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From The Ground Up: Know what to do to protect plants in frosts, freezes

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By Lisa Slattery, Iowa State University Extension

It’s officially fall and our temperatures will go up and down for a little bit before real cold sets in. Protecting plants from colder temperatures to prolong the growing season is possible, but you need to understand when it truly is the end.

So what’s the difference between a frost and a freeze and how does each temperature event effect plants?

A frost is a short-term event, usually in the late night/early morning on a clear night, when the temperature dips. A frost can occur in temperatures between 32 and 36 degrees. Frosts can be light or hard, depending on the temperature. In a light frost, the tops of plants may be killed, but the lower parts may remain green. A hard frost can kill the entire plant. Frost can occur in pockets or it can be widespread.

A freeze is a more significant event, when a cold air mass moves into an area, bringing freezing temperatures, and winds over 5 mph. A freeze warning is called when there is an 80 percent chance or better that the temperature will be 32 degrees or lower.

Frost can occur without seriously damaging plants, but freezes can be more destructive. Sudden and prolonged freezing will be more damaging than gradual cooling of short duration. Plants in some areas are more likely to be affected. Plants at the bottom of slopes, where frost pockets can form, are more likely to be damaged, while hilltops often remain frost-free until a more severe frost occurs.

Some plants are more tolerant of cold temperatures; woody plants are less affected than succulent plants and fruits and flowers can be more sensitive than leaves.

If you know a frost is coming, you can protect your tender plants in two ways:

First, cover your plants the night before with a sheet or blanket. If you have row covers, you can use those too. Plastic doesn’t work well. The covering keeps the cold air from settling on your plants. Covering plants usually works when temperatures drop into the upper 20s. Plastic doesn’t work as a covering

Second, it sounds odd, but watering your plants also helps. If the drop in temperature isn’t too great, watering plants really early the next morning may help those that were left uncovered. Water sprayed directly on plants forms a protective barrier that insulates the plants. This method is used by Florida orange growers when a frost is forecast. But spraying plants with water doesn’t work in a hard freeze.

Most important, keep track of weather alerts from your local weather stations, so you know when to protect your garden and save your plants from damage.

For gardening questions, call the Linn County Extension Master Gardener Hortline at (319) 447-0647.

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