Frost Protection- A sad story
Mark Halcomb, UT Area Nursery Specialist

Nursery Plants will be damaged and seedlings will be killed if the forecast is correct (for April 4-8, 2007 with lows in west KY forecast to be 23 degrees Fahrenheit). Here is some information that might assist you in making decisions.

How cold it gets, how long the temperature stays that low, and the wind are serious factors affecting the degree of injury.

Should you be concerned? Yes, based on the forecast I have heard.

Should you attempt to provide protection? If you can not afford to loose the plants, then Yes, if you can.

We are concerned with germinated seedlings, rooted cuttings, first year buds and just the tender growth on plants in the nursery and landscape. Be sure the soil or media is moist. A dry soil will allow more damage to occur.

One method of cold protection involves figuring how to hold the earth's heat in around the crop and prevent the wind from blowing the heat away. If using a cover, holding the edge firmly to the ground is critical for success. Use concrete blocks, cross-ties, timbers, pots of gravel, soil in pots, etc.

Another method of protection uses the heat generated by the action of freezing water, when plenty of water is available. Turn the overhead irrigation on at 32 degrees and off when the ice melts. This may mean leaving the water on all night. Will you have sufficient water to do this each night needed? No need to do it 2 nights and not be able to the 3rd. It usually means someone staying up to determine when to crank the pump, stay with it to monitor the engine, and determine when to shut it off. An accurate thermometer should be placed near the crop foliage that is to be protected, not on the barn door 6' above the ground.

Frost covers (there are also overwintering blankets) are the easiest and fastest ways to cover large areas of plants. But it is too late to have something shipped to you and local stores are sold out. Greenhouse poly laid over the crop will offer protection. I would prefer white. But it must be supported and not allowed to touch foliage. Twine or wire could be tied several inches above the crop, to steel rods or steel fence posts. One or a stack of empty containers could be used to provide the support.

Shade cloth would be better than nothing, but I would expect it to allow much of the heat to escape through the holes. It could be used to insulate and prevent poly from touching tender foliage. ( poly laid over shade cloth)

Seedlings and first year buds in rows can be covered by burlap or straw. I have seen a 36" wide roll of burlap cut in half (18") with a band saw (not easy) and laid over and centered on the row; supported by a twine held 6" above the soil. The twine was tied to stakes driven down the center of the row, every 10' or so. The stakes were probably 12" in soil and 6 or 7" above the soil. Edges of burlap was pegged to the ground every 3-4' with 16d nails. Or soil could hold the edge down. The burlap could be used in future years. Using soil would shorten the life. Store away from rats. The burlap can be installed in March and left til early May. I have seen it done. Enough sunlight got through the burlap. Do not use the thickest, heaviest.

I do not have any ideas for acres and acres of peach or dogwood seedlings. If you do, please share.

Wheat straw can be scattered loosely over the crop and probably left during a cloudy cold day, but removed if the next day gets hot. I have heard of newspaper and paper towels laid over the rows. Just depends how big the area is. There are bed sheets and towels for desperate times. Sawdust can be applied to cover the seedlings, but will probably need to be removed the next day. It can be removed with a commercial leaf blower on wheels, but can not be found to use another night.

I can provide more details on several of the ideas.
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After the fact statements by my counterpart in North Carolina May 21, 2002 following a low of 20 and 24 degrees, May 20, 2002

SUBJECT: Frost Follow-up/Freeze Hindsight

I have been told it was 24 F. in one mountain nursery on Monday, May 20 and 20 F. in another. There is scattered damage to new plant growth throughout our region and devastating damage in some places.

This may be a good time to remind folks of certain facts:

1. Hardy plants are not as hardy when they are actively growing versus dormant. Sugar maples and Colorado blue spruce can have new growth killed by frosts even though they are hardy to below zero temperatures once established in landscapes and dormant.

2. Hardy plant seedlings and rooted cuttings that are actively flushing growth are often killed by frost. They have little or no reserves to draw upon for regrowth.

3. The degree of tolerance for cold temperatures varies from species to species and among cultivars within the same species.

4. When in doubt, the safe thing to do is to protect. Often all that is needed is a temporary cover as light as Reemay. Any cover that can hold in some ground heat when we are faced with circumstances like we have experienced in the past few days, can help protect. Shade cloth protected some hostas from the cold this week where temperatures in the mid 20's were recorded!

5. There is a temptation to "ice down" a crop with irrigation to protect them under conditions like we experienced. In most situations, this is a bad idea and pulling a cover over the plants is a better idea.
If your growers decide to use irrigation to protect, they need to continue to irrigate until the temperatures rise to above freezing again. In addition to cold injury, the weight of ice on new growth in a nursery can cause great destruction.

6. Do not apply more fertilizer to plants that have been cold damaged thinking that fertilizer will accelerate new growth. If the plants have already been fertilized this spring, the controlled release fertilizers are still present and there is often less photosynthesis/transpiration taking
place because of damaged or killed foliage. Do not add more salts to the production system thinking you are helping the plants.

7. Wait a day or two to prune. A lot of plants will look limp after cold and may appear ready to have leaves die but those stems and leaves will recover from frost or chilling injury . . . if they are not removed by pruning.

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Antitranspirants and Frost Protection
I get a few calls each spring about using antitranspirants (Vapor Gard, Wilt Pruf) to protect small seedlings (2 inches tall) in the row from late frosts. I have heard of a few successes and a few failures. I suspect the antitranspirants may not have helped at all. The producer may have credited the antitranspirants when maybe the temperature at the soil may not have gotten low enough to cause injury.
Dr. Ken Tilt, nursery specialist at Auburn, recalls that while he was Tennessee's Nursery Specialist, he too heard of accounts when the antitranspirant was not effective, sometimes effective and occasionally even harmful.
Success or failure could be affected by the species, plant size, actual temperature at the soil line, humidity and wind.
I asked Dr. Stephen Garton, current UT Ext. Nursery Specialist, about this in 2003. He responded by saying, "This is not a simple thing but on the whole I agree that frost protection of tender plant structures with antitranspirants is not effective and therefore not recommended. Like every other generality there will be exceptions and there are probably people who have tried such treatments on one or two species in specific situations who are convinced that the antitranspirant was beneficial. In the absence of good scientific information, we cannot endorse such practices."

Mark A. Halcomb
UT Area Nursery Specialist
Warren Co. Extension
201 Locust St., Suite #10
McMinnville, Tenn. 37110