2016 Cold Stratification

*This month I sort of took the path of least resistance and wrote my column for the Wenatchee World as a shorter version of the blog post I wrote for Mother Earth News....*

The snow is rapidly melting, leaving behind a landscape that seems almost barren and asleep. However, for many native plants, it this act of freezing and thawing that awakens them and actually increases their ability to survive and reproduce. Cold stratification is the term used to describe this very basic need; the need for winter.  Winter has the ability to soften the outer seed coat of some of nature’s toughest seeds through the action of freezing and thawing in a moist environment. For many plants that require stratification, this process can take up to 2 months and typically happens between 34 and 41 degrees Fahrenheit. During that time, the seed coat softens and embryonic growth is stimulated. Eventually, the embryo bursts through the softened coat and begins the process of germination.

For those of us who enjoy starting our own flowers, there are some classic perennials that require a period of stratification to increase germination. One example is Echinacea. Without a period of stratification, the germination rate for this garden favorite can plummet to less than 30%. However, with stratification, it is possible to germinate nearly 100% of all seeds that are started.  Cold stratification is a process that is easily replicated at home in a controlled environment. After the seed is planted into a potting mix, water thoroughly until the soil is completely saturated but no longer dripping out the bottom drain holes (I like to plant one seed per cell in a 78 cell container). Then, wrap the top of the container in plastic wrap and secure loosely with duct tape. Put a piece of tape on the top of the plastic wrap with a label indicating both the date the seed was planted and the date that you are removing the container from cold stratification. Also include the name of the cultivar that was planted in the container. Place the container onto a cookie sheet or nested in another tray that will catch any excess moisture and eliminate any dripping or mess. When all of these steps are complete, slide the tray into a spare refrigerator (like the drink fridge you keep in the garage) and place a note on the outside door of the fridge with the date the tray should be removed from cold stratification. Typically, 30 days is enough stratification time for Echinacea. Other species may take longer. During those 30 days, check on the container and make sure that the soil is still sufficiently moist. If need be, pull out the container and water thoroughly. This should only need to happen once in the 30 day period since the plastic wrap will help to contain the moisture.

After the period of stratification has finished, pull the container out from the fridge, remove the plastic wrap and continue the seed starting ritual like usual including any heat mats or lighting that you typically use for your vegetable starts. 

For those of us who like to collect native seeds (ex: Balsam Root), an easier method is to take the saved seed (good quality, mature seed heads), plant it into a ½ gallon or similar sized pot, place the pot in the shade outside your house for the summer and then water the pot intermittently over the fall and allow it to freeze and/or get snowed on over the winter. Come spring, move the pot into a sunnier location and water regularly without overwatering. Take note of the rate of germination and experiment with overwintering your seeds in different locations around your yard to see if germination increases or decreases with location.  Stratification can be a lot of fun! Good luck and Happy Gardening.