Down the hill from our house there are several giant mullein plants in various stages of growth. The most obvious are the old, dead stalks that bore flowers last summer. These stalks are hard, dry, dead. They have completed their mission in life. They have produced seeds to perpetuate the species.
Here and there along the roadside and creek bank are flat rosettes of furry green leaves. These are new plants that started growth late last summer from the seeds produced by the old, dry stalks. They will start new rapid growth next spring as soon as it is warm enough, producing a new flowering stalk and more seeds to start the cycle all over again. The old stalks will tumble and the old leaves will rot away beneath the snow, returning minerals to the soil to foster the new growth. How can this be? you ask. Why did winter’s cold kill the old plant but still leave the young plants alive to grow next spring?
Scientists have been doing a great many experiments recently to determine the answer to this and other questions about plant survival. One of the things they have studied is the purpose or effect that furry hairs have upon plants. There are many different kinds of hairs on plant leaves, ranging from single-cell hairs to multicellular hairs. Some hairs are just a single straight cell, while others are branched and tiered, almost tree-like in form. The furry mullein leaves are in this category. Find a green mullein leaf and bend it slightly to separate the hairs, and then look at them with a good hand lens to see their unusual shape.
For many years it has been believed that these hairs somehow kept the plants warmer and helped prevent them from freezing. However, it seems these hairs actually are of most use to plants in semi-arid, sunny locations. They increase the surface of the leaf and reduce the flow of warm air across the surface of the leaf, thus preventing the leaf from drying out; they also help keep more humid air trapped close to the leaf surface.
So, what keeps these overwintering leaves from freezing? It appears that they become dormant as the weather turns colder and the length of daylight shortens. These dormant plants apparently become more cold resistant because they store large quantities of sugar and carbohydrates in their roots. Some plants also actually develop the ability to endure colder temperatures, but there is a limit as to how low a temperature they can survive.
Plants also have cold-tolerant genes. Plant biologists have cloned these genes and put them into other plants to make them more cold hardy in attempts to produce strains of tomatoes, corn and potatoes that will grow farther north. These genes also figure in nurserymen advising gardeners to buy hardier stock from local or more northern nurseries, for the same species grown in the South will not develop the same cold acclimation. Our long late warm fall weather this year, followed by a sudden severe cold spell, probably was the death knell for some of our borderline plants — time will tell. Plants that survive our erratic winter weather have to be very hardy; we can help them by slowing down on watering in the fall, for this keeps producing new young growth that freezes easily. However, they should not be drought-stressed, as that too makes them more prone to freezing.
If storing sugars and carbohydrates keeps plants warm in winter, I think I should be able to be out walking in shorts, but it doesn’t seem to work for me. I am weary of winter, wind and cold. However, February brings the first migrants back, and on nice days it is a good time to prune shrubs and trees before they start new growth.