This recent spate of freezing temperatures has got me thinking about how some plants in my garden seem to be unfazed while others turn to mush. I find it fascinating that the flowers on my witch hazel, Cornelian cherry, Sarcococca, and even my beloved winter daphne haven’t skipped a beat while the early blooms of the Christmas Cheer and Olive rhodies and Camellias have turned completely brown. Some plants just seem to be better adapted to dealing with freezing temperatures. Why is this?
Genetics play a large part in this story. Plants have evolved to tolerate the conditions of where they are growing. Alpine plants often have small foliage like the needle-like leaves of conifers. These small leaves have a limited amount of surface area and are often covered with waxy materials that reduce their exposure to the elements. Even larger leafed broadleaf evergreens like PJM rhodies will lay down a fresh layer of cuticular wax in the summer in anticipation of the approaching winter.
Plants have “learned” to combat cellular freezing through a process of “supercooling” and “dehydration”. In supercooling, proteins are formed that protect the intracellular tissues down as low as -40. For further protection, plants produce “dehydrin proteins” that cause the plant to evacuate water from the cellular protoplasm into the intracellular spaces where ice crystals can form without damaging the plant. The concentrated fluids left in the cell contain sugars and amino acids that will freeze at a lower temperature and thus act like a kind of anti-freeze. This system works well, provided the plant has already acclimated to the fall and winter weather, the mercury doesn’t go down too low, and the plant hasn’t been lured out of dormancy due to unseasonably warm weather (which can often happen this time of year).
Another technique a plant will employ is called “thermonasty”. This is where a plant “moves” due to a stimulus such as light, heat, cold, or humidity. This is what causes a sunflower to follow the sun during the day. In the case of cold, plants will roll up their leaves into a tight circle and droop downward in order to reduce surface exposure. This is real obvious in rhododendrons.
In addition to the above actions, plants can simply remove water from their tissues by either respiring it or translocating it down to their root system, thus minimizing the possibility of forming ice crystals. With less water in their tissues they can appear to be wilting, which is exactly what my hellebores have been looking like in the mornings when I walk through my garden. Later in the day after the sun comes out and the temperature rises, they look just fine.
This winter has been very mild but cold enough (in my opinion) to adequately harden off plants so they can withstand most of our coldest nights. This might change at the beginning of this coming week and we need to be ready to add some extra protection to our darlings. If you live in the outlying areas, you might see temps as low as in the teens and that can cause problems with any plant that is pushing out new growth, like hydrangeas or early blooming trees like peaches or plums. Plan on covering the plants with blankets or “row cover” if you don’t want to see damaged shoots or blooms (don’t worry about your bulbs and pansies, they will be fine). Hopefully, this cold spell will be short lived and we can get back to enjoying an early spring.