When snow finally arrived this winter, and high winds followed, I knew the trees would be on the move. Not just dancing in the breeze, but spreading into new territory. Many of our native species drop small, lightweight seeds that are blown great distances over the surface of the snow.
Seeds of birches - white, yellow, and gray - are familiar to those who walk the winter woods. Up to 1,000 seeds are attached to each tight seed cluster, or catkin. Seeds are enclosed in tiny scales that resemble a fleur de lys, or “iris blossom.” When cold weather intensifies, scales detach from the center spike of the catkin in an ascending spiral. As the scales flit over the snow, the smaller, winged seeds chafe off and sail even farther. Winter-drop birch seeds have the highest rates of germination. If windblown seeds collect in the lee of a snowdrift, they sometimes sprout a line of seedlings come springtime.
This form of seed dispersal is so successful that many of our native trees deliberately drop seeds in winter, including such cone-bearing species as pitch pine, hemlock, and larch. I often find seeds of hophornbeam atop the snow amid those of hemlock - two trees that tend to associate in our forests. Down in the wet places, the cone-like seeds of alder drop near beaver lodges, and each seedball of the riparian sycamore releases 200 or more parasoled wings to ride the chill breath of Old Man Winter.
Yet winter seed dispersal is not without risk. If every seed sprouted at the flush of our first February thaw, or during a prolonged, languid autumn, then tender seedlings would risk being killed as soon as the cold weather returned. In order to prevent this from happening, seeds and buds enter a state of dormancy, a word that comes from the Latin dormire, “to sleep.”
Wherever they land, many seeds require extended exposure to the cold in order to sprout come springtime. During the late summer, sugar maple seeds spin away from the tree on whirligigs called samaras. There they lie, waiting to sprout, but first they must experience up to six months of winter cold. No cold, no baby sugar maples.
The seeds of many beloved wildflowers also lie in wait for the life-bringing kiss of cold, including those of columbine, baneberry, vervain, betony, hellebore, Jack-in-the-pulpit, and wild rose. Members of the diverse crowfoot family - such as hepatica, anemone, monkshood, marsh marigold (cowslip) and buttercup - produce seeds that will not germinate unless they’ve experienced prolonged temperatures between 15 and 25°F.
Dormancy for buds begins when days grow short, sunlight becomes less intense, and temperatures drop. Sugars, minerals, and proteins move out of the leaves and into winter storage in roots, trunks, and branches. Leaves fall from twigs, where buds have already formed. Each bud contains an embryonic leaf or flower enclosed in protective scales that help to keep it from drying out or freezing.
Dormant buds have everything they need to grow into leaves and flowers, but hormones called dormins repress their growth and induce a state of rest. Abscisic acid, in particular, inhibits buds and seeds. Some seeds, like acorns and walnuts, have a thick, hard coat that isolates them from winter weather.
Even if the weather warms up in mid-winter, most dormant seeds and buds will not start growing. Dormancy will only break when the days grow longer and warmer, the rain returns, and soil thaws so roots can again absorb water and minerals. Then plant-growth chemicals, such as ethylene, begin to encourage roots, stems, and buds to grow.
The system doesn’t always work perfectly. A late-arriving winter on the heels of a mild autumn, like this past year, may cause some buds to break dormancy prematurely and start to unfurl. Some cherry trees blossomed in Washington, D.C., this December, as did apple orchards and vineyards in Europe. Once this happens, delicate plant tissues lie open and exposed to the wrath of the returning winter, and they stand little chance of surviving. Even buds that haven’t actually opened may break dormancy and freeze inside when winter returns.
Fortunately, dormancy won’t release its hold on many plants until the plants are exposed to enough cold weather to have made it safely into the warm vernal days. A lilac bush grown in a warm greenhouse all winter will not form leaves or flowers come springtime. But a single branch on the same lilac, which has been protruding through a hole in the glass and exposed to the cold, will alone sprout leaves and bloom. This proves, beyond doubt, that springtime’s riot of green foliage and rainbow blossoms requires a steady diet of cold, hard winter.