin Meissen, CC BY-SA 2.0