What does Frost do to Plants?

As I was walking to work today my heart broke when I saw this:

The blooms from all the Magnolia trees in the park next too my building were dead. Most had fallen off the tree, and some were just barely there. Mother nature tricked the trees into blooming early this year, making them vulnerable, with almost 80 degree temperatures last week. This week we had two hard frosts for two nights in a row, and the poor magnolias couldn’t stand it.

A Frost Primer

Frost occurs when water is deposited on an object from the air and then is cooled below the dew point and the freezing point (32 degrees Fahrenheit).

During this time of year (spring) in the northeastern United States, it usually occurs on cloudless nights with little wind. At night clouds act as Earth’s blanket, reflecting the heat of the Earth back to the ground. A lack of clouds and cool temperatures mean that the heat of the Earth just escapes, leaving the plants defenseless.

You will find varying degrees of frost, as well as plants with different tolerances. Some plants, like the magnolia pictured above, are not frost tolerant. Others, like daffodils which are also in bloom around here, look like they didn’t receive too much damage at all.

Here is a little guide for the different degrees of frost and what you should worry about:

Light frost: Occurs with temperatures between 29 degrees F and 32 degrees F (-2 degrees C to 0 degrees C). Only tender plants will die at these temperatures, while other plants will come back easily.

Moderate frost: Occurs with temperatures between 25 degrees F and 28 degrees F (-4 degrees C to -2 degrees C). Lots of damage to most vegetation, especially fruit blossoms and tender plants. Even semi-hardy plants will not survive these temperatures well.

Severe or hard frost: Occurs with temperatures below 24 degrees F (-4 degrees C). Almost no actively growing plant can escape these frosts without some kind of damage.

So What Exactly does Frost do to the Plants?

Frost injures plants in two ways. First, if cold enough, it freezes the cell walls damaging them. The second blow occurs if the days are warm and the plants heat up quickly. Then, the weakened cells burst and the plants die. It’s not just the cold that hurts them but the quick heating process as well. In order to save your plants you need to protect them from both.

If it’s too late and your plants are already damaged, make sure check out this post for how to revive frost damaged plants.

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