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Avoiding damage to plants during spring frosts, freezes

By Fred M. Hall

Wichita County AgriLife Extension Agent

With landscapes and gardens turning green the weatherman has once again pulled the dastardly deed of predicting temperatures below freezing. Gardeners are scrambling to protect plants from the impending doom, but few may really understand the science of how freezing temperatures damage plants. On of the best Extension publications on this type of damage is "Frosts and Freezes" by Skip Richter. He notes that when the water inside a plant freezes, it causes ice crystals to form that pierce the cell walls of the plant. When the temperature warms up, the cells leak out their fluids and die.

Richter points out that frost appears on the surface of plant tissues as well as on most any other exposed surface. During the night these surfaces radiate heat to the sky. When their temperature drops to the freezing point. the water vapor next to it freezes on the surface.

It is possible for frost to form when the air temperature is above freezing. Solid surfaces lose heat faster than air on a cold night. As a result we see frost on a windshield when few other things around the landscape show frost. The surface of a leaf can also drop a little below the temperature of the air around it on a cold night, causing it to drop below freezing and frost to form on the surface. Remember that frost is a sign that the plant tissues have dropped below freezing.

Anything that reflects the radiating heat back down will prevent or at least greatly reduce frost formation. In winter, walk out on a frosty morning and notice that while there is frost on the lawn around your landscape, underneath the live oak tree or underneath a picnic table there is little if any frost. Clouds perform the same radiant heat-reflecting function. On a clear night temperatures drop fast. On a cloudy night much heat is reflected back to the ground, slowing the drop and in many cases preventing a frost or freeze.

The terms frost and freeze to refer to different types of temperature-related events according to Richter. Typically frost forms on a still night when the temperature drops to near or just below freezing. A freeze, on the other hand, refers to a more extended period below freezing and may or may not include wind.

Most of the time in the fall or spring season, gardeners are dealing with a marginal freeze where the temperature drops briefly to just below freezing at the end of the night and then moves back up above freezing soon after the sun rises. This is enough to destroy a fall or spring garden or fruit blooms and the hope of a spring crop.

We can do a lot to protect plants from a marginal freeze because the temperatures are usually not too low and the duration is brief. Hopefully there is also not much wind, thus making protective measures easier and more effective.

When a hard freeze hits with a strong wind and lasts for a day or more, though, there is usually little we can do to protect our gardens. The wind displaces any heat that might have helped protect the plants and speeds cooling of plant tissues. The extended time below freezing makes our simplest protective measures inadequate to the task.

There are a number of techniques we can use to help avoid freeze damage to our plants. Here are a few of the more common ones.

--Watering: All plants under drought stress can be more susceptible to cold damage. By watering plants several days or more before cold weather threatens you can relieve stress if they are suffering from drought.

Water is also a great "heat sink." That is, it holds warmth and releases it slowly--more slowly than plant surfaces or air. Watering your plants right before a freeze creates a source of warmth that will slowly lose its heat over the course of a long cold evening. This alone is not going to provide protection from a hard freeze but can be used with covers to make a small difference on a marginal night, and every little bit helps.

The second way water is used is by sprinkling plants on a cold night. The basic concept involves the physics of water. If you were to chart the drop in temperature of water you would see that it drops steadily to about 32 degrees and then levels off before dropping again after the water freezes. It takes a lot of energy to push water to change from liquid to solid. That is the key to using water to protect plants. However, protecting plants with sprinklers, while possible in some situations, is seldom a viable option.

--Covering plants: Covering plants is the simplest, most practical way to protect against a frost or freeze. Gardeners head out with sheets, blankets, plastic, rowcovers and anything else that they can get their hands on to wrap up plants for a cold night.

Keep in mind, however, that a blanket doesn't keep a plant warm, at least not to any significant degree. Blankets keep us warm because our bodies produce heat that the blanket helps hold in. If you wrap up the branches of a small tree or shrub with a blanket, you aren't doing it much good. In fact they may keep some of the heat available to the plant away from it.

Here's what Richter means. The main source of heat for a plant is the soil. On a cold night heat from the soil rises up around the plants. If you use a blanket to trap this heat within the plant's canopy you can make a very significant difference on a cold night. When I talk about trapping heat, I don't necessarily mean warm air, just air that is warmer than freezing. If you keep the temperature around plants from dropping below freezing, you have accomplished your goal. Even cold soil is actually significantly warmer than freezing and thus a source of "heat" on a cold night.

To cover plants effectively, lay the cover over the plant and allow it to drape down to the soil on all sides. Then secure it with boards, bricks, rocks or soil to hold in the air. This is especially helpful in preventing a breeze from cooling things down faster. The next day, remove the covers to allow the sun to warm the soil surface a little and then replace the covers as the sun goes down.

Cardboard boxes and large round garbage cans can be used to cover plants. Plastic sheeting or any material that radiates its heat out quickly will "burn" (actually freeze) plant tissues where it touches them. It also tends to not reflect the radiant heat back down as well. Plastic is good, however, in holding in the air on a windy night, so if you cover the plastic with a blanket or sheet you can increase the amount of heat reflected back to the plant and soil.

--Adding heat: If it is going to get too cold for a simple cover to protect your plants, adding a source of heat beneath the cover can make a big difference. Anything that provides some heat is going to be helpful, especially if you have a good cover that is secured to prevent wind from moving the warmer air out from beneath it.

Two common ways to add heat are by adding a mechanic's light or a string of Christmas lights beneath the cover. When I say Christmas lights I mean the big ones, not the little twinkling things, as they don't put out much if any heat. Don't allow a hot light bulb to come close to plant tissues or they can suffer damage.

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