How Plants Survive Winter

  It's cold—the plants in the greenhouse are thriving-their glossy leaves in stark contrast to the view beyond the glass. Outside, brown and gray bare stems and drooping evergreen leaves are rimmed in ice. Have you ever wondered-what mechanisms make those outdoor plants survive beyond the greenhouse walls? And why is one plant designated a Zone 7 and another a Zone 3?

  I checked out these questions and learned that one of the main factors for cold survival is how plants handle water. Then I called Ross Penhallagon, at the Oregon State University Extension Service office. He confirmed what I had discovered.

  Ross told me that the first aspect of cold survival is acclimation. Plants do best when they get several weeks of exposure to near-freezing temperatures before a frost. This signals them to prepare.

  Dry Strategies Work

  One preparation method? Plants purposely put themselves in a state of desiccation. They remove excess water from their leaves-in the case of evergreens like rhododendrons and conifers-or from buds and stems. It's similar to draining exposed pipes so that ice won't expand and burst them. In plants, the forming ice plays havoc with unprotected cellular structures-why tomato vines are mush after a first frost.

  But tougher vegetation evolved in colder climates has adapted. Some savvy plants move water from within their cells to between the cells, where there is more room for expansion. All excess water goes from the roots into the surrounding soil. What's left behind has higher concentrations of sugars that act like antifreeze. Colder zone plants produce more of this antifreeze protection. Conifers have quite a bit. Greenhouse tropicals have none.

  How Gardeners Can Help

  When plants are in this self-induced dry state, protection from wind can be a plus. However, it's common gardening advice to water plants well before a frost. Maybe that's true in parts of the country where freezes are seldom expected—orange groves are watered overhead so forming ice protects the plants from lower than 32 degree temperatures. But unless the ground is bone dry, Ross confirmed for me that watering the soil before a frost may do more harm than good. After all, the plants are doing their best to get rid of water, right?

  So when you're in your cozy greenhouse, raise your watering can in salute to those denizens beyond the glass that display nifty strategies to survive the cold.