From the Ground Up: Know the difference between a frost, freeze

Even though the growing season is coming to an end it's nice to understand the difference between a frost and a freeze and how to protect tender plants. In fall and spring, when plants are not yet dormant, or are coming out of dormancy, they are susceptible to extremes of cold.

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, blanket, or tarp. If you have row covers, you can use those, too. The covering keeps the cold air from settling on your plants. Covering plants usually works when temperatures drop into the upper 20s. Iowa State University says, 'Plastic ... doesn't work as well as the other coverings mentioned.”

Second, it sounds crazy, but watering your plants also helps. You can sprinkle them as a 'morning after” solution. If the drop in temperature is not too great, watering plants early the next morning may protect those that were left uncovered. Wet soil holds more water than dry soil, and water sprayed directly on plants forms a layer of ice that insulates the plants. This is method the Florida orange growers use 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.