Hartley Magazine

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How Plants Freeze

In wintertime, the greenhouse is in its glory with colorful and glowing plants. It takes good care—water, heat and light—for these greenhouse beauties to survive. But just beyond the glass, the landscape stretches in patterns of gray and brown, every branch and drooping evergreen leaf outlined in a thin casing of ice. What mechanisms make those outside plants able to live through the winter? And why is one plant designated a Zone 7 and another a Zone 3?

I researched these questions and discovered that how plants handle water is an important factor for their survival. I called Ross Penhallagon, at the Oregon State University Extension Service office and he confirmed my findings.

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

Clever Desiccation Strategies

Some of this preparation involves removing water from their leaves, buds and stems. It’s similar to draining exposed pipes so ice won’t expand and burst them. In plants, the forming ice plays similar havoc with vulnerable cellular structures—why tomato vines are mush after a frost.
But hardier vegetation has evolved in colder climes. Some savvy plants move water from within their cells to between the cells, where there is more room to survive the expansion. All excess water goes out the roots into the surrounding soil. What’s left behind has higher concentrations of sugars that act like antifreeze. Plant survivors in colder zones have more of this antifreeze protection. Conifers, for instance, have quite a bit. Greenhouse tropicals have none.

Garden Myth Busted

When plants are in this self-induced dry state, gardeners can provide protection from winter winds. However, the common advice about watering plants well before a frost is often misguided. That might be true in parts of the country where freezes are seldom expected—orange groves are watered overhead because forming ice helps protect against extreme cold—but unless the ground is bone dry, Ross confirmed for me that watering the soil before a freeze may do more harm than good. After all, the plants are doing their best to get rid of excess water, right?

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