Tuesday, February 13, 2018

Adaptations - Stratification

Wild Leek waiting out the winter months

A simple definition of the word adaptation is a physical trait or behavior that helps a living organism survive in its habitat.  Every living organism has adaptations.  Physical adaptations are easy to see - think the spots on a fawn or the thorns on a rosebush.  In animals behavioral adaptations can be easy to observe, such as hibernation during periods of extreme cold or heat.  Behavioral adaptations in plants are more difficult to recognize.

One behavioral adaptation that can be observed is stratification.  While some seeds of some plants are capable of germinating immediately upon maturation, for other species this would be less than ideal.  For instance, early germination might mean that a plant begins to grow at the wrong time of the year.  A plant that requires 90 days of warm weather to grow and develop flowers and seeds would not want to sprout and begin growing in September when it will be killed by cold within weeks.

Instead, many seeds experience a period of dormancy once they mature.  Stratification is an adaptation that allows the seeds to break that period of dormancy and then sprout.  Many seeds require a period of cold stratification.  This means that they must experience an extended period of cold temperatures before they will come out of dormancy.  One good example of a plant that goes through cold stratification is Common Milkweed.  I have planted milkweed seeds in spring that did not undergo stratification.  Rather than sprouting, they remained in the ground dormant all summer before finally sprouting the following spring (after a winter in the ground).  Many summer and fall wildflowers require this type of stratification.

Common Milkweed seeds need to undergo cold stratification before they can sprout.

Other wildflowers require a more complex hot-cold stratification, requiring a long period of warm weather followed by a period of cold weather.  Many spring wildflowers such as Bloodroot (Sanguinaria canadensis) fit in this category.  Other species such as Wild Leek (Allium tricoccum) require a multi-year hot-cold-hot-cold stratification process.  All of these different stratification requirements can be very frustrating when trying to start a wildflower garden from seed.

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