Cold Stratification Pretreatment
Pollinator Conservation Efforts
What is Cold stratification?
Cold stratification is the process of subjecting seeds to both cold and moist conditions. Seeds of many trees, shrubs and perennials require these conditions before germination will ensue.
In the wild
In the wild, seed dormancy is usually overcome by the seed spending time in the ground through a winter period and having its hard seed coat softened by frost and weathering action. By doing so the seed is undergoing a natural form of "cold stratification" or pretreatment. This cold moist period triggers the seed's embryo; its growth and subsequent expansion eventually break through the softened seed coat in its search for sun and nutrients.
Cold Stratification Pretreatment Process
If you plant your seeds outside by early march, and they get at least 30 days of cold weather provided by mother nature, you can skip this process.
In its most basic form, when the stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool (ideally +1° to +3°C [34 to 37 degrees Fahrenheit] not freezing) and moist environment for a period found to be sufficient for the species in question. This period of time may vary from one to three months.
Treatment for Milkweed (Asclepias) seeds should be started 30 days prior to spring planting date.
1. Rinse or complete a short soak. Pour into a coffee filter, paper towel or fine screen to drain. Soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place.
2. Arrange seed in a single layer and allow all excess water to drain off.
3. Fold seed loosely into the coffee filter or paper towel to allow for weekly spot checks. The seed and paper should be damp but not wet.
4. Add a dry paper towel to your labeled resealable bag to help to maintain even moisture while pulling excessive moisture away. Do not allow the stratification medium to completely dry out or stay soggy.
5. Place the sealed bag in your refrigerator (not freezer) and monitor weekly, or as needed, until it is time to remove for sowing. Replace coffee filter or paper towel often; repeat from step 1. Once seed has completed the recommended stratification period, or if excessive early sprouting occurs, plant immediately.
After undergoing the recommended period of stratification, the seeds are ready to be removed and sown in the nursery bed for germination.
Alternatively, the seed may be sown in small pots filled with moist soil and then the whole thing enclosed inside a plastic bag before placing inside a common refrigerator.