How Plants Overwinter
(This column was first published in the November 4, 2002 issue of The Buffalo News.)
The abrupt transition between summer and fall always impresses me here on the Niagara Frontier. It seems only yesterday when we sweated through the tail end of that hot summer with daily temperature highs in the 80s and even 90s. Our highs now are in the 40s and 30s and we seem largely to have bypassed the 70s, 60s and 50s.
Each fall here we have to race to get our summer and winter clothing exchanged, our screens and storm windows switched, and our shovels out to prepare for that first snowfall -- falling as I write.
But if we're busy, so too are our botanic neighbors.
Trees and shrubs are hardening. The shorter day length has informed them that winter is approaching and they must respond. Deciduous trees are significantly reducing their surface area and the associated water loss by shedding their leaves. Evergreens are closing the tiny pores on their needles to prevent similar water loss.
Meanwhile all trees are instituting internal chemical changes that will protect them from freezing. In some cases they even synthesize chemicals much like the anti-freeze we use in our automobile radiators. Failure to accomplish this in time can lead to those loud cracks you occasionally hear in the forest after an early hard freeze. The cause of those noises: internal water is turning to ice and in the process expanding. More important than the sound, sometimes branches or even tree trunks are split in the process.
Those of us old enough to recall home delivery of milk in glass bottles can recall how much water expands when frozen. Left out in cold weather the water in the milk turned to ice, forcing off the bottle cap and shoving a one or two inch neck of white slush out of the top.
In many ways perennial wildflowers mirror the processes of their woody cousins. As with those on trees, their buds, already signaling the new leaves and flowers of next spring, are protected by thick, waxy scales. Here too anti-freeze-like chemicals are at work.
Annual wildflowers, plants that last only one growing season, must employ different strategies. For them life is carried forward only in their progeny, their seeds. They are distributed in various ways. Some are spread by wind. My favorite example is the milkweed whose pods will continue to release those white parachutes through much of the winter, thus broadcasting the dainty seeds that are attached. Others, like beggar's ticks, hitch rides on animals by attaching themselves to fur -- or, in our case, to clothing. And still others like jewelweed develop mechanisms that, when triggered, fire their seeds some distance. These touch-me-nots are autumn favorites of small children.
Some of those seeds will survive the cold weather under the leaves that now cover the ground and they will germinate to produce what seems like miraculous new life in the spring.
A third kind of plant is the biennial. Their two-year life cycle is quite different from that of annuals and perennials. They need two seasons to develop the seeds that annuals produce in one so they must survive that midlife winter. They usually do so by developing large taproots and in that first year spreading only horizontally to form what are called basal rosettes. Blanketed under the snow these leaves remain green through the winter. I often come across the showiest of these rosettes on winter hikes: the big, furry and light green leaves of the common mullein.
Carrots are also biennial. Queen Anne's lace is the wild variety. Of course, we don't let her tame cousin live out its second year.-- Gerry Rising