Posts Tagged ‘freeze protection’

Weather Worries for Fruit Growers

Thursday, March 29th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Peaches, plums, and apricots have bloomed, several weeks ahead of normal. Strawberries have been blooming for weeks in plasticulture systems. These fruit crops are at great risk of losses due to freeze events. Other fruit may also flower early and be at risk. For example, pears are in bloom now and cherries and blueberries are starting to bloom.

Normally, the average date of the last frost in Delaware is somewhere between April 20-25. We still have four weeks of worry ahead for our fruiting crops.

For all these fruit crops the most susceptible stage of injury is when flowers have just opened. Closed buds have higher cold tolerance as do small fruit. For most fruits, critical temperature for losses after fruits have formed is 28° F.

 Plasticulture strawberries blooming 3- 29-2012.

Frost and freeze protection methods vary with fruits and the type of freeze expected. Advective freezes occur with freezing temperatures and high winds. This is the most difficult to protect against. For strawberries, two layers of floating row covers may be the most effective strategy for advective freezes. Double covers have been shown to be more effective than single heavy covers in this case. Irrigation along with double covers can provide even more protection if done properly.

Radiational freezes occur on cold, still nights. In this case cold air is near the ground and warmer air is above. Wind machines and helicopters have been successfully used to stir the air and raise the temperatures in orchards in this case. Row covers in strawberries will protect against radiational freezes too.

Irrigation has also been successfully used for frost protection but it has to be done properly. How irrigation works is that as ice forms on plants heat is released. The key is to keep ice formation occurring through the night and continue through melt in the morning. Remember that initially, until ice starts forming, there will actually be evaporative cooling of the plant. The latent heat of fusion (water freezing) will release heat (approximately 144 BTUs/lb of water), whereas evaporative cooling will absorb heat from the plant (absorbing approximately 1,044 BTUs/lb of water) and lower plant temperatures. Therefore, irrigation must start well above critical temperatures. Also, the volume of water needed needs to be matched with the expected temperature drop and wind speed. In addition, uniformity of water application is critical. This is difficult to do in high wind situations.

This past week temperatures dropped below freezing in parts of Delaware on three nights, with some areas in the mid-twenties. NOAA has predicted an increased risk for lower than normal temperatures in the Mid-Atlantic region for the next 2 weeks.

Strawberries, Row Covers & Freeze Protection

Friday, March 2nd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Row cover management in plasticulture strawberries has been difficult this year due to the mild winter. In normal winters, row covers applied in December serve as winter protection to limit stand losses, dessication damage, and low temperature damage to buds. While plants are in a dormant state or when buds are not yet active in strawberries, the buds can tolerate temperatures down to 10°F.

Removing row covers during warmer winter periods can help to delay bud activity and reduce susceptibility to later freezes. Replace row covers in times when freezes are expected. Highest yield potentials are usually obtained by uncovering and covering in the late winter and spring based on expected temperatures when compared to the practice of keeping row covers on continuously into the flowering stage.

Once buds have begun to emerge, even when tight, they can only tolerate temperatures down to 22°F. As they begin to open, the critical temperature for damage increases (popcorn stage 26°F, open blossom 30°F).

For growers that have not been taking row covers on and off and will be leaving them on until bloom, the potential for losses due to freeze events will be greater during March due to the increased bud activity. Prior to forecasted freeze events, check the plant bud stage, and apply additional freeze production to limit losses. This may include double covering with row covers (2 layers), or the use of low volume sprinklers through the night and into the morning as a frost protection over the row covers. Loss of buds or flowers due to freeze events will reduce yields and profits substantially. A single 1.2 ounce floating row cover will give about 4 degrees of protection.

Critical Temperatures for Strawberry Buds and Blossoms and Freeze Protection

Thursday, March 4th, 2010

Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu

As we move closer the spring season, it is important to consider critical temperatures for strawberry buds and blossoms and freeze protection, especially where floating row covers are being used to obtain earlier production.

While plants are in a dormant state or when buds are not yet active in strawberries, the buds can tolerate temperatures down to 10ºF. As buds become active but before flowers open, the buds can survive down to temperatures of 22-27ºF (depends on just how close they are to opening).

Removing row covers during warmer winter periods can help to delay bud activity and reduce susceptibility to freezes. Replace row covers in times when freezes are expected. Highest yield potentials are usually obtained by uncovering and covering in the late winter and spring based on expected temperatures when compared to the practice of keeping row covers on continuously into the flowering or early fruiting stage (another issue is that pollinating insects are restricted with covers resulting in smaller fruit size if left on too long).

As flowers open, strawberries can only tolerate drops in temperature down to 30ºF. Small green fruit can stand temperatures down to 28ºF.

During flowering and fruiting be prepared to freeze protect using row covers and in very cold conditions, sprinklers. There are usually several days where temperatures drop into the low 20s during the spring and freeze protection is critical. Removing row covers too early with no provision to replace them during freeze events will often result in severe yield losses.

There are two types of freeze events that you need to be concerned with. Radiation freezes are where heat from air is lost to a black, cloudless sky with little or no wind conditions. We can be very successful at protecting strawberries in these conditions with row covers and/or sprinklers. Advection freezes are where cold air is moving into the area with high wind. This cold air blowing across the crop with significant wind speed is much harder to deal with and where some losses are likely, even with freeze protection measures.

For row covers, the heavier, the more protection against freeze. However, the heavier the cover, the less light, and the more growth is restricted. A balance is what is desired. Generally, covers in the 0.9-1.25 ounce/sq. yd. range give adequate freeze protection without restricting light too much.

Sprinklers can also be used to freeze protect, but they must be used correctly. The idea is to slowly build up ice over the period when temperatures are below freezing over the plant or row covers. As ice is formed, some heat is released to the plant surface and to the surrounding air, due to the heat of fusion. To do this, sprinklers must be turned on before temperatures are at 34-35ºF. Use low volume sprinklers and apply irrigation throughout the night, building ice all night. Continue into the morning until ice has melted. An application rate of 0.15 inch per hour with no wind will provide protection to 22ºF. At colder temperatures or higher wind speeds more water will be needed.

It is recommended that for protection against frost above freezing, use sprinklers or row covers alone, for freezing temperatures in the mid to high 20s use sprinklers or row covers alone, for temperatures in the low 20s or below, use both sprinklers and row covers. A combination of row covers and sprinklers has been shown to protect well below 20ºF.