Winter officially ends tomorrow and most of my established native plants are leafing out and showing off their new growth. The native seeds I sowed late in the fall have ended their dormancy and are starting to break through their outer coatings in a process called germination. Why do seeds wait before sprouting, and what shapes these conditions?
Through natural selection, plants all over the world have adapted incredibly specific requirements before germination will occur. I won’t claim to fully understand these complex mechanisms, but evolution places immense selective pressure on these species, shaping their survival strategies—especially during their most vulnerable stages of life. I remember some years ago reading about the convergent evolution in ecosystems pressured by fire, and it blew my mind (all of plant evolution does!). The environmental stress of fires led trees and shrubs to evolve amazing physical traits: thicker bark, lignotubers, serotinous cones, and other highly adaptive characteristics. One such tree is the Lodgepole Pine, Pinus contorta, which grows at elevations of 6,000 feet and requires temperatures above 113 degrees F (forest fires) to melt the resin that coats its cones. This adaptation leads to lower mortality rates of seedlings as post-wildfire conditions allow light from the newly exposed canopy to reach the ground, increase nutrient availability in the soil from ash deposits, and removes competition.
While fire has shaped ecosystems in some regions, cold winters have exerted their own selective pressures on the plants here in the Mid-Atlantic. All of the native plants here evolved to survive hard winters with temperatures regularly dropping below 20 degrees Fahrenheit, and the ground freezing with the average frost line depth in PA being 44 inches! In the fall, as peaceful, colorful, and romantic as it can be for humans, plants and trees undergo massive biochemical changes in a race against time (exciting, right? I’ll save most of this for a future post). Before the freeze comes, plants need to prepare for dormancy by moving food, chemicals, nutrients, and much more all throughout their systems. Starches are stored in the roots, tubers, or bulbs. They decrease the water content in their cells and increase the concentration of sugars and other compounds, effectively lowering the freezing point!
The seeds of a majority of these plants need to go through a cycle of cold and moisture, freezing and thawing. The freeze-thaw cycle breaks down the seed coat, allowing moisture to finally penetrate the embryo and trigger germination. Before all of the right conditions are met, the seed lies dormant and waits, preventing early germination and death.
For many of my favorite native plants, trees, and shrubs, this cold stratification selection process occurred over the last ten to fifty thousand years, or since the end of the last ice age. It’s important to remember that plants have been evolving and adapting to their environment for millions of years, and evolution is gradual.
Understanding the ‘how’ behind germination makes it easy to mimic these conditions at home. For me, understanding the science behind this natural process eases my uncertainty. So go ahead — sow seeds in the fall, let winter shape them, and watch as spring coaxes them to life.
