So far this winter has been cold but not terribly snowy here in southeast Michigan. Low temperatures have been down to the single digits Fahrenheit (F), and we’ve been through at least a couple of straight weeks with highs only in the teens and twenties F. A few weeks ago we got a few inches of snow that persisted until now. Yesterday and today high temperatures got up around 50° F and the snow melted, so I took a stroll through a local floodplain forest.
I noticed at least a dozen different herbaceous plants that were still quite green and healthy. Several of them were sedges like the ones above, and nine others were the forbs shown below.
These plants may be winter-hardy, or they may be very cold-hardy. Winter-hardy plants remain green through the winter and either go dormant or grow intermittently on warmer days. Cold-hardy plants can tolerate temperatures down to a certain point before dying. All of the plants shown here seem to be cold-hardy to at least 5° F, our lowest temperature thus far this season.
Southeast Michigan is in USDA Hardiness Zone 5b, indicating that winter lows can reach -10° to -15° F. Since we’re still about 20° shy of those lows, some of these plants could still die off this winter.
Plants are injured by sub-freezing temperatures due to the formation of ice crystals in their cells. Water in the cells can freeze, and the resulting ice crystals can rupture the membranes of the plant cells, causing cell death. Excessive ice formation can destroy enough of the cells to kill the plant.
These hardy herbs are protected from the cold by several factors. First, their compact structures minimize exposed surface area. Second, the thick leaf cover (and sometimes snow) on the ground helps to insulate them. Third, and most importantly, these plants have two main physiologic mechanisms for cold tolerance.
The first mechanism is called “supercooling.” Water below 32° F requires nucleation points for initial ice crystal formation. These plants can build up dissolved solutes that prevent ice crystal nucleation. In this manner, the water in plant tissues can drop well below 32° F before damaging ice crystals form (Seiler and Groninger 2009).
The second mechanism is known as “intracellular dehydration.” With this approach, plant cells readily release their water into the spaces between the cells. The cells dehydrate, and the water then freezes between the cells instead of within them. This prevents the cells from being damaged by ice crystals (Seiler and Groninger 2009).
These plants require an acclimation period in order to initiate these mechanisms. Reduced daylight and lower temperatures give the plants cues to make the necessary physiological and biochemical changes. A plant that is suddenly exposed to sub-freezing temperatures without acclimation will likely die, since it has not yet prepared itself for the cold.
I plan to keep on eye on these plants throughout the winter to see how well they stand up to even colder temperatures. I already know the sedges and garlic mustard will survive, but I don’t know about the other herbs.
Source: Seiler, J.R. and J.W. Groninger. 2009. Dormancy and Coldhardiness. In: Forest Biology. Virginia Polytechnic Institute and State University.