Plants Cleverly Endure The Big Chill

As a warm-blooded creature, all I can conclude is that January is about basic survival.  Howling wind, falling snow and plunging mercury has my old house’s furnace working double-time.  I’m huddled under an afghan, counting on my laptop to keep me warm.  Perhaps it would be easier to go dormant like the plants outside, which are just as alive as me yet somehow better equipped to beat the chill.  So how can species such as oaks and maples, composed largely of water, survive double digit negative numbers while water itself freezes at a rather balmy 32 F?

For such woody plants, building up cold tolerance is a process.  Stage one hardiness is triggered by the shortening daylight and cooler temperatures of autumn, which stop growth, encourage the shedding of leaves, and trigger other chemical reactions which are not completely understood.  Just this decrease in light and temperature helps, allowing a stem to remain alive down to about 0 F, but no more.         

Next comes something called “deep supercooling.”  This ability to suppress the freezing of sap within plant cells is also somewhat of a mystery, but it allows many species to survive down to -40 F.  Part of the secret here are recently discovered “anti-freeze” proteins, which inhibit the growth and recrystallization of ice within the living part of the cells.  Certain animals can produce anti-freeze proteins, too, which led some scientists to insert an arctic flounder anti-freeze gene into sweet corn back in the ‘90’s, hoping to make a hardier crop.  While such tinkering with Mother Nature gives some people the chills, growing corn in Alaska would be super-cool.

The toughest of the tough – plants like paper birch, willow and red-twig dogwood – have another trick up their trunks to survive even colder conditions:  dehydration.  As temperatures plunge, water moves from inside the living cells into the dead cell walls, causing two good things to happen.  First, water freezing in the cell walls causes no harm to the plant, since the cells walls are dead.  Second, the sugars and other compounds inside the cell become concentrated, lowering the freezing point.  This parallels the practice of putting anti-freeze solution in car radiators, so your Dodge Aspen is more similar to a quaking aspen than you might otherwise think.   

Evergreen rhododendrons have a few tricks of their own, too.  They look pitiful when their leaves droop and curl on a frigid day, but it’s just a defense mechanism.  Rhodos loose water through stomates, tiny openings on the undersides of their leaves, and the drooping shelters stomates from the wind.  The trouble really starts when the soil freezes, cutting off the water supply from the roots, and causing browning of the foliage, a.k.a. winter injury, to occur.  But amazingly, some cultivars have even this figured out.  Rhododendron carolinianum ‘P.J.M.,’ one of the hardiest available, can lose up to 70% of its leaf moisture and be just fine, while the more tender Rhododendron catawbiense ‘Grandiflorum’ is injured when only 50% dehydrated.

Still shivering?  Pass the arctic flounder, please.